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• DNS no longer works after server reboot

  - by Burning the Codeigniter

  Strangely enough, when I reboot my Ubuntu 12.04 server, the DNS no longer works, which makes the domain unavailable to access to my site. Normally the DNS should be working after a reboot, but this doesn't happen anymore. I use nginx to serve content, but nginx is already configured to work with my domains. What are the typical practises must I do after a reboot and how can I solve this issue I experience? I already have BIND, networking and resolvconf to boot when the server boots up. ; <<>> DiG 9.8.1-P1 <<>> mysite.com ;; global options: +cmd ;; connection timed out; no servers could be reached This is my output with dig $ttl 38400 mysite.com. IN SOA ns1.mysite.com. webmaster.mysite.com. ( 1055026205 6H 1H 5D 20M ) mysite.com. IN A xx.xx.xx.xx # Server IP *.mysite.com. IN A xx.xx.xx.xx # Server IP www.mysite.com. IN CNAME mysite.com. ns1.mysite.com. IN A xx.xx.xx.xx # Server 2nd IP ns2.mysite.com. IN A xx.xx.xx.xx # Server 3rd IP mysite.com. IN NS ns1.mysite.com. mysite.com. IN NS ns2.mysite.com. mail.mysite.com. IN MX 1 mysite.com. This is the contents of /etc/resolv.conf # Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8) # DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN nameserver 85.17.150.123 nameserver 85.17.96.69 nameserver 62.212.64.122 search localdomain After using more dig commands, outputs: ; <<>> DiG 9.7.3-P3 <<>> @85.17.150.123 mysite.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 24847 ;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0 ;; WARNING: recursion requested but not available ;; QUESTION SECTION: ;mysite.com. IN A ;; Query time: 2145 msec ;; SERVER: 85.17.150.123#53(85.17.150.123) ;; WHEN: Mon Nov 5 16:31:32 2012 ;; MSG SIZE rcvd: 30 ; <<>> DiG 9.7.3-P3 <<>> @85.17.96.69 mysite.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 27879 ;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0 ;; WARNING: recursion requested but not available ;; QUESTION SECTION: ;mysite.com. IN A ;; Query time: 949 msec ;; SERVER: 85.17.96.69#53(85.17.96.69) ;; WHEN: Mon Nov 5 16:32:59 2012 ;; MSG SIZE rcvd: 30 ; <<>> DiG 9.7.3-P3 <<>> @62.212.64.122 mysite.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 29293 ;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0 ;; WARNING: recursion requested but not available ;; QUESTION SECTION: ;mysite.com. IN A ;; Query time: 825 msec ;; SERVER: 62.212.64.122#53(62.212.64.122) ;; WHEN: Mon Nov 5 16:33:39 2012 ;; MSG SIZE rcvd: 30 With Google DNS (8.8.8.8): ; <<>> DiG 9.7.3-P3 <<>> @8.8.8.8 mysite.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 38498 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;mysite.com. IN A ;; Query time: 3982 msec ;; SERVER: 8.8.8.8#53(8.8.8.8) ;; WHEN: Mon Nov 5 16:37:27 2012 ;; MSG SIZE rcvd: 30

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• Linked List exercise, what am I doing wrong?

  - by Sean Ochoa

  Hey all. I'm doing a linked list exercise that involves dynamic memory allocation, pointers, classes, and exceptions. Would someone be willing to critique it and tell me what I did wrong and what I should have done better both with regards to style and to those subjects I listed above? /* Linked List exercise */ #include <iostream> #include <exception> #include <string> using namespace std; class node{ public: node * next; int * data; node(const int i){ data = new int; *data = i; } node& operator=(node n){ *data = *(n.data); } ~node(){ delete data; } }; class linkedList{ public: node * head; node * tail; int nodeCount; linkedList(){ head = NULL; tail = NULL; } ~linkedList(){ while (head){ node* t = head->next; delete head; if (t) head = t; } } void add(node * n){ if (!head) { head = n; head->next = NULL; tail = head; nodeCount = 0; }else { node * t = head; while (t->next) t = t->next; t->next = n; n->next = NULL; nodeCount++; } } node * operator[](const int &i){ if ((i >= 0) && (i < nodeCount)) throw new exception("ERROR: Invalid index on linked list.", -1); node *t = head; for (int x = i; x < nodeCount; x++) t = t->next; return t; } void print(){ if (!head) return; node * t = head; string collection; cout << "["; int c = 0; if (!t->next) cout << *(t->data); else while (t->next){ cout << *(t->data); c++; if (t->next) t = t->next; if (c < nodeCount) cout << ", "; } cout << "]" << endl; } }; int main (const int & argc, const char * argv[]){ try{ linkedList * myList = new linkedList; for (int x = 0; x < 10; x++) myList->add(new node(x)); myList->print(); }catch(exception &ex){ cout << ex.what() << endl; return -1; } return 0; }

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• Undefined referencec to ...

  - by Patrick LaChance

  I keep getting this error message every time I try to compile, and I cannot find out what the problem is. any help would be greatly appreciated: C:\DOCUME~1\Patrick\LOCALS~1\Temp/ccL92mj9.o:main.cpp:(.txt+0x184): undefined reference to 'List::List()' C:\DOCUME~1\Patrick\LOCALS~1\Temp/ccL92mj9.o:main.cpp:(.txt+0x184): undefined reference to 'List::add(int)' collect2: ld returned 1 exit status code: //List.h ifndef LIST_H define LIST_H include //brief Definition of linked list class class List { public: /** \brief Exception for operating on empty list */ class Empty : public std::exception { public: virtual const char* what() const throw(); }; /** \brief Exception for invalid operations other than operating on an empty list */ class InvalidOperation : public std::exception { public: virtual const char* what() const throw(); }; /** \brief Node within List */ class Node { public: /** data element stored in this node */ int element; /** next node in list / Node next; /** previous node in list / Node previous; Node (int element); ~Node(); void print() const; void printDebug() const; }; List(); ~List(); void add(int element); void remove(int element); int first()const; int last()const; int removeFirst(); int removeLast(); bool isEmpty()const; int size()const; void printForward() const; void printReverse() const; void printDebug() const; /** enables extra output for debugging purposes */ static bool traceOn; private: /** head of list */ Node* head; /** tail of list */ Node* tail; /** count of number of nodes */ int count; }; endif //List.cpp I only included the parts of List.cpp that might be the issue include "List.h" include include using namespace std; List::List() { //List::size = NULL; head = NULL; tail = NULL; } List::~List() { Node* current; while(head != NULL) { current = head- next; delete current-previous; if (current-next!=NULL) { head = current; } else { delete current; } } } void List::add(int element) { Node* newNode; Node* current; newNode-element = element; if(newNode-element head-element) { current = head-next; } else { head-previous = newNode; newNode-next = head; newNode-previous = NULL; return; } while(newNode-element current-element) { current = current-next; } if(newNode-element <= current-element) { newNode-previous = current-previous; newNode-next = current; } } //main.cpp include "List.h" include include using namespace std; //void add(int element); int main (char** argv, int argc) { List* MyList = new List(); bool quit = false; string value; int element; while(quit==false) { cinvalue; if(value == "add") { cinelement; MyList-add(element); } if(value=="quit") { quit = true; } } return 0; } I'm doing everything I think I'm suppose to be doing. main.cpp isn't complete yet, just trying to get the add function to work first. Any help will be greatly appreciated.

