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  • Need theoretical help, how to comprehend an if-else dependency net

    - by macbie
    I am going to face a following issue: I'm writing a program that manages some properties, some of them are general and some are specific. Each property is a pair of key and value, and for example: if it is given a general property and other specific property with exactly the same key and value has been existed before then the general property will swap the specific one in the register. If there are two the same general properties - both will remain in the register. And so on; it is like a net of dependencies. In my case I can handle with it intuitively and foresee all cases, but only because the system is not too vast. What if it would? I have met such problems a few times in many different programs and languages (i.e working with C semaphores) and my question is: How to approach this kind of problem? Is this connected with finite state machine, graph theory or something similar? How to be sure that I have considered the whole system and each possible case? Could you recommend some resources (books, sites) to learn from?

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  • theoretical and practical matrix multiplication FLOP

    - by mjr
    I wrote traditional matrix multiplication in c++ and tried to measure and compare its theoretical and practical FLOP. As I know inner loop of MM has 2 operation therefore simple MM theoretical Flops is 2*n*n*n (2n^3) but in practice I get something like 4n^3 + number of operation which is 2 i.e. 6n^3 also if I just try to add up only one array a[i][j]++ practical flops then calculate like 3n^3 and not n^3 as you see again it is 2n^3 +1 operation and not 1 operation * n^3 . This is in case if I use 1D array in three nested loops as Matrix multiplication and compare flop, practical flop is the same (near) the theoretical flop and depend exactly as the number of operation in inner loop.I could not find the reason for this behaviour. what is the reason in both case? I know that theoretical flop is not the same as practical one because of some operations like load etc. system specification: Intel core2duo E4500 3700g memory L2 cache 2M x64 fedora 17 sample results: Matrix matrix multiplication 512*512 Real_time: 1.718368 Proc_time: 1.227672 Total flpops: 807,107,072 MFLOPS: 657.429016 Real_time: 3.608078 Proc_time: 3.042272 Total flpops: 807,024,448 MFLOPS: 265.270355 theoretical flop: 2*512*512*512=268,435,456 Practical flops= 6*512^3 =807,107,072 Using 1 dimensional array float d[size][size]:512 or any size for (int j = 0; j < size; ++j) { for (int k = 0; k < size; ++k) { d[k]=d[k]+e[k]+f[k]+g[k]+r; } } Real_time: 0.002288 Proc_time: 0.002260 Total flpops: 1,048,578 MFLOPS: 464.027161 theroretical flop: *4n^2=4*512^2=1,048,576* practical flop : 4n^2+overhead (other operation?)=1,048,578 3 loop version: Real_time: 1.282257 Proc_time: 1.155990 Total flpops: 536,872,000 MFLOPS: 464.426117 theoretical flop:4n^3 = 536,870,912 practical flop: *4n^3=4*512^3+overheads(other operation?)=536,872,000* thank you

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  • How ZFS handles online replacement in a RAID-Z (theoretical)

    - by Kevin
    This is a somewhat theoretical question about ZFS and RAID-Z. I'll use a three disk single-parity array as an example for clarity, but the problem can be extended to any number of disks and any parity. Suppose we have disks A, B, and C in the pool, and that it is clean. Suppose now that we physically add disk D with the intention of replacing disk C, and that disk C is still functioning correctly and is only being replaced out of preventive maintenance. Some admins might just yank C and install D, which is a little more organized as devices need not change IDs - however this does leave the array degraded temporarily and so for this example suppose we install D without offlining or removing C. Solaris docs indicate that we can replace a disk without first offlining it, using a command such as: zpool replace pool C D This should cause a resilvering onto D. Let us say that resilvering proceeds "downwards" along a "cursor." (I don't know the actual terminology used in the internal implementation.) Suppose now that midways through the resilvering, disk A fails. In theory, this should be recoverable, as above the cursor B and D contain sufficient parity and below the cursor B and C contain sufficient parity. However, whether or not this is actually recoverable depnds upon internal design decisions in ZFS which I am not aware of (and which the manual doesn't say in certain terms). If ZFS continues to send writes to C below the cursor, then we are fine. If, however, ZFS internally treats C as though it were gone, resilvering D only from parity between A and B and only writing A and B below the cursor, then we're toast. Some experimenting could answer this question but I was hoping maybe someone on here already knows which way ZFS handles this situation. Thank you in advance for any insight!

