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  • Recursive vs. Iterative algorithms

    - by teehoo
    I'm implementing the Euclidian algorithm for finding the GCD (Greatest Common Divisor) of two integers. Two sample implementations are given: Recursive and Iterative. http://en.wikipedia.org/wiki/Euclidean_algorithm#Implementations My Question: In school I remember my professors talking about recursive functions like they were all the rage, but I have one doubt. Compared to an iterative version don't recursive algorithms take up more stack space and therefore much more memory? Also, because calling a function requires uses some overhead for initialization, aren't recursive algorithms more slower than their iterative counterpart?

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  • slowing down a loop in a recursive function

    - by eco_bach
    I have a difficult problem with a recursive function. Essentially I need to 'slow down' a for loop within a function that repeatedly calls itself(the function); Is this possible, or do I need to somehow extract the recursive nature of the function? function callRecursiveFuncAgain(ob:Object):void{ //do recursive stuff; for (var i:int = 0; i < 4; i++) { _nextObj=foo callRecursiveFuncAgain(_nextObj); } }

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  • Apply a recursive CTE on grouped table rows (SQL server 2005).

    - by Evan V.
    Hi all, I have a table (ROOMUSAGE) containing the times people check in and out of rooms grouped by PERSONKEY and ROOMKEY. It looks like this: PERSONKEY | ROOMKEY | CHECKIN | CHECKOUT | ROW ---------------------------------------------------------------- 1 | 8 | 13-4-2010 10:00 | 13-4-2010 11:00 | 1 1 | 8 | 13-4-2010 08:00 | 13-4-2010 09:00 | 2 1 | 1 | 13-4-2010 15:00 | 13-4-2010 16:00 | 1 1 | 1 | 13-4-2010 14:00 | 13-4-2010 15:00 | 2 1 | 1 | 13-4-2010 13:00 | 13-4-2010 14:00 | 3 13 | 2 | 13-4-2010 15:00 | 13-4-2010 16:00 | 1 13 | 2 | 13-4-2010 15:00 | 13-4-2010 16:00 | 2 I want to select just the consecutive rows for each PERSONKEY, ROOMKEY grouping. So the desired resulting table is: PERSONKEY | ROOMKEY | CHECKIN | CHECKOUT | ROW ---------------------------------------------------------------- 1 | 8 | 13-4-2010 10:00 | 13-4-2010 11:00 | 1 1 | 1 | 13-4-2010 15:00 | 13-4-2010 16:00 | 1 1 | 1 | 13-4-2010 14:00 | 13-4-2010 15:00 | 2 1 | 1 | 13-4-2010 13:00 | 13-4-2010 14:00 | 3 13 | 2 | 13-4-2010 15:00 | 13-4-2010 16:00 | 1 I want to avoid using cursors so I thought I would use a recursive CTE. Here is what I came up with: ;with CTE (PERSONKEY, ROOMKEY, CHECKIN, CHECKOUT, ROW) as (select RU.PERSONKEY, RU.ROOMKEY, RU.CHECKIN, RU.CHECKOUT, RU.ROW from ROOMUSAGE RU where RU.ROW = 1 union all select RU.PERSONKEY, RU.ROOMKEY, RU.CHECKIN, RU.CHECKOUT, RU.ROW from ROOMUSAGE RU inner join CTE on RU.ROWNUM = CTE.ROWNUM + 1 where CTE.CHECKIN = RU.CHECKOUT and CTE.PERSONKEY = RU.PERSONKEY and CTE.ROOMKEY = RU.ROOMKEY) This worked OK for very small datasets (under 100 records) but it's unusable on large datasets. I'm thinking that I should somehow apply the cte recursevely on each PERSONKEY, ROOMKEY grouping on my ROOMUSAGE table but I am not sure how to do that. Any help would be much appreciated, Cheers!

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  • How to prevent recursive windows when connecting to vncserver on localhost

    - by blog.adaptivesoftware.biz
    I have a VNCServer (vino) configured on my Ubuntu 8.10 box. I would like to connect to this server from a vncclient running on this same machine (the reason for doing this strange thing is mentioned below). Understandably, when I connect to a vncserver on the same box, my vncclient shows recursive windows. Is there a way I can connect to the vncserver on the same machine and not have the recursive windows problem? Perhaps if I could start the vncserver on one display and the client on another display then will it work? How can I do something like this? Note - Reason for running vnc client and server on the same machine: When I start our Java Swing unit test suite, a bunch of swing UI's are created and destroyed as the tests run. These windows fly in the foreground making it impossible to work while the test suite is running. I am hoping to start the test suite within a vncclient so that I can continue working while the tests run.