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• Lockless queue implementation ends up having a loop under stress

  - by Fozi

  I have lockless queues written in C in form of a linked list that contains requests from several threads posted to and handled in a single thread. After a few hours of stress I end up having the last request's next pointer pointing to itself, which creates an endless loop and locks up the handling thread. The application runs (and fails) on both Linux and Windows. I'm debugging on Windows, where my COMPARE_EXCHANGE_PTR maps to InterlockedCompareExchangePointer. This is the code that pushes a request to the head of the list, and is called from several threads: void push_request(struct request * volatile * root, struct request * request) { assert(request); do { request->next = *root; } while(COMPARE_EXCHANGE_PTR(root, request, request->next) != request->next); } This is the code that gets a request from the end of the list, and is only called by a single thread that is handling them: struct request * pop_request(struct request * volatile * root) { struct request * volatile * p; struct request * request; do { p = root; while(*p && (*p)->next) p = &(*p)->next; // <- loops here request = *p; } while(COMPARE_EXCHANGE_PTR(p, NULL, request) != request); assert(request->next == NULL); return request; } Note that I'm not using a tail pointer because I wanted to avoid the complication of having to deal with the tail pointer in push_request. However I suspect that the problem might be in the way I find the end of the list. There are several places that push a request into the queue, but they all look generaly like this: // device->requests is defined as struct request * volatile requests; struct request * request = malloc(sizeof(struct request)); if(request) { // fill out request fields push_request(&device->requests, request); sem_post(device->request_sem); } The code that handles the request is doing more than that, but in essence does this in a loop: if(sem_wait_timeout(device->request_sem, timeout) == sem_success) { struct request * request = pop_request(&device->requests); // handle request free(request); } I also just added a function that is checking the list for duplicates before and after each operation, but I'm afraid that this check will change the timing so that I will never encounter the point where it fails. (I'm waiting for it to break as I'm writing this.) When I break the hanging program the handler thread loops in pop_request at the marked position. I have a valid list of one or more requests and the last one's next pointer points to itself. The request queues are usually short, I've never seen more then 10, and only 1 and 3 the two times I could take a look at this failure in the debugger. I thought this through as much as I could and I came to the conclusion that I should never be able to end up with a loop in my list unless I push the same request twice. I'm quite sure that this never happens. I'm also fairly sure (although not completely) that it's not the ABA problem. I know that I might pop more than one request at the same time, but I believe this is irrelevant here, and I've never seen it happening. (I'll fix this as well) I thought long and hard about how I can break my function, but I don't see a way to end up with a loop. So the question is: Can someone see a way how this can break? Can someone prove that this can not? Eventually I will solve this (maybe by using a tail pointer or some other solution - locking would be a problem because the threads that post should not be locked, I do have a RW lock at hand though) but I would like to make sure that changing the list actually solves my problem (as opposed to makes it just less likely because of different timing).

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• VMWare player - compiling server modules - Ubuntu 13.10

  - by user211976

  While running Ubuntu 13.04 whenever the Linux kernel had been updated, this used to make vmware player happy: sudo apt-get install linux-headers-$(uname -r) sudo vmware-modconfig --console --install-all Yesterday I upgraded to Ubuntu 13.10 and lo and behold, the above workaround does not work anymore: Unable to install all modules. See log for details. I assume by "See log" it means the files in /tmp/vmware-root/*log root@hugin:/tmp/vmware-root# ls -ltr /tmp/vmware-root/ totalt 16 -rw-r--r-- 1 root root 3815 nov 6 13:54 vmware-apploader-17267.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-17693.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-17742.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-18701.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-18750.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-19100.log -rw-r--r-- 1 root root 0 nov 6 13:54 vmware-vmis-19149.log -rw-r--r-- 1 root root 9250 nov 6 13:54 vmware-modconfig-17267.log root@hugin:/tmp/vmware-root# tail /tmp/vmware-root/vmware-modconfig-17267.log 2013-11-06T13:54:28.950+01:00| modconfig| I120: Copied Module.symvers from "/tmp/modconfig-wpDrtf/vmci-only/Module.symvers" to "/tmp/modconfig-wpDrtf/vsock-only/Module.symvers". 2013-11-06T13:54:28.950+01:00| modconfig| I120: Building module with command "/usr/bin/make -j8 -C /tmp/modconfig-wpDrtf/vsock-only auto-build HEADER_DIR=/lib/modules/3.11.0-12-generic/build/include CC=/usr/bin/gcc IS_GCC_3=no" 2013-11-06T13:54:31.048+01:00| modconfig| I120: Successfully built vsock. Module is currently at "/tmp/modconfig-wpDrtf/vsock.o". 2013-11-06T13:54:31.048+01:00| modconfig| I120: Found the vsock symvers file at "/tmp/modconfig-wpDrtf/vsock-only/Module.symvers". 2013-11-06T13:54:31.048+01:00| modconfig| I120: Installing vsock from /tmp/modconfig-wpDrtf/vsock.o to /lib/modules/3.11.0-12-generic/misc/vsock.ko. 2013-11-06T13:54:31.048+01:00| modconfig| I120: Registering file "/lib/modules/3.11.0-12-generic/misc/vsock.ko". 2013-11-06T13:54:31.400+01:00| modconfig| I120: "/usr/lib/vmware-installer/2.1.0/vmware-installer" exited with status 0. 2013-11-06T13:54:31.400+01:00| modconfig| I120: Registering file "/usr/lib/vmware/symvers/vsock-3.11.0-12-generic". 2013-11-06T13:54:31.764+01:00| modconfig| I120: "/usr/lib/vmware-installer/2.1.0vmware-installer" exited with status 0. 2013-11-06T13:54:31.786+01:00| modconfig| I120: We are now shutdown. Ready to die! root@hugin:/tmp/vmware-root# tail /tmp/vmware-root/vmware-apploader-17267.log 2013-11-06T13:54:20.911+01:00| appLoader| I120: libglib-2.0.so.0 <SYSTEM> 2013-11-06T13:54:20.911+01:00| appLoader| I120: libz.so.1 <SYSTEM> 2013-11-06T13:54:20.911+01:00| appLoader| I120: libvmware-modconfig-console.so <SHIPPED> 2013-11-06T13:54:20.912+01:00| appLoader| I120: Shipped glib version is 2.24 2013-11-06T13:54:20.912+01:00| appLoader| I120: System glib version is 2.38 2013-11-06T13:54:20.912+01:00| appLoader| I120: Using system version of glib. 2013-11-06T13:54:20.912+01:00| appLoader| I120: Loading system version of libgcc_s.so.1. 2013-11-06T13:54:20.912+01:00| appLoader| I120: Loading system version of libglib-2.0.so.0. 2013-11-06T13:54:20.912+01:00| appLoader| I120: Loading system version of libz.so.1. 2013-11-06T13:54:20.912+01:00| appLoader| I120: Loading shipped version of libxml2.so.2.