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  • NAT / PAT Theoretical Question

    - by dbasnett
    Given the following simplistic network Would it be possible to construct NAT / PAT pools such that the PC's attached to the network could be identified by their port number. I understand that if I assign a public IP to each user I can identify them. What I am trying to do is to conserve Public IP's, but maintain the ability to identify the user from the public network. If your answer is vendor specific that is OK with me. Thanks in advance.

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  • Theoretical Wi-Fi decay

    - by lithiium
    Is there a way to (theoretically at least) calculate the decay on bandwith of a Wifi related to the streght signal? For example, I know that I can theoretically expect 54Mbps of a 802.11g at 100%, which will be the bandwith expected at a 30% of signal? is it lineal? is it the same? I could not find any source for this, but considering the error replay involved, I guess it should be possible to calculate something like this. Anybody knows?

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  • client-server syncing methodology [theoretical]

    - by Kenneth Ballenegger
    I'm in the progress of building an web-app that syncs with an iOS client. I'm currently tackling trying to figure out how to go about about syncing. I've come up with following two directions: I've got a fairly simple server web-app with a list of items. They are ordered by date modified and as such syncing the order does not matter. One direction I'm considering is to let the client deal with syncing. I've already got an API that lets the client get the data, as well as do certain actions on it, such as update, add or remove single items. I was considering: 1) on each sync asking the server for all items modified since the last successful sync and updating the local records based on what's returned by the server, and 2) building a persistent queue of create / remove / update requests on the client, and keeping them until confirmation by the server. The risk with this approach is that I'm basically asking each side to send changes to the other side, hoping it works smoothly, but risking a diversion at some point. This would probably be more bandwidth-efficient, though. The other direction I was considering was a more traditional model. I would have a "sync" process in which the client would send its whole list to the server (or a subset since last modified sync), the server would update the data on the server (by fixing conflicts by keeping the last modified item, and keeping deleted items with a deleted = 1 field), and the server would return an updated list of items (since last successful sync) which the client would then replace its data with. Thoughts?

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  • (type theoretical) How is ([] ==) [] typed in haskell?

    - by Ingo
    It sounds silly, but I can't get it. Why can the expression [] == [] be typed at all? More specifically, which type (in class Eq) is inferred to the type of list elements? In a ghci session, I see the following: Prelude> :t (==[]) (==[]) :: (Eq [a]) => [a] -> Bool But the constraint Eq [a] implies Eq a also, as is shown here: Prelude> (==[]) ([]::[IO ()]) <interactive>:1:1: No instance for (Eq (IO ())) arising from use of `==' at <interactive>:1:1-2 Probable fix: add an instance declaration for (Eq (IO ())) In the definition of `it': it = (== []) ([] :: [IO ()]) Thus, in []==[], the type checker must assume that the list element is some type a that is in class Eq. But which one? The type of [] is just [a], and this is certainly more general than Eq a = [a].

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  • Genetics algorithms theoretical question

    - by mandelart
    Hi All! I'm currently reading "Artificial Intelligence: A Modern Approach" (Russell+Norvig) and "Machine Learning" (Mitchell) - and trying to learn basics of AINN. In order to understand few basic things I have two 'greenhorn' questions: Q1: In a genetic algorithm given the two parents A and B with the chromosomes 001110 and 101101, respectively, which of the following offspring could have resulted from a one-point crossover? a: 001101 b: 001110 Q2: Which of the above offspring could have resulted from a two-point crossover? and why? Please advise.