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  • Public Facing Recursive DNS Servers - iptables rules

    - by David Schwartz
    We run public-facing recursive DNS servers on Linux machines. We've been used for DNS amplification attacks. Are there any recommended iptables rules that would help mitigate these attacks? The obvious solution is just to limit outbound DNS packets to a certain traffic level. But I was hoping to find something a little bit more clever so that an attack just blocks off traffic to the victim IP address. I've searched for advice and suggestions, but they all seem to be "don't run public-facing recursive name servers". Unfortunately, we are backed into a situation where things that are not easy to change will break if we don't do so, and this is due to decisions made more than a decade ago before these attacks were an issue.

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  • Java REPL onsole

    - by Don
    Hi, I'm looking for a console that I can use to test out snippets of Java code. Either a desktop app, or a web app (like the Groovy web console). Ideally, the console should automatically import commonly used package like java.io.* java.util.* So that I can copy and paste code from a class without having to add a bunch of imports. Does such a thing exist? Thanks, Donal

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  • Are there any free embedable REPL interpreters for a website? [migrated]

    - by Google
    I am hoping to find a free REPL interpreter for a number of languages. I am starting a new web page to help developers learn with a number of tutorials. I want to embed an interpreter on the site so users can have a tutorial and interpreter side by side. I really like free interpreters such as the one at repl.it. However, I am unable to find a free/embeddable interpreter. I was hoping to avoid making one myself-- they seem tricky to make safely! Thanks!

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  • Can a destructor be recursive?

    - by Cubbi
    Is this program well-defined, and if not, why exactly? #include <iostream> #include <new> struct X { int cnt; X (int i) : cnt(i) {} ~X() { std::cout << "destructor called, cnt=" << cnt << std::endl; if ( cnt-- > 0 ) this->X::~X(); // explicit recursive call to dtor } }; int main() { char* buf = new char[sizeof(X)]; X* p = new(buf) X(7); p->X::~X(); // explicit call to dtor delete[] buf; } My reasoning: although invoking a destructor twice is undefined behavior, per 12.4/14, what it says exactly is this: the behavior is undefined if the destructor is invoked for an object whose lifetime has ended Which does not seem to prohibit recursive calls. While the destructor for an object is executing, the object's lifetime has not yet ended, thus it's not UB to invoke the destructor again. On the other hand, 12.4/6 says: After executing the body [...] a destructor for class X calls the destructors for X's direct members, the destructors for X's direct base classes [...] which means that after the return from a recursive invocation of a destructor, all member and base class destructors will have been called, and calling them again when returning to the previous level of recursion would be UB. Therefore, a class with no base and only POD members can have a recursive destructor without UB. Am I right?

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  • Java iterative vs recursive

    - by user1389813
    Can anyone explain why the following recursive method is faster than the iterative one (Both are doing it string concatenation) ? Isn't the iterative approach suppose to beat up the recursive one ? plus each recursive call adds a new layer on top of the stack which can be very space inefficient. private static void string_concat(StringBuilder sb, int count){ if(count >= 9999) return; string_concat(sb.append(count), count+1); } public static void main(String [] arg){ long s = System.currentTimeMillis(); StringBuilder sb = new StringBuilder(); for(int i = 0; i < 9999; i++){ sb.append(i); } System.out.println(System.currentTimeMillis()-s); s = System.currentTimeMillis(); string_concat(new StringBuilder(),0); System.out.println(System.currentTimeMillis()-s); } I ran the program multiple time, and the recursive one always ends up 3-4 times faster than the iterative one. What could be the main reason there that is causing the iterative one slower ?

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  • What does this code do? Recursive Iterator in php?

    - by Ali
    I'm working on a zend framework based email project and I'm following some code samples online.. I can't understand this line of code which apparently loops through different 'parts' of an email message. I have no idea how it works btw and suspect that theres some error taking place which my parser isn't showing right. foreach (new RecursiveIteratorIterator($mail->getMessage($i)) as $ii=>$part) what does the above foreach loop mean?

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  • Recursive Syntax in Oslo

    - by Kevin Lawrence
    I'm writing my first DSL with Oslo and I am having a problem with a recursive syntax definition. The input has sections which can contain questions or other sections recursively (composite pattern) like this: Section: A Question: 1 Question: 2 Section: B Question: 1 End End My definition for a Section looks like this syntax Section = "Section:" id:Text body:(SectionBody)* "End Section"; Which works (but doesn't handle recursive sections) if I define SectionBody like this syntax SectionBody = (Question); but doesn't work with a recursive definition like this syntax SectionBody = (Question | Section); What am I missing?