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• IIS Logfile Visualization with XNA

  - by BobPalmer

  In my office, I have a wall mounted monitor who's whole purpose in life is to display perfmon stats from our various servers.  And on a fairly regular basis, I have folks walk by asking what the lines mean.    After providing the requisite explaination about CPU utilization, disk I/O bottlenecks, etc. this is usually followed by some blank stares from the user in question, and a distillation of all of our engineering wizardry down to the phrase 'So when the red line goes up that's bad then?'   This of course would not do.  So I talked to my friends and our network admin about an option to show something more eye catching and visual, with which we could catch at a glance a feel for what was up with our site.    He initially pointed me out to a video showing GLTail and Chipmunk done in Ruby.  Realizing this was both awesome, and that I needed an excuse to do something in XNA, I decided to knock out a proof of concept for something very similar, but with a few tweaks.   Here's a link to a video of the current prototype:   http://www.youtube.com/watch?v=jM_PWZbtH2I   Essentially this app opens up a log file (even an active one) and begins pulling out the lines of text.  (Here's a good Code Project link that covers how to do tail reading from an active text file: http://www.codeproject.com/KB/files/tail.aspx).   As new data is added, a bubble is generated in the application - a GET statement comes from the left, and a POST from the right.  I then run it through a series of expression checkers, and based on the kind of statement and the pattern, a bubble of an appropriate color is generated.   For example, if I get a 500, a huge red bubble pops out.  Others are based on the part of the system the page is from - i.e. green bubbles are from our claims management subsystem, and blue bubbles are from the pages our scheduling staff use to schedule patients.  Others include the purple bubbles for security and login, and yellow bubbles for some miscellaneous pages.   The little grey bubbles represent things like images, JS, CSS, etc - and their small size makes them work like grease to keep the larger page bubbles moving.   The app is also smart enough that if it is starting to bog down with handling the physics and interactions, it will suspend new bubbles until enough have dropped off that performance can resume (you can see this slight stuttering in the sample video).   The net result is that anyone will be able to look up on the wall monitor, and instantly get a quick feel for how things are going on the floor.  Website slow?  You can get a feel for both volume and utilized modules with one glance.  Website crashing?  Look for a wall of giant red bubbles.  No activity at all?  Maybe the site is down.  Now couple this with utilization within a farm, and cross referenced with a second app showing the same kind of data from your SQL database...   As for the app itself, it's a windows XNA project with the code in C#.   The physics are handled by the Farseer physicis eingine for XNA (http://www.codeplex.com/FarseerPhysics) which is just pure goodness.  The samples are great, and I had the app up and working in two evenings (half of that was fine tuning, and the other was me coding with a kid in my lap).   My next steps include wiring this to SQL (I have some ideas...), and adding a nice configuration module.  For example, you could use polygons, etc to tie to your regex - or more entertaining things like having a little human ragdoll to represent a user login.     Once that's wrapped up and I have a chance to complete some hardening, I will be releasing the whole thing into the wild as opensource.     Feel free to ping me if you have any questions! -Bob