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  • Any task-control algorithms programming practices?

    - by NumberFour
    Hi, I was just wondering if there's any field which concerns the task-control programming (or at least that's the way I call it). For a better explanation of task-control consider the following scenario: An application (master-thread) waits for a command - which might be a particular action or a set of actions the application should perform. When a command is received the master-thread creates a task (= spawns an independent thread which actually does the action) and adds a record in it's task-list - thus keeping track of the time of execution, thread handle, task priority...etc. The master-thread awaits for any other incoming commands while taking care of all the tasks - e.g: kills tasks running too long, prioritizes tasks with higher priorities, kills a task on a request of another task, limits the number of currently running tasks, allows task scheduling, cleans finished tasks (threads) and so on. The model is pretty similar to what we can see in OS dealing with running processes. Are there any good practices programming such task-models or is there some theoretical work done in this field? Maybe my question is too generalized, but at least I wanted to know whether there are any experiences working on such models or if there's a better approach. Thanks for any answers.

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  • How would you handle making an array or list that would have more entries than the standard implemen

    - by faceless1_14
    I am trying to create an array or list that could handle in theory, given adequate hardware and such, as many as 100^100 BigInteger entries. The problem with using an array or standard list is that they can only hold Integer.MAX_VALUE number of entries. How would you work around this limitations? A whole new class/interface? A wrapper for list? another data type entirely?

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  • Is there any difference between md5 and sha1 in this situation?

    - by Vili
    It is known that 1. if ( md5(a) == md5(b) ) 2. then ( md5(a.z) == md5(b.z) ) 3. but ( md5(z.a) != md5(z.b) ) where the dots concatenate the strings. What happens in the second row if we change all the md5 to sha1? So: 1. if ( sha1(c) == sha1(d) ) 2. then ( sha1(c.z) ?= sha1(d.z) ) I couldn't find two different strings with same sha1, that's why I'm asking this. Are there any other interesting "rules" about sha1?

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  • P-NP-Problem: What are the most promising methods?

    - by phimuemue
    Hello everybody, I know that P=NP has not been solved up to now, but can anybody tell me something about the following: What are currently the most promising mathematical / computer scientific methods that could be helpful to tackle this problem? Or are there even none such methods known to be potentially helpful up to now? Is there any (free) compendium on this topic where I can find all / most of the research done in this area?

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  • P=NP?-Problem: What are the most promising methods?

    - by phimuemue
    Hello everybody, I know that P=NP has not been solved up to now, but can anybody tell me something about the following: What are currently the most promising mathematical / computer scientific methods that could be helpful to tackle this problem? Or are there even none such methods known to be potentially helpful up to now? Is there any (free) compendium on this topic where I can find all / most of the research done in this area?

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  • What is the peak theoretical WiFi G user density? [closed]

    - by Bigbio2002
    I've seen a few WiFi capacity planning questions, and this one is related, but hopefully different enough not to be closed. Also, this is related specifically to 802.11g, but a similar question could be made for N. In order to squeeze more WiFi users into a space, the transmit power on the APs need to be reduced and the APs squeezed closer together. My question is, how far can you practically take this before the network becomes unusable? There will come a point where the transmit power is so weak that nobody will actually be able to pick up a connection, or be constantly roaming to/from APs spaced a few feet apart as they walk around. There are also only 3 available channels to use as well, which is a factor to consider. After determining the peak AP density, then multiply by users-per-AP, which should be easier to find out. After factoring all of this in and running some back-of-the-envelope calculations, I'd like to be able to get a figure of "XX users per 10ft^2" or something. This can be considered the physical limit of WiFi, and will keep people from asking about getting 3,000 people in a ballroom conference on WiFi. Can anyone with WiFi experience chime in, or better yet, provide some calculations for a more accurate figure? Assumptions: Let's assume an ideal environment with no reflection (think of a big, square, open room, with the APs spaced out on a plane), APs are placed on the ceiling so humans won't absorb the waves, and the only interference are from the APs themselves and the devices. As for what devices specifically, that's irrelevant for the first point of the question (AP density, so only channel and transmit power should matter). User experience: Wikipedia states that Wireless G has about 22Mbps maximum effective throughput, or about 2.75MB/s. For the purpose of this question, anything below 100KB/s per user can be deemed to be a poor user experience. As for roaming, I'll assume the user is standing in the same place, so hopefully that will be a non-issue.

