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• Using hash functions with Bloom filters

  - by dangerstat

  Hi, A bloom filter uses a hash function (or many) to generate a value between 0 and m given an input string X. My question is how to you use a hash function to generate a value in this way, for example an MD5 hash is typically represented by a 32 length hex string, how would I use an MD5 hashing algorithm to generate a value between 0 and m where I can specify m? I'm using Java at the moment so an example of to do this with the MessageDigest functionality it offers would be great, though just a generic description of how to do about it would be fine too. Thanks

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• Does the 80/20 rule of time management apply to developers?

  - by Dean

  Jeff's recent article linked to a time management example of the First Fit Decreasing algorithm, which talked about the Pareto principle (or, the 80/20 rule) of time management, that is, that 80% of the work we produce in 20% of our time. Now we've all heard the programmer quote: The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time. But all jokes aside, it is often as if 20% of your code is to do what you want, and the other 80% is to handle exceptions... so does the 80/20 rule really apply to developers? Does anyone have any examples of why it does / does not apply to us?

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• How should I use random.jumpahead in Python

  - by Peter Smit

  I have a application that does a certain experiment 1000 times (multi-threaded, so that multiple experiments are done at the same time). Every experiment needs appr. 50.000 random.random() calls. What is the best approach to get this really random. I could copy a random object to every experiment and do than a jumpahead of 50.000 * expid. The documentation suggests that jumpahead(1) already scrambles the state, but is that really true? Or is there another way to do this in 'the best way'? (No, the random numbers are not used for security, but for a metropolis hasting algorithm. The only requirement is that the experiments are independent, not whether the random sequence is somehow predictable or so)

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• Writing a search engine

  - by wvd

  Hello all, The title might be a bit misleading, but I couldn't figure out a better title. I'm writing a simple search engine which will search on several sites for the specific domain. To be concrete: I'm writing a search engine for hardstyle livesets/aftermovies/tracks. To do I will search on the sites who provide livesets, tracks, and such. The problem here is speed, I need to pass the search query to 5-7 sites, get the results and then use my own algorithm to display the results in a sorted order. I could just "multithread" it, but it's easier said then done so I have a few questions. What would be the best solution to this problem? Should I just multithread/process this application, so I'm going to get a bit of speed-up? Are there any other solutions or I am doing something really wrong? Thanks, William van Doorn

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• What's the difference between the 'DES' class and The 'DESCryptoServiceProvider' class?

  - by IbrarMumtaz

  All I can make out is that one of them is the BC for all 'DES' algorithms to be derived from and the later is a wrapper for the Cryptographic service provider implementation of the DES algorithm. The reason why I ask is that I am going over .Net Security and the MS official training book simply refers to the DES class but the another official MS book refers to the DESCrypto' class. What's the difference between these two? When would you use either of them? What do I need to know as far as the 70-536 exam is concerned. I am asking my question from an educational P.O.V as far as the 70-536 exam is concerned. Thanks In Advance. Ibrar

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• Can this be done with the ORM? - Django

  - by RadiantHex

  Hi folks, I have a few item listed in a database, ordered through Reddit's algorithm. This is it: def reddit_ranking(post): t = time.mktime(post.created_on.timetuple()) - 1134000000 x = post.score if x>0: y=1 elif x==0: y=-0 else: y=-1 if x<0: z=1 else: z=x return (log(z) + y * t/45000) I'm wondering if there is any clever way of using Django's ORM, in order to UPDATE the models in bulk. Without doing this: items = Item.objects.filter(created_on__gte=datetime.now()-timedelta(days=7)) for item in items: item.reddit_rank = reddit_rank(item) item.save() I know about the F() object, but I can't figure out if this function can be performed inside the ORM. Any ideas? Help would be very much appreciated!

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• How to perform a literal match with regex using wildcard

  - by kashif4u

  I am trying to perform literal match with regular expression using wildcard. string utterance = "Show me customer id 19"; string pattern 1 = "*tom*"; string patter 2 = "*customer id [0-9]*"; Desired results: if (Regex.IsMatch(utterance, pattern 1 )) { MATCH NOT FOUND } if (Regex.IsMatch(utterance, pattern 2 )) { MATCH FOUND } I have tried looking for literal match solution/syntax in wildcard but having difficulty. Could you also enlighten me with with an example on possible Pattern Matching Strength algorithm i.e. if code match 90 select? Note: I have table with 100000 records to perform literal matches from user utterances. Thanks in advance.