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• PC to USB transfer slow

  - by Vipin Ms

  I'm having trouble with USB transfer,not with external hard disk. Transfer starts with like, for the transfer of 700MB file it starts with 30mb/s and towards the end it stops at 0s and stays put for like 3-4 mins to transfer the last bit. I have tried different USB devices, but no luck. Is it a bug? Another important point is, in Kubuntu there is no such issue. So is it something related to Gnome? I'm using Ubuntu 11.10 64bit. Somebody please help, it's really annoying. Here are the details. PC all of my drives are in ext4. USB I tried ext3,ntfs and fat32. All having the same problem. Here are my USB controllers details: root@LAB:~# lspci|grep USB 00:1a.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 (rev 03) 00:1a.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5 (rev 03) 00:1a.2 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #6 (rev 03) 00:1a.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2 (rev 03) 00:1d.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 (rev 03) 00:1d.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 (rev 03) 00:1d.2 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 (rev 03) 00:1d.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 (rev 03) Here is an example of one transfer. I connected one of my 4GB usb device. Nov 24 12:01:25 LAB kernel: [ 1175.082175] userif-2: sent link up event. Nov 24 12:01:25 LAB kernel: [ 1695.684158] usb 2-2: new high speed USB device number 3 using ehci_hcd Nov 24 12:01:25 LAB mtp-probe: checking bus 2, device 3: "/sys/devices/pci0000:00/0000:00:1d.7/usb2/2-2" Nov 24 12:01:26 LAB mtp-probe: bus: 2, device: 3 was not an MTP device Nov 24 12:01:26 LAB kernel: [ 1696.132680] usbcore: registered new interface driver uas Nov 24 12:01:26 LAB kernel: [ 1696.142528] Initializing USB Mass Storage driver... Nov 24 12:01:26 LAB kernel: [ 1696.142919] scsi4 : usb-storage 2-2:1.0 Nov 24 12:01:26 LAB kernel: [ 1696.143146] usbcore: registered new interface driver usb-storage Nov 24 12:01:26 LAB kernel: [ 1696.143150] USB Mass Storage support registered. Nov 24 12:01:27 LAB kernel: [ 1697.141657] scsi 4:0:0:0: Direct-Access SanDisk U3 Cruzer Micro 8.02 PQ: 0 ANSI: 0 CCS Nov 24 12:01:27 LAB kernel: [ 1697.168827] sd 4:0:0:0: Attached scsi generic sg2 type 0 Nov 24 12:01:27 LAB kernel: [ 1697.169262] sd 4:0:0:0: [sdb] 7856127 512-byte logical blocks: (4.02 GB/3.74 GiB) Nov 24 12:01:27 LAB kernel: [ 1697.169762] sd 4:0:0:0: [sdb] Write Protect is off Nov 24 12:01:27 LAB kernel: [ 1697.169767] sd 4:0:0:0: [sdb] Mode Sense: 45 00 00 08 Nov 24 12:01:27 LAB kernel: [ 1697.171386] sd 4:0:0:0: [sdb] No Caching mode page present Nov 24 12:01:27 LAB kernel: [ 1697.171391] sd 4:0:0:0: [sdb] Assuming drive cache: write through Nov 24 12:01:27 LAB kernel: [ 1697.173503] sd 4:0:0:0: [sdb] No Caching mode page present Nov 24 12:01:27 LAB kernel: [ 1697.173510] sd 4:0:0:0: [sdb] Assuming drive cache: write through Nov 24 12:01:27 LAB kernel: [ 1697.175337] sdb: sdb1 After that I initiated one transfer. lsof -p 3575|tail -2 mv 3575 root 3r REG 8,8 1719599104 4325379 /media/Misc/The Tree of Life (2011) DVDRip XviD-MAXSPEED/The Tree of Life (2011) DVDRip XviD-MAXSPEED www.torentz.3xforum.ro.avi mv 3575 root 4w REG 8,17 1046347776 15 /media/SREE/The Tree of Life (2011) DVDRip XviD-MAXSPEED/The Tree of Life (2011) DVDRip XviD-MAXSPEED www.torentz.3xforum.ro.avi Here are the total time spent on that transfer. root@LAB:/media/SREE# time mv /media/Misc/The\ Tree\ of\ Life\ \(2011\)\ DVDRip\ XviD-MAXSPEED/ /media/SREE/ real 11m49.334s user 0m0.008s sys 0m5.260s root@LAB:/media/SREE# df -T|tail -2 /dev/sdb1 vfat 3918344 1679308 2239036 43% /media/SREE /dev/sda8 ext4 110110576 60096904 50013672 55% /media/Misc Do you think this is normal?? Approximately 12 minutes for 1.6Gb transfer? Thanks.

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• Type checking and recursive types (Writing the Y combinator in Haskell/Ocaml)

  - by beta

  When explaining the Y combinator in the context of Haskell, it's usually noted that the straight-forward implementation won't type-check in Haskell because of its recursive type. For example, from Rosettacode [1]: The obvious definition of the Y combinator in Haskell canot be used because it contains an infinite recursive type (a = a -> b). Defining a data type (Mu) allows this recursion to be broken. newtype Mu a = Roll { unroll :: Mu a -> a } fix :: (a -> a) -> a fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x)) And indeed, the “obvious” definition does not type check: ?> let fix f g = (\x -> \a -> f (x x) a) (\x -> \a -> f (x x) a) g <interactive>:10:33: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 Expected type: t2 -> t0 -> t1 Actual type: (t2 -> t0 -> t1) -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a <interactive>:10:57: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a (0.01 secs, 1033328 bytes) The same limitation exists in Ocaml: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; Error: This expression has type 'a -> 'b but an expression was expected of type 'a The type variable 'a occurs inside 'a -> 'b However, in Ocaml, one can allow recursive types by passing in the -rectypes switch: -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. By using -rectypes, everything works: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> utop # let fact_improver partial n = if n = 0 then 1 else n*partial (n-1);; val fact_improver : (int -> int) -> int -> int = <fun> utop # (fix fact_improver) 5;; - : int = 120 Being curious about type systems and type inference, this raises some questions I'm still not able to answer. First, how does the type checker come up with the type t2 = t2 -> t0 -> t1? Having come up with that type, I guess the problem is that the type (t2) refers to itself on the right side? Second, and perhaps most interesting, what is the reason for the Haskell/Ocaml type systems to disallow this? I guess there is a good reason since Ocaml also will not allow it by default even if it can deal with recursive types if given the -rectypes switch. If these are really big topics, I'd appreciate pointers to relevant literature. [1] http://rosettacode.org/wiki/Y_combinator#Haskell

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• Writing an unthemed view while still using Orchard shapes and helpers

  - by Bertrand Le Roy

  This quick tip will show how you can write a custom view for a custom controller action in Orchard that does not use the current theme, but that still retains the ability to use shapes, as well as zones, Script and Style helpers. The controller action, first, needs to opt out of theming: [Themed(false)] public ActionResult Index() {} .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Then, we still want to use a shape as the view model, because Clay is so awesome: private readonly dynamic _shapeFactory; public MyController(IShapeFactory shapeFactory) { _shapeFactory = shapeFactory; } [Themed(false)] public ActionResult Index() { return View(_shapeFactory.MyShapeName( Foo: 42, Bar: "baz" )); } As you can see, we injected a shape factory, and that enables us to build our shape from our action and inject that into the view as the model. Finally, in the view (that would in Views/MyController/Index.cshtml here), just use helpers as usual. The only gotcha is that you need to use “Layout” in order to declare zones, and that two of those zones, Head and Tail, are mandatory for the top and bottom scripts and stylesheets to be injected properly. Names are important here. @{ Style.Include("somestylesheet.css"); Script.Require("jQuery"); Script.Include("somescript.js"); using(Script.Foot()) { <script type="text/javascript"> $(function () { // Do stuff }) </script> } } <!DOCTYPE html> <html> <head> <title>My unthemed page</title> @Display(Layout.Head) </head> <body> <h1>My unthemed page</h1> <div>@Model.Foo is the answer.</div> </body> @Display(Layout.Tail) </html> Note that if you define your own zones using @Display(Layout.SomeZone) in your view, you can perfectly well send additional shapes to them from your controller action, if you injected an instance of IWorkContextAccessor: _workContextAccessor.GetContext().Layout .SomeZone.Add(_shapeFactory.SomeOtherShape()); Of course, you’ll need to write a SomeOtherShape.cshtml template for that shape but I think this is pretty neat.