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  • What theoretical and/or experimental programming-language features are there?

    - by Gary Rake
    I'm designing a programming language, purely for fun, and want to add as many experimental features as I can, just to make programming in it something completely different, and that not in a bad way like Brainf*ck or Malbolge. However, I seem to be quite bad at coming up with new things for it but I'm sure that there are tons of things out there that have been talked about but never really tried out. What experimental language features or concepts not implemented in mainstream languages are there at the moment? E.g: If I asked this in, let's say, 1960, an answer could be "Object-oriented programming". I'm sure that there are a lot of unimplemented ideas computer-scientists have (recently) come up with, at least I was told so.

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  • Efficient, partial, point-in-time database restores

    - by GavinPayneUK
    This article is about a situation that many of us could describe the theoretical approach to solving, but then struggle to understand why SQL Server wasn’t following that theoretical approach when you tried it for real. Earlier this week, I had a client ask about the best way to perform: a partial database restore, 1 of 1300 filegroups; to a specific point in time; using a differential backup, and therefore; without restoring each transaction log backup taken since the full backup. The last point...(read more)

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  • Sockets: Transport endpoint is not connected on send

    - by TheoretiCAL
    I'm trying to learn socket programming from http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html and am attempting to build a SOCK_STREAM client/server. My client: #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <netdb.h> #define SERVERPORT "4951" // the port users will be connecting to int main(int argc, char *argv[]) { int sockfd; struct addrinfo hints, *servinfo, *p; int rv; int numbytes; if (argc != 3) { fprintf(stderr,"usage: talker hostname message\n"); exit(1); } memset(&hints, 0, sizeof hints); hints.ai_family = AF_UNSPEC; hints.ai_socktype = SOCK_STREAM; if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) { fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv)); return 1; } // loop through all the results and make a socket for(p = servinfo; p != NULL; p = p->ai_next) { if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) { perror("talker: socket"); continue; if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) { close(sockfd); perror("client: connect"); continue; } } break; } if (p == NULL) { fprintf(stderr, "talker: failed to bind socket\n"); return 2; } if ((numbytes = send(sockfd, argv[2], strlen(argv[2]), 0) == -1)) { perror("talker: send"); exit(1); } freeaddrinfo(servinfo); printf("talker: sent %d bytes to %s\n", numbytes, argv[1]); close(sockfd); return 0; } Server: #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <netdb.h> #define MYPORT "4951" // the port users will be connecting to #define MAXBUFLEN 100 static int backlog = 10; // get sockaddr, IPv4 or IPv6: void *get_in_addr(struct sockaddr *sa) { if (sa->sa_family == AF_INET) { return &(((struct sockaddr_in*)sa)->sin_addr); } return &(((struct sockaddr_in6*)sa)->sin6_addr); } int main(void) { int sockfd; struct addrinfo hints, *servinfo, *p; int rv; int numbytes; int new_fd; socklen_t addr_size; struct sockaddr_storage their_addr; char buf[MAXBUFLEN]; char s[INET6_ADDRSTRLEN]; memset(&hints, 0, sizeof hints); hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4 hints.ai_socktype = SOCK_STREAM; hints.ai_flags = AI_PASSIVE; // use my IP if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) { fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv)); return 1; } // loop through all the results and bind to the first we can for(p = servinfo; p != NULL; p = p->ai_next) { if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) { perror("listener: socket"); continue; } int yes=1; // lose the pesky "Address already in use" error message if (setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) { perror("setsockopt"); exit(1); } if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) { close(sockfd); perror("listener: bind"); continue; } if (listen(sockfd,backlog) == -1){ close(sockfd); perror("listener:listen"); continue; } break; } if (p == NULL) { fprintf(stderr, "listener: failed to bind socket\n"); return 2; } freeaddrinfo(servinfo); printf("listener: waiting to recv..\n"); while(1){ addr_size = sizeof their_addr; if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size))==-1){ perror("accept"); exit(1); } if ((numbytes = recv(new_fd, buf, MAXBUFLEN-1 , 0) == -1)) { perror("recv"); exit(1); } printf("listener: got packet from %s\n", inet_ntop(their_addr.ss_family, get_in_addr((struct sockaddr *)&their_addr), s, sizeof s)); printf("listener: packet is %d bytes long\n", numbytes); buf[numbytes] = '\0'; printf("listener: packet contains \"%s\"\n", buf); close(sockfd); } return 0; } Upon executing the client, I get " send: Transport endpoint is not connected" and I'm not sure where I went wrong. Thanks.