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  • wget recursive limited within subdomain

    - by Paul Seangwongree
    I want to download the following subdomain with the recursive option using wget: www.example.com/A/B So if that URL has links to www.example.com/A/B/C and www.example.com/A/B/D, these two should also be downloaded. But I don't want anything outside the www.example.com/A/B subdomain to be downloaded. For example, if www.example.com/A/B/C has a link back to www.example.com, the page www.example.com should not be downloaded. What wget command should I use?

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  • Correct way to model recursive relationship in Django

    - by Yuval A
    My application has two node types: a parent node which can hold recursive child nodes. Think of it like the post-comment system in SO, but comments can be recursive: parent_1 child_11 child_12 child_121 child_3 parent_2 child_21 child_211 child_2111 Important to note that the parent nodes have different attributes and behavior than the child nodes. What is the correct (and presumably most efficient) way of modeling this relationship in Django?

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  • Recognizing Tail-recursive functions with Flex+Bison and convert code to an Iterative form

    - by Viet
    I'm writing a calculator with an ability to accept new function definitions. Being aware of the need of newbies to try recursive functions such as Fibonacci, I would like my calculator to be able to recognize Tail-recursive functions with Flex + Bison and convert code to an Iterative form. I'm using Flex & Bison to do the job. If you have any hints or ideas, I welcome them warmly. Thanks!

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  • Recursive insert-method for a linked list

    - by user3726477
    I'm learning C# and I've made a recursive insert-method for a linked list: public static int recursiveInsert(ref int value, ref MyLinkedList list) { if (list == null) return new MyLinkedList(value, null); else { list.next = recursiveInsert(ref int value, ref list.next); return list; } } How would you modify this method to make the recursive call look like this: recursiveInsert(value, ref list.next) instead of: list.next = recursiveInsert(ref int value, ref list.next);

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  • Scala: recursive search avoiding cycles

    - by user1826663
    How can I write a recursive search that I avoid cycles. My class is this: class Component(var name: String, var number: Int, var subComponent: Set[Component]) Now I need a way to check whether a component is contained within its subcomponent or between subcomponent of its subcomponent and so on.Avoiding possible cycles caused by other Component. My method of recursive search must have the following signature, where subC is the Set [component] of comp. def content (comp: Component, subC: Set[Component]) : Boolean = { } Thanks for the help.

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  • Simple recursive DNS resolver for debugging (app or VM)

    - by notpeter
    I have an issue which I believe is caused by incorrect DNS queries (doubled subdomains like _record.host.subdomain.tld.subdomain.tld) when querying for SRV records. So I need to an alternate DNS server with heavy logging so I can see every query (especially stupid ones), acting as a recursive resolver with the ability create records which override real DNS records so I can not only find the records it's (wrongly) looking for, but populate those records as well. I know I could install a DNS server on yet another linux box, but I feel like this is the sort of thing that someone may already setup a simple python script or single use vm just for this purpose.

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  • Tail-recursive merge sort in OCaml

    - by CFP
    Hello world! I’m trying to implement a tail-recursive list-sorting function in OCaml, and I’ve come up with the following code: let tailrec_merge_sort l = let split l = let rec _split source left right = match source with | [] -> (left, right) | head :: tail -> _split tail right (head :: left) in _split l [] [] in let merge l1 l2 = let rec _merge l1 l2 result = match l1, l2 with | [], [] -> result | [], h :: t | h :: t, [] -> _merge [] t (h :: result) | h1 :: t1, h2 :: t2 -> if h1 < h2 then _merge t1 l2 (h1 :: result) else _merge l1 t2 (h2 :: result) in List.rev (_merge l1 l2 []) in let rec sort = function | [] -> [] | [a] -> [a] | list -> let left, right = split list in merge (sort left) (sort right) in sort l ;; Yet it seems that it is not actually tail-recursive, since I encounter a "Stack overflow during evaluation (looping recursion?)" error. Could you please help me spot the non tail-recursive call in this code? I've searched quite a lot, without finding it. Cout it be the let binding in the sort function? Thanks a lot, CFP.

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  • A F# tail-recursive question

    - by ksharp
    Recently, I'm learning F#. I try to solve problem in different ways. Like this: (* [0;1;2;3;4;5;6;7;8] -> [(0,1,2);(3,4,5);(6,7,8)] *) //head-recursive let rec toTriplet_v1 list= match list with | a::b::c::t -> (a,b,c)::(toTriplet_v1 t) | _ -> [] //tail-recursive let toTriplet_v2 list= let rec loop lst acc= match lst with | a::b::c::t -> loop t ((a,b,c)::acc) | _ -> acc loop list [] //tail-recursive(???) let toTriplet_v3 list= let rec loop lst accfun= match lst with | a::b::c::t -> loop t (fun ls -> accfun ((a,b,c)::ls)) | _ -> accfun [] loop list (fun x -> x) let funs = [toTriplet_v1; toTriplet_v2; toTriplet_v3]; funs |> List.map (fun x -> x [0..8]) |> List.iteri (fun i x -> printfn "V%d : %A" (i+1) x) I thought the results of V2 and V3 should be the same. But, I get the result below: V1 : [(0, 1, 2); (3, 4, 5); (6, 7, 8)] V2 : [(6, 7, 8); (3, 4, 5); (0, 1, 2)] V3 : [(0, 1, 2); (3, 4, 5); (6, 7, 8)] Why the results of V2 and V3 are different?