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• Intermittent internet connectivity

  - by Rob Oplawar

  UPDATED: I recently built a new computer and set it up to dual-boot Windows 7 and Ubuntu 11.10. In Windows, using the same hardware, my LAN connectivity is solid. In Ubuntu, however, my network interface periodically dies and resets itself; I'll have a solid connection for 30 seconds, and then it will go out for 30 seconds. When I tail the log: tail -f /var/log/kern.log I see "eth0 link up" messages appear periodically, corresponding with the return of connectivity. I posted the original question months ago, and misinterpreted what was going on. With a working Internet connection in Windows, I ignored the problem for some months. See my answer below for the solution (drivers). ORIGINAL POST In Ubuntu, although I maintain a solid connection to my LAN (pinging the router IP address consistently returns a good result), my internet connectivity drops in and out. When I continuously ping 74.125.227.18 (a google.com server), I get responses for a while, then I start getting "Destination Host Unreachable" for a while, then I get responses again. This happens consistently, dropping the connection for about 30 seconds out of every minute or two. Whether I configure my network via the network manager or via /etc/network/interfaces seems to make no difference. I configure with the following settings: address 192.168.1.101 network 192.168.1.0 gateway 192.168.1.99 (my router's IP address) netmask 255.255.255.0 (confirmed as the right netmask for the router) broadcast 192.168.1.255 (also confirmed with the router). ifconfig confirms that these settings are working: eth0 Link encap:Ethernet HWaddr 50:e5:49:40:da:a6 inet addr:192.168.1.101 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::52e5:49ff:fe40:daa6/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:11557 errors:0 dropped:11557 overruns:0 frame:11557 TX packets:13117 errors:0 dropped:211 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:9551488 (9.5 MB) TX bytes:1930952 (1.9 MB) Interrupt:41 Base address:0xa000 I get the same issue when I use automatic DHCP address settings, although I did confirm that there is no other machine on the network with the static IP address I want to use. As I said, the connection to the local network stays solid - I never have any trouble pinging 192.168.1.* - it's internet addresses that I intermittently cannot reach. It's not a DNS issue because pinging known IP addresses directly shows the same behavior. Also, I don't think it's a hardware issue, as I never have any internet connectivity problems on the same machine in Windows. The network hardware is built into the motherboard: Gigabyte Z68XP-UD3P. I managed to bring the OS fully up to date, according to the update manager, but it didn't fix the issue, and with my limited understanding of network architecture I'm at my wit's end. The only clue I can see is that ifconfig is reporting a lot of dropped packets, but I'm not sure what to do about it. UPDATE: It seems my problem is a little more generic than I described; now when I try pinging my router and google simultaneously, they both go unreachable at the same time. Running ifdown eth0 and then ifup eth0 brings it back temporarily; if I just wait it comes back after a couple of minutes. I'll broaden my search through intermittent network connectivity problems.

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• Can aptitude for learning Programming paradigms be influenced by culture or native language's gramma

  - by DVK

  It is well known that different people have different aptitudes regarding various programming paradigms (e.g. some people have trouble learning non-procedural, especially functional languages. Some people have trouble understanding pointers - see Joel Spolsky's blog for musings on that. Some people have trouble grasping recursion). I was recently reading about a study that looked at how the grammar of someone's native language affected their speed of learning math. Can't find that article now but a quick googling found this reference. That led me to wondering whether someone's native culture or first language might affect their aptitude towards various programming paradigms. I'm more curious about positive influences - e.g. some trait that make it easier/faster for someone to learn a particular paradigm, for example native language grammar being very recursion-oriented. To be clear, I'm looking for how culture/language grammare may affect the difference between aptitude of the same person towards various paradigms as opposed to how it affects overall aptitude towards programming between different persons. Important: the only answers I'm interested in are either references to scientific studies, or personal observations from someone intimately familiar with a particular culture/language, including from their own experience. E.g. I'm not interested in your opinion of how Chinese being your first language affects anything unless you speak Chinese or worked with extremely large set of Chinese-native programmers extensively. I'm OK with your guesstimates not based on scientific studies, but please be sure to supply your reasoning about plausible causes of your observation. I'm not interested in culture-bashing (any such commends will be deleted or flagged for deletion). I'm also not particularly interested in culture-building - we all know Linus is from Finland and Tetris was written in Russia and Larry Wall is an American. Any culture/nation can produce a brilliant mind in any discipline. I'm interested in averages.

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• Remove file from git repository (history)

  - by Devenv

  (solved, see bottom of the question body) Looking for this for a long time now, what I have till now is: http://dound.com/2009/04/git-forever-remove-files-or-folders-from-history/ and http://progit.org/book/ch9-7.html Pretty much the same method, but both of them leave objects in pack files... Stuck. What I tried: git filter-branch --index-filter 'git rm --cached --ignore-unmatch file_name' rm -Rf .git/refs/original rm -Rf .git/logs/ git gc Still have files in the pack, and this is how I know it: git verify-pack -v .git/objects/pack/pack-3f8c0...bb.idx | sort -k 3 -n | tail -3 And this: git filter-branch --index-filter "git rm -rf --cached --ignore-unmatch file_name" HEAD rm -rf .git/refs/original/ && git reflog expire --all && git gc --aggressive --prune The same... Tried git clone trick, it removed some of the files (~3000 of them) but the largest files are still there... I have some large legacy files in the repository, ~200M, and I really don't want them there... And I don't want to reset the repository to 0 :( SOLUTION: This is the shortest way to get rid of the files: check .git/packed-refs - my problem was that I had there a refs/remotes/origin/master line for a remote repository, delete it, otherwise git won't remove those files (optional) git verify-pack -v .git/objects/pack/#{pack-name}.idx | sort -k 3 -n | tail -5 - to check for the largest files (optional) git rev-list --objects --all | grep a0d770a97ff0fac0be1d777b32cc67fe69eb9a98 - to check what files those are git filter-branch --index-filter 'git rm --cached --ignore-unmatch file_names' - to remove the file from all revisions rm -rf .git/refs/original/ - to remove git's backup git reflog expire --all --expire='0 days' - to expire all the loose objects (optional) git fsck --full --unreachable - to check if there are any loose objects git repack -A -d - repacking the pack git prune - to finally remove those objects

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• Substitute values (for specific dates) from a second data frame to the first data frame