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  • How do you deal with translating theory into practice?

    - by Mr. Shickadance
    Hello all! Being a computer scientist in a research field I am often tasked with working alongside professionals outside of the software domain (think math people, electrical engineer etc), and then translating their theories and ideas into real-world implementations. I often find it difficult when they present a theoretical problem which appears to be somewhat disconnected from reality. I am not saying that the theory is bogus, only that it is difficult to translate into real-world situations. For example, recently I have been working with software defined radios. We are exploring many different areas, but often the math specialists in my group would present a problem which is heavily grounded in theory (signal processing, physics, whatever). I often struggle at times where it is hard to draw direct parallels between the theory and the real-world implementation that I need to develop. Say we are working on an energy detector, the theory person in my group would say "you need to measure the noise variance with no signal present." This leads me to think "how the hell do I isolate noise from a signal in reality?" There are many examples, but I hope you see where I am going. So, my question is how does one deal with implementation of theoretical concepts when the theory seems detached from reality? Or at least when the connections are not so clear. Or perhaps, the person with the 'theory' may be ignorant of real restrictions? Note: I found this to be a hard question to ask - hopefully you are following me. If you have suggestions on how I could improve it, by all means let me know! Thanks for looking! EDIT: To be a bit more clear, I understand in situations like this that I must learn that specific domain myself to an extent (i.e. signal processing), but I am more concerned with when those theoretical concepts do not appear to be as grounded in practice as one would like.

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  • Audio recording, a tool for human-aided drum quantizing.

    - by basilio.mp
    I have this situation: the drummer records the track (8 tracks in a multitrack session). Now, how do I check how distant are the recorded beats from their theoretical position i.e.: there is always some error in human recorded tracks, but is there any software that can show me the ideal (theoretical, quantized) beat and the recorded one and could alert me if the error is too big. P.S.: I'm searching for a standalone tool, or for a plugin that can work with Adobe Audition 3 or Nuendo 3.

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  • gpgpu vs. physX for physics simulation

    - by notabene
    Hello First theoretical question. What is better (faster)? Develop your own gpgpu techniques for physics simulation (cloth, fluids, colisions...) or to use PhysX? (If i say develop i mean implement existing algorithms like navier-strokes...) I don't care about what will take more time to develop. What will be faster for end user? As i understand that physx are accelerated through PPU units in gpu, does it mean that physical simulation can run in paralel with rastarization? Are PPUs different units than unified shader units used as vertex/geometry/pixel/gpgpu shader units? And little non-theoretical question: Is physx able to do sofisticated simulation equal to lets say Autodesk's Maya fluid solver? Are there any c++ gpu accelerated physics frameworks to try? (I am interested in both physx and gpgpu, commercial engines are ok too).

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