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  • recursive grep started at / hangs

    - by Martin
    I have used following grep search pattern on multiple platforms: grep -r -I -D skip 'string_to_match' / For example on FreeBSD 8.0, FreeBSD 6.4 and Debian 6.0(squeeze). Command does a recursive search starting from root directory, assumes that binary files do not have the 'string_to_match' and skips devices, sockets and named pipes. FreeBSD 8.0 and FreeBSD 6.4 use GNU grep version 2.5.1 and Debian 6.0 uses GNU grep version 2.6.3. On FreeBSD 6.4, last information printed to stderr was "grep: /dev/cuad0: Device busy". After this grep just idles as according to "top -m io -o total" the I/O usage of grep is nonexistent. Same behavior is true under FreeBSD 8.0, but last information sent to stderr is "grep: /tmp/.wine-0: Permission denied" on my installation. In case of Debian, last output to stderr is "grep: /proc/sysrq-trigger: Input/output error". If I check the I/O usage of grep process under Debian, it is following: root@Debian:~# iotop -bp 22439 Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 22439 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % grep -r -I -D skip 10.10.10.99 / ^Croot@Debian:~# What might cause this? Is there a way to view which file grep is currently processing in case lsof is not present? I'm able to use lsof under Debian and looks like the problematic file name there is "0xc6b2c230 file struct, ty=0, op=0xc0d34120". I'm not sure what this is.. I'm not able to use lsof or fstat under FreeBSD. PS: I know I could use find utility, but this is not the question.

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  • named responding recursive on norecurse queries

    - by Keks
    I have a server on which named is running. It is intercepted with another named server which it is not aware of. Querying the first named server results in timeouts. The server tries to resolve the query recursively. During that the firewall redirects the DNS Request from the first named server to the second one (the query from the first one is addressed to a e.g. a root server and has its "Recursion desired" bit set to 0). Despite that the second named responds to this request with a entirely or at least 1 level more resolved response than the first named server expects. So it ends up with a timeout even though it got a correct name server or even the full IP for the queried domain. In the first case the first name server tries to follow the authority domain ignoring the coresponding glue record and ends up in a loop it aborts: queried: google.com -> got from named#2: ns1.google.com -> ignore glue record and query: ns1.google.com -> got authority from named#2: google.com In the second case it ignores the answer section with the correct IP and instead tries to follow the name servers from the authority section, which ends up in the same dead end as case 1. So how can it be that the second named responds with recursive results even though the bit was explicitly set to 0 in the request from the first named?

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  • Cannot make bind9 forward DNS query to subdomain unless recursive enabled

    - by PP.
    I am trying to develop my own dynamic DNS. I'm running my own custom DNS for the subdomain on port 5353. ASCII diagram: INET --->:53 Bind 9 --->:5353 node.js | V zone_files I have example.com. The node.js DNS is for dyn.example.com. In my /etc/bind/named.conf.local I have: zone "example.com" { type master; file "/etc/bind/db.com.example"; allow-transfer { zonetxfrsafe; }; }; zone "dyn.example.com" IN { # DYNAMIC type forward; forwarders { 127.0.0.1 port 5353; }; forward only; }; I've even gone so far as to add a NS in my example.com zone file: $TTL 86400 @ IN SOA ns.example.com. hostmaster.example.com. ( 2013070104 ; Serial 7200 ; Refresh 1200 ; Retry 2419200 ; Expire 86400 ) ; Negative Cache TTL ; NS ns ; inet of our nameserver ns A 1.2.3.4 ; NS record for subdomain dyn NS ns When I attempt to get a record from the subdomain server it doesn't get forwarded: dig @127.0.0.1 test.dyn.example.com However if I turn recursive on in /etc/bind/named.conf.options: options { recursion yes; } .. then I CAN see the request going to the subdomain server. But I don't want recursion yes; in my Bind configuration as it is poor security practice (and allows all-and-sundry requests that are not related to my managed zones). How does one forward (proxy) zone queries for just one zone? Or do I give up on Bind altogether and find a DNS server that can actually forward specific queries?

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