  - by user1665355

  I have two time series data frames: The first one: head(df1) : GMT MSCI ACWI DJGlbl Russell 1000 Russell Dev S&P GSCI Industrial S&P GSCI Precious 1999-03-01 -0.7000000 0.2000000 -0.1000000 -1.5000000 -1.0000000 -0.4000000 1999-03-02 -0.5035247 0.0998004 -0.7007007 -0.2030457 0.4040404 -0.3012048 1999-03-03 -0.2024291 0.2991027 0.0000000 -0.6103764 0.1006036 -0.1007049 1999-03-04 0.7099391 0.2982107 1.5120968 -0.1023541 0.5025126 0.4032258 1999-03-05 2.4169184 0.8919722 2.1847071 2.7663934 -1.2000000 0.0000000 1999-03-08 0.3933137 0.3929273 0.5830904 -0.0997009 -0.2024291 1.1044177 tail(df1) : GMT MSCI ACWI DJGlbl Russell 1000 Russell Dev S&P GSCI Industrial S&P GSCI Precious 2011-12-23 0.68241470 0.84790673 0.9441385 0.6116208 0.5822862 -0.2345300 2011-12-26 -0.05213764 0.00000000 0.0000000 0.0000000 0.0000000 0.0000000 2011-12-27 0.20865936 0.05254861 0.3117693 0.2431611 0.0000000 -0.7233273 2011-12-28 -0.62467465 -1.20798319 -1.1655012 -0.9702850 -2.0414381 -2.4043716 2011-12-29 0.52383447 0.47846890 0.8647799 0.5511329 -0.0933126 -1.2504666 2011-12-30 0.26055237 1.03174603 -0.4676539 1.2180268 1.9613948 1.7388017 The second one: head(df2) : GMT MSCI.ACWI DJGlbl Russell.1000 Russell.Dev S.P.GSCI.Industrial S.P.GSCI.Precious 1999-06-01 0.00000000 0.24438520 0.0000000 0 -0.88465521 0.008522842 1999-07-01 0.12630441 0.06755621 0.0000000 0 0.29394697 0.000000000 1999-08-02 0.07441812 0.18922829 0.0000000 0 0.02697299 -0.107155063 1999-09-01 -0.36952701 0.08684107 0.1117509 0 0.24520976 0.000000000 1999-10-01 0.00000000 0.00000000 0.0000000 0 0.00000000 1.941266205 1999-11-01 0.41879925 0.00000000 0.0000000 0 0.00000000 -0.197897901 tail(df2) : GMT MSCI.ACWI DJGlbl Russell.1000 Russell.Dev S.P.GSCI.Industrial S.P.GSCI.Precious 2011-07-01 0.00000000 0.0000000 0.0000000 0.0000000 0.00000000 -0.1141162 2011-08-01 0.00000000 0.0000000 0.0000000 0.0000000 0.02627347 0.0000000 2011-09-01 -0.02470873 0.2977585 -0.0911891 0.6367605 0.00000000 0.2830977 2011-10-03 0.42495188 0.0000000 0.4200743 -0.4420027 -0.41012646 0.0000000 2011-11-01 0.00000000 0.0000000 0.0000000 -0.6597739 0.00000000 0.0000000 2011-12-01 0.50273034 0.0000000 0.0000000 0.6476393 0.00000000 0.0000000 The first df cointains daily observations. The second df contains only the "first day of each month" forecasted values. I would like to substitute the values from the second df into the first one. In other words, the "first day of each month" values in the first df will be substituted for the "first day of each month" values from the second df. I tried to write an lapply loop that substitutes the values and was only trying to use match function. But I failed. I could not find the similar question at StackOverflow either... Greatful for any suggestions!

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• php cli script hangs with no messages

  - by julio

  Hi-- I've written a PHP script that runs via SSH and nohup, meant to process records from a database and do stuff with them (eg. process some images, update some rows). It works fine with small loads, up to maybe 10k records. I have some larger datasets that process around 40k records (not a lot, I realize, but it adds up to a lot of work when each record requires the download and processing of up to 50 images). The larger datasets can take days to process. Sometimes I'll see in my debug logs memory errors, which are clear enough-- but sometimes the script just appears to "die" or go zombie on me. My tail of the debug log just stops, with no error messages, the tail of the nohup log ends with no error, and the process is still showing in a ps list, looking like this-- 26075 pts/0 S 745:01 /usr/bin/php ./import.php but no work is getting done. Can anyone give me some ideas on why a process would just quit? The obvious things (like a php script timeout and memory issues) are not a factor, as far as I can tell. Thanks for any tips PS-- this is hosted on a godaddy VDS (not my choice). I am sort of suspecting that godaddy has some kind of limits that might kick in on me despite what overrides I put in the code (such as set_time_limit(0);).

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• Functional way to get a matrix from text

  - by Elazar Leibovich

  I'm trying to solve some Google Code Jam problems, where an input matrix is typically given in this form: 2 3 #matrix dimensions 1 2 3 4 5 6 7 8 9 # all 3 elements in the first row 2 3 4 5 6 7 8 9 0 # each element is composed of three integers where each element of the matrix is composed of, say, three integers. So this example should be converted to #!scala Array( Array(A(1,2,3),A(4,5,6),A(7,8,9), Array(A(2,3,4),A(5,6,7),A(8,9,0), ) An imperative solution would be of the form #!python input = """2 3 1 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 0 """ lines = input.split('\n') print lines[0] m,n = (int(x) for x in lines[0].split()) array = [] row = [] A = [] for line in lines[1:]: for elt in line.split(): A.append(elt) if len(A)== 3: row.append(A) A = [] array.append(row) row = [] from pprint import pprint pprint(array) A functional solution I've thought of is #!scala def splitList[A](l:List[A],i:Int):List[List[A]] = { if (l.isEmpty) return List[List[A]]() val (head,tail) = l.splitAt(i) return head :: splitList(tail,i) } def readMatrix(src:Iterator[String]):Array[Array[TrafficLight]] = { val Array(x,y) = src.next.split(" +").map(_.trim.toInt) val mat = src.take(x).toList.map(_.split(" "). map(_.trim.toInt)). map(a => splitList(a.toList,3). map(b => TrafficLight(b(0),b(1),b(2)) ).toArray ).toArray return mat } But I really feel it's the wrong way to go because: I'm using the functional List structure for each line, and then convert it to an array. The whole code seems much less efficeint I find it longer less elegant and much less readable than the python solution. It is harder to which of the map functions operates on what, as they all use the same semantics. What is the right functional way to do that?