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• openssl ssl encryption

  - by deddihp

  Hello, I want to discuss about openssl write and read method. Assume I have an data structure like below: /-----------------------------------------------------\ | my_header | PAYLOAD | \-----------------------------------------------------/ | | \ / \ / not encrypted encrypted I think the proper algorithm would be like this : SEND: build my_header with my own header. encrypt PAYLOAD with encryption function attach my_header and PAYLOAD (encrypted) to one buffer send it using common POSIX function just like send or sendto RECV: using common POSIX function just like recv or recvfrom. extract my_header and PAYLOAD(encrypted) decrypt PAYLOAD with decryption function at last i got my_header and PAYLOAD(decrypted). How is your approach if you face a problem like above. Since openssl encrypt all of data that is sent to SSL_write function (CMIIW). Thanks

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• Query size of block device file in Python

  - by ??O?????

  Hello. I have a Python script that reads a file (typically from optical media) marking the unreadable sectors, to allow a re-attempt to read said unreadable sectors on a different optical reader. I discovered that my script does not work with block devices (e.g. /dev/sr0), in order to create a copy of the contained ISO9660/UDF filesystem, because os.stat().st_size is zero. The algorithm currently needs to know the filesize in advance; I can change that, but the issue (of knowing the block device size) remains, and it's not answered here, so I open this question. I am aware of the following two related SO questions: Determine the size of a block device (/proc/partitions, ioctl through ctypes) how to check file size in python? (about non-special files) Therefore, I'm asking: in Python, how can I get the file size of a block device file?

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• question about frequency of updating access

  - by I__

  i have a table in an access database this access database is used on a regular basis, basically from 9-5 someone else has a copy of this exact table. sometimes records are added, sometimes deleted, and sometimes data within the records is updated. i need to update the access database table with the offsite table every hour or so. what is the best algorithm of updating the data? there are about 5000 records. would it severely lock up the table for a few seconds every hour? i would like to publicly apologize for my rude comment to david fenton

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• is there a such thing as a randomly accessible pseudo-random number generator? (preferably open-sour

  - by lucid

  first off, is there a such thing as a random access random number generator, where you could not only sequentially generate random numbers as we're all used to, assuming rand100() always generates a value from 0-100: for (int i=0;i<5;i++) print rand100() output: 14 75 36 22 67 but also randomly access any random value like: rand100(0) would output 14 as long as you didn't change the seed rand100(3) would always output 22 rand100(4) would always output 67 and so on... I've actually found an open-source generator algorithm that does this, but you cannot change the seed. I know that pseudorandomness is a complex field; I wouldn't know how to alter it to add that functionality. Is there a seedable random access random number generator, preferably open source? or is there a better term for this I can google for more information? if not, part 2 of my question would be, is there any reliably random open source conventional seedable pseudorandom number generator so I could port it to multiple platforms/languages while retaining a consistent sequence of values for each platform for any given seed?

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• Throughput measurements

  - by dotsid

  I wrote simple load testing tool for testing performance of Java modules. One problem I faced is algorithm of throughput measurements. Tests are executed in several thread (client configure how much times test should be repeated), and execution time is logged. So, when tests are finished we have following history: 4 test executions 2 threads 36ms overall time - idle * test execution 5ms 9ms 4ms 13ms T1 |-*****-*********-****-*************-| 3ms 6ms 7ms 11ms T2 |-***-******-*******-***********-----| <-----------------36ms---------------> For the moment I calculate throughput (per second) in a following way: 1000 / overallTime * threadCount. But there is problem. What if one thread will complete it's own tests more quickly (for whatever reason): 3ms 3ms 3ms 3ms T1 |-***-***-***-***----------------| 3ms 6ms 7ms 11ms T2 |-***-******-*******-***********-| <--------------32ms--------------> In this case actual throughput is much better because of measured throughput is bounded by the most slow thread. So, my question is how should I measure throughput of code execution in multithreaded environment.

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• Why is win32com so much slower than xlrd?

  - by Josh

  I have the same code, written using win32com and xlrd. xlrd preforms the algorithm in less than a second, while win32com takes minutes. Here is the win32com: def makeDict(ws): """makes dict with key as header name, value as tuple of column begin and column end (inclusive)""" wsHeaders = {} # key is header name, value is column begin and end inclusive for cnum in xrange(9, find_last_col(ws)): if ws.Cells(7, cnum).Value: wsHeaders[str(ws.Cells(7, cnum).Value)] = (cnum, find_last_col(ws)) for cend in xrange(cnum + 1, find_last_col(ws)): #finds end column if ws.Cells(7, cend).Value: wsHeaders[str(ws.Cells(7, cnum).Value)] = (cnum, cend - 1) break return wsHeaders And the xlrd def makeDict(ws): """makes dict with key as header name, value as tuple of column begin and column end (inclusive)""" wsHeaders = {} # key is header name, value is column begin and end inclusive for cnum in xrange(8, ws.ncols): if ws.cell_value(6, cnum): wsHeaders[str(ws.cell_value(6, cnum))] = (cnum, ws.ncols) for cend in xrange(cnum + 1, ws.ncols):#finds end column if ws.cell_value(6, cend): wsHeaders[str(ws.cell_value(6, cnum))] = (cnum, cend - 1) break return wsHeaders

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• HowTo make a marching ants border in Morphic?