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• displaying database content in wxHaskell

  - by snorlaks

  Hello, Im using tutorials from wxHaskell and want to display content of table movies in the grid. HEre is my code : {-------------------------------------------------------------------------------- Test Grid. --------------------------------------------------------------------------------} module Main where import Graphics.UI.WX import Graphics.UI.WXCore hiding (Event) import Database.HDBC.Sqlite3 (connectSqlite3) import Database.HDBC main = start gui gui :: IO () gui = do f <- frame [text := "Grid test", visible := False] -- grids g <- gridCtrl f [] gridSetGridLineColour g (colorSystem Color3DFace) gridSetCellHighlightColour g black appendColumns g (movies) -- Here is error: -- Couldn't match expected type [String]' -- against inferred typeIO [[String]]' --appendRows g (map show [1..length (tail movies)]) --mapM_ (setRow g) (zip [0..] (tail movies)) gridAutoSize g -- layout set f [layout := column 5 [fill (dynamic (widget g))] ] focusOn g set f [visible := True] -- reduce flicker at startup. return () where movies = do conn <- connectSqlite3 "Spop.db" r <- quickQuery' conn "SELECT id, title, year, description from Movie where id = 1" [] let myResult = map convRow r return myResult setRow g (row,values) = mapM_ ((col,value) - gridSetCellValue g row col value) (zip [0..] values) {-------------------------------------------------------------------------------- Library?f --------------------------------------------------------------------------------} gridCtrl :: Window a - [Prop (Grid ())] - IO (Grid ()) gridCtrl parent props = feed2 props 0 $ initialWindow $ \id rect - \props flags - do g <- gridCreate parent id rect flags gridCreateGrid g 0 0 0 set g props return g appendColumns :: Grid a - [String] - IO () appendColumns g [] = return () appendColumns g labels = do n <- gridGetNumberCols g gridAppendCols g (length labels) True mapM_ ((i,label) - gridSetColLabelValue g i label) (zip [n..] labels) appendRows :: Grid a - [String] - IO () appendRows g [] = return () appendRows g labels = do n <- gridGetNumberRows g gridAppendRows g (length labels) True mapM_ ((i,label) - gridSetRowLabelValue g i label) (zip [n..] labels) convRow :: [SqlValue] - [String] convRow [sqlId, sqlTitle, sqlYear, sqlDescription] = [intid, title, year, description] where intid = (fromSql sqlId)::String title = (fromSql sqlTitle)::String year = (fromSql sqlYear)::String description = (fromSql sqlDescription)::String What should I do to get rif of error in code above (24th line)

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• Practical Scheme Programming

  - by Ixmatus

  It's been a few months since I've touched Scheme and decided to implement a command line income partitioner using Scheme. My initial implementation used plain recursion over the continuation, but I figured a continuation would be more appropriate to this type of program. I would appreciate it if anyone (more skilled with Scheme than I) could take a look at this and suggest improvements. I'm that the multiple (display... lines is an ideal opportunity to use a macro as well (I just haven't gotten to macros yet). (define (ab-income) (call/cc (lambda (cc) (let ((out (display "Income: ")) (income (string->number (read-line)))) (cond ((<= income 600) (display (format "Please enter an amount greater than $600.00~n~n")) (cc (ab-income))) (else (let ((bills (* (/ 30 100) income)) (taxes (* (/ 20 100) income)) (savings (* (/ 10 100) income)) (checking (* (/ 40 100) income))) (display (format "~nDeduct for bills:---------------------- $~a~n" (real->decimal-string bills 2))) (display (format "Deduct for taxes:---------------------- $~a~n" (real->decimal-string taxes 2))) (display (format "Deduct for savings:-------------------- $~a~n" (real->decimal-string savings 2))) (display (format "Remainder for checking:---------------- $~a~n" (real->decimal-string checking 2)))))))))) Invoking (ab-income) asks for input and if anything below 600 is provided it (from my understanding) returns (ab-income) at the current-continuation. My first implementation (as I said earlier) used plain-jane recursion. It wasn't bad at all either but I figured every return call to (ab-income) if the value was below 600 kept expanding the function. (please correct me if that apprehension is incorrect!)

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• recursively implementing 'minimum number of coins' in python

  - by user5198

  This problem is same as asked in here. Given a list of coins, their values (c1, c2, c3, ... cj, ...), and the total sum i. Find the minimum number of coins the sum of which is i (we can use as many coins of one type as we want), or report that it's not possible to select coins in such a way that they sum up to S. I"m just introduced to dynamic programming yesterday and I tried to make a code for it. # Optimal substructure: C[i] = 1 + min_j(C[i-cj]) cdict = {} def C(i, coins): if i <= 0: return 0 if i in cdict: return cdict[i] else: answer = 1 + min([C(i - cj, coins) for cj in coins]) cdict[i] = answer return answer Here, C[i] is the optimal solution for amount of money 'i'. And available coins are {c1, c2, ... , cj, ...} for the program, I've increased the recursion limit to avoid maximum recursion depth exceeded error. But, this program gives the right answer only someones and when a solution is not possible, it doesn't indicate that. What is wrong with my code and how to correct it?

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• WPF - DataTemplate to show only one hierarchical level in recursive data

  - by Paull

  Hi all, I am using a tree to display my data: persons grouped by their teams. In my model a team can contain another team, so the recursion. I want do display details about the selected node of the tree using a contentpresenter. If the selection is a person, everything is fine: I can show the person name or datails without problem using a simple datatemplate. If the selection is a team I would like to display the team name followed by a list of member names. If one of these members is another team I would like to display just the team name, without recursion... My code here is wrong because it displays data in a recursive way, what is the right way of doing it? Thanks in advance for any help! best regards, Paolo <ContentPresenter Content="{Binding Path=SelectedItem, ElementName=PeopleTree}" > <ContentPresenter.Resources> <DataTemplate DataType="{x:Type my:PersonViewModel}"> <TextBlock Text="{Binding PersonName}"/> </DataTemplate> <DataTemplate DataType="{x:Type my:TeamViewModel}"> <StackPanel> <TextBlock Text="{Binding TeamName}" /> <ListBox ItemsSource="{Binding Members}" /> </StackPanel> </DataTemplate> </ContentPresenter.Resources> </ContentPresenter>

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• How to access a superclass's class attributes in Python?

  - by Brecht Machiels

  Have a look at the following code: class A(object): defaults = {'a': 1} def __getattr__(self, name): print('A.__getattr__') return self.get_default(name) @classmethod def get_default(cls, name): # some debug output print('A.get_default({}) - {}'.format(name, cls)) try: print(super(cls, cls).defaults) # as expected except AttributeError: #except for the base object class, of course pass # the actual function body try: return cls.defaults[name] except KeyError: return super(cls, cls).get_default(name) # infinite recursion #return cls.__mro__[1].get_default(name) # this works, though class B(A): defaults = {'b': 2} class C(B): defaults = {'c': 3} c = C() print('c.a =', c.a) I have a hierarchy of classes each with its own dictionary containing some default values. If an instance of a class doesn't have a particular attribute, a default value for it should be returned instead. If no default value for the attribute is contained in the current class's defaults dictionary, the superclass's defaults dictionary should be searched. I'm trying to implement this using the recursive class method get_default. The program gets stuck in an infinite recursion, unfortunately. My understanding of super() is obviously lacking. By accessing __mro__, I can get it to work properly though, but I'm not sure this is a proper solution. I have the feeling the answer is somewhere in this article, but I haven't been able to find it yet. Perhaps I need to resort to using a metaclass?