  - by Helene Bilbo

  I am looking for a marching ants border or line in Morphic: Wikipedia describes a possbile algorithm: The easiest way to achieve this animation is by drawing the selection using a pen pattern that contains diagonal lines. If the selection outline is only one pixel thick, the slices out of the pattern will then look like a dashed line, and the animation can easily be achieved by simply shifting the pattern one pixel sideways and redrawing the outline. As there is probably none readymade, has anybody a hint for me how to create such a border or line? (The marching ants pattern as a GIF animation is licensed under CC by Uli Kusterer)

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• Efficiency of while(true) ServerSocket Listen

  - by Submerged

  I am wondering if a typical while(true) ServerSocket listen loop takes an entire core to wait and accept a client connection (Even when implementing runnable and using Thread .start()) I am implementing a type of distributed computing cluster and each computer needs every core it has for computation. A Master node needs to communicate with these computers (invoking static methods that modify the algorithm's functioning). The reason I need to use sockets is due to the cross platform / cross language capabilities. In some cases, PHP will be invoking these java static methods. I used a java profiler (YourKit) and I can see my running ServerSocket listen thread and it never sleeps and it's always running. Is there a better approach to do what I want? Or, will the performance hit be negligible? Please, feel free to offer any suggestion if you can think of a better way (I've tried RMI, but it isn't supported cross-language. Thanks everyone

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• Candidate Elimination Question---Please help!

  - by leon

  Hi , I am doing a question on Candidate Elimination Algorithm. I am a little confused with the general boundary G. Here is an example, I got G and S to the fourth case, but I am not sure with the last case. Sunny,Warm,Normal,Strong,Warm,Same,EnjoySport=yes Sunny,Warm,High,Strong,Warm,Same,EnjoySport=yes Rainy,Cold,High,Strong,Warm,Change,EnjoySport=no Sunny,Warm,High,Strong,Cool,Change,EnjoySport=yes Sunny,Warm,Normal,Weak,Warm,Same,EnjoySport=no What I have here is : S 0 :{0,0,0,0,0,0} S 1 :{Sunny,Warm,Normal,Strong,Warm,Same} S 2 , S 3 : {Sunny,Warm,?,Strong,Warm,Same} S 4 :{Sunny,Warm,?,Strong,?,?} G 4 :{Sunny,?,?,?,?,?,?,Warm,?,?,?,?} G 3 :{Sunny,?,?,?,?,?,?,Warm,?,?,?,?,?,?,?,?,?,Same} G 0 , G 1 , G 2 : {?,?,?,?,?,?} What would be the result of G5? Is it G5 empty? {}? or {???Strong??) ? Thanks

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• pyplot: really slow creating heatmaps

  - by cvondrick

  I have a loop that executes the body about 200 times. In each loop iteration, it does a sophisticated calculation, and then as debugging, I wish to produce a heatmap of a NxM matrix. But, generating this heatmap is unbearably slow and significantly slow downs an already slow algorithm. My code is along the lines: import numpy import matplotlib.pyplot as plt for i in range(200): matrix = complex_calculation() plt.set_cmap("gray") plt.imshow(matrix) plt.savefig("frame{0}.png".format(i)) The matrix, from numpy, is not huge --- 300 x 600 of doubles. Even if I do not save the figure and instead update an on-screen plot, it's even slower. Surely I must be abusing pyplot. (Matlab can do this, no problem.) How do I speed this up?

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• Encode/compress sequence of repeating integers

  - by Alex

  Hey there! I have very long integer sequences that look like this (arbitrary length!): 0000000001110002220033333 Now I need some algorithm to convert this string into something compressed like a9b3a3c3a2d5 Which means "a 9 times, then b 3 times, then a 3 times" and so on, where "a" stands for 0, "b" for 1, "c" for 2 and "d" for 3. How would you do that? So far nothing suitable came to my mind, and I had no luck with google because I didn't really know what to search for. What is this kind of encoding / compression called? PS: I am going to do the encoding with PHP, and the decoding in JavaScript.

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• In Haskell, how can you sort a list of infinite lists of strings?

  - by HaskellNoob

  So basically, if I have a (finite or infinite) list of (finite or infinite) lists of strings, is it possible to sort the list by length first and then by lexicographic order, excluding duplicates? A sample input/output would be: Input: [["a", "b",...], ["a", "aa", "aaa"], ["b", "bb", "bbb",...], ...] Output: ["a", "b", "aa", "bb", "aaa", "bbb", ...] I know that the input list is not a valid haskell expression but suppose that there is an input like that. I tried using merge algorithm but it tends to hang on the inputs that I give it. Can somebody explain and show a decent sorting function that can do this? If there isn't any function like that, can you explain why? In case somebody didn't understand what I meant by the sorting order, I meant that shortest length strings are sorted first AND if one or more strings are of same length then they are sorted using < operator. Thanks!