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• Natural problems to solve using closures

  - by m.u.sheikh

  I have read quite a few articles on closures, and, embarassingly enough, I still don't understand this concept! Articles explain how to create a closure with a few examples, but I don't see any point in paying much attention to them, as they largely look contrived examples. I am not saying all of them are contrived, just that the ones I found looked contrived, and I dint see how even after understanding them, I will be able to use them. So in order to understand closures, I am looking at a few real problems, that can be solved very naturally using closures. For instance, a natural way to explain recursion to a person could be to explain the computation of n!. It is very natural to understand a problem like computing the factorial of a number using recursion. Similarly, it is almost a no-brainer to find an element in an unsorted array by reading each element, and comparing with the number in question. Also, at a different level, doing Object-oriented programming also makes sense. So I am trying to find a number of problems that could be solved with or without closures, but using closures makes thinking about them and solving them easier. Also, there are two types to closures, where each call to a closure can create a copy of the environment variables, or reference the same variables. So what sort of problems can be solved more naturally in which of the closure implementations?

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• Clicking inside a polygon in Google Maps

  - by amarsh-anand

  The included JavaScript snippet is supposed to do the following: As the user clicks on the map, initialize headMarker and draw a circle (polygon) around it As the user clicks inside the circle, initialize tailMarker and draw the path between these two markers 1 is happening as expected. But as the user clicks inside the circle, in the function(overlay,point), overlay is non-null while point is null. Because of this, the code fails to initialize tailMarker. Can someone tell me a way out. GEvent.addListener(map, "click", function(overlay,point) { if (isCreateHeadPoint) { // add the head marker headMarker = new GMarker(point,{icon:redIcon,title:'0'}); map.addOverlay(headMarker); isCreateHeadPoint = false; // draw the circle drawMapCircle(point.lat(),point.lng(),1,'#cc0000',2,0.8,'#0',0.1); } else { // add the tail marker tailMarker = new GMarker(point,{icon:greenIcon,title:''}); map.addOverlay(tailMarker); isCreateHeadPoint = true; // load thes path from head to tail direction.load("from:" + headMarker.getPoint().lat()+ ", " + headMarker.getPoint().lng()+ " " + "to:" + tailMarker.getPoint().lat() + "," + tailMarker.getPoint().lng(), {getPolyline:true}); } });

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• XSLT 1.0: restrict entries in a nodeset

  - by Mike

  Hi, Being relatively new to XSLT I have what I hope is a simple question. I have some flat XML files, which can be pretty big (eg. 7MB) that I need to make 'more hierarchical'. For example, the flat XML might look like this: <D0011> .... .... and it should end up looking like this: <D0011> .... .... I have a working XSLT for this, and it essentially gets a nodeset of all the b elements and then uses the 'following-sibling' axis to get a nodeset of the nodes following the current b node (ie. following-sibling::*[position() =$nodePos]). Then recursion is used to add the siblings into the result tree until another b element is found (I have parameterised it of course, to make it more generic). I also have a solution that just sends the position in the XML of the next b node and selects the nodes after that one after the other (using recursion) via a *[position() = $nodePos] selection. The problem is that the time to execute the transformation increases unacceptably with the size of the XML file. Looking into it with XML Spy it seems that it is the 'following-sibling' and 'position()=' that take the time in the two respective methods. What I really need is a way of restricting the number of nodes in the above selections, so fewer comparisons are performed: every time the position is tested, every node in the nodeset is tested to see if its position is the right one. Is there a way to do that ? Any other suggestions ? Thanks, Mike

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• Pass in a value into Python Class through command line

  - by chrissygormley

  Hello, I have got some code to pass in a variable into a script from the command line. The script is: import sys, os def function(var): print var class function_call(object): def __init__(self, sysArgs): try: self.function = None self.args = [] self.modulePath = sysArgs[0] self.moduleDir, tail = os.path.split(self.modulePath) self.moduleName, ext = os.path.splitext(tail) __import__(self.moduleName) self.module = sys.modules[self.moduleName] if len(sysArgs) > 1: self.functionName = sysArgs[1] self.function = self.module.__dict__[self.functionName] self.args = sysArgs[2:] except Exception, e: sys.stderr.write("%s %s\n" % ("PythonCall#__init__", e)) def execute(self): try: if self.function: self.function(*self.args) except Exception, e: sys.stderr.write("%s %s\n" % ("PythonCall#execute", e)) if __name__=="__main__": test = test() function_call(sys.argv).execute() This works by entering ./function <function> <arg1 arg2 ....>. The problem is that I want to to select the function I want that is in a class rather than just a function by itself. The code I have tried is the same except that function(var): is in a class. I was hoping for some ideas on how to modify my function_call class to accept this. Thanks for any help.

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• Best implementation of Java Queue?

  - by Georges Oates Larsen

  I am working (In java) on a recursive image processing algorithm that recursively traverses the pixels of the image, outward from a center point. Unfortunately... That causes stack overflows, so I have decided to switch to a Queue-based algorithm. Now, this is all fine and dandy -- But considering the fact that its queue will be analyzing THOUSANDS of pixels in a very short amount of time, while constantly popping and pushing, WITHOUT maintaining a predictable state (It could be anywhere between length 100, and 20000); The queue implementation needs to have significantly fast popping and pushing abilities. A linked list seems attractive due to its ability to push elements unto its self without rearranging anything else in the list, but in order for it to be fast enough, it would need easy access to both its head, AND its tail (or second-to-last node if it were not doubly-linked). Sadly, though I cannot find any information related to the underlying implementation of linked lists in Java, so it's hard to say if a linked list is really the way to go... This brings me to my question... What would be the best implementation of the Queue interface in Java for what I intend to do? (I do not wish to edit or even access anything other than the head and tail of the queue -- I do not wish to do any sort of rearranging, or anything. On the flip side, I DO intend to do a lot of pushing and popping, and the queue will be changing size quite a bit, so preallocating would be inefficient)

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