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• what does this C++ line of code mean "sol<?=f((1<<n)-1,i,0)+abs(P[i])*price;"

  - by KItis

  Could anyone help me to understand following line of code. sol I am studying an algorithm written using c++ and it has following operator " following is the error message returned. Hello.cpp: In function ‘int main()’: Hello.cpp:115: error: ‘memset’ was not declared in this scope Hello.cpp:142: error: expected primary-expression before ‘?’ token Hello.cpp:142: error: expected primary-expression before ‘=’ token Hello.cpp:142: error: expected ‘:’ before ‘;’ token Hello.cpp:142: error: expected primary-expression before ‘;’ token may be " Thanks in advance for the time you spent reading this post.

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• PHP Code to Generate Simple Flowchart

  - by revbackup

  I am making a flowchart out the subjects in the curriculum of our school. a flowchart is generated through its preRequisite.... for example FIRST YEAR FIRST SEMESTER SUBJECTS ---- PREREQUISITE MATH 1 ---- NONE MATH 2 ---- NONE ENGL 1 ---- NONE SOCIO 1 ----- NONE POLSCI 1 ----- NONE FIRSTE YEAR SECOND SEMESTER SUBJECTS ---- PREREQUISITE MATH 3 ----- MATH 1 MATH 4 ----- MATH 2, MATH 1 ENGL 2 ----- ENGL 1 POLSCI 2 ----- POLSCI 1 So, I must print it this way, just using simple PHP but difficult Logic.: MATH1 -----> MATH3 -----> MATH4 MATH 2 ----->MATH 4 ENGL1 -----> ENGL 2 SOCIO 1 POLSCI 1 -----> POLSCI 2 Can anyone give me a good algorithm for this, because this is really difficult. I am planning to echo the results in an HTML table, and it makes it more complicated. Do you have suggestions how to solve this problem properly and display the results also properly???? Thank you in advance!

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• SQLAlchemy custom sorting algorithms when using SQL indexes

  - by David M

  Is it possible to write custom collation functions with indexes in SQLAlchemy? SQLite for example allows specifying the sorting function at a C level as sqlite3_create_collation(). An implementation of some of the Unicode collation algorithm has been provided by James Tauber here, which for example sorts all the "a"'s close together whether they have accents on them or not. Other examples of why this might be useful is for different alphabet orders (languages other than English) and sorting numeric values (sorting 10 after 9 rather than codepoint order.) Is this possible in SQLAlchemy? If not, is it supported by the pysqlite3 or MySQLdb modules, or for any other SQL database modules supported by python for that matter? Any information would be greatly appreciated.

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• Document ranking strategy for P2P file sharing system.

  - by ablmf

  Recently, I got a task of building a P2P file sharing system. There is one requirement : the system should have a document ranking algorithm so that it could be used help users find more valuable files. Several strategy might be useful : force user to give a score to a file before it was downloaded The documents containing certain key words would get higher rank Manager could modify file ranking manually the more a file was downloaded, it would get a higher rank. Do you know any other strategy or methods that also appropriate? Or is there any real world example?

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• Counting viable sublist lengths from an array.

  - by Ben B.

  This is for a genetic algorithm fitness function, so it is important I can do this as efficiently as possible, as it will be repeated over and over. Lets say there is a function foo(int[] array) that returns true if the array is a "good" array and false if the array is a "bad" array. What makes it good or bad does not matter here. Given the full array [1,6,8,9,5,11,45,16,9], lets say that subarray [1,6,8] is a "good" array and [9,5,11,45] is a "good" array. Furthermore [5,11,45,16,9] is a "good" array, and also the longest "good" subarray. Notice that while [9,5,11,45] is a "good" array, and [5,11,45,16,9] is a "good" array, [9,5,11,45,16,9] is a "bad" array. We wants the length counts of all "good" arrays, but not subarrays of "good" arrays. Furthermore, as described above, a "good" array might begin in the middle of another "good" array, but the combination of the two might be a "bad" array.

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• Vectorizing sums of different diagonals in a matrix

  - by reve_etrange

  I want to vectorize the following MATLAB code. I think it must be simple but I'm finding it confusing nevertheless. r = some constant less than m or n [m,n] = size(C); S = zeros(m-r,n-r); for i=1:m-r for j=1:n-r S(i,j) = sum(diag(C(i:i+r-1,j:j+r-1))); end end The code calculates a table of scores, S, for a dynamic programming algorithm, from another score table, C. The diagonal summing is to generate scores for individual pieces of the data used to generate C, for all possible pieces (of size r). Thanks in advance for any answers! Sorry if this one should be obvious...

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