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• Sum of path products in DAG

  - by Jules

  Suppose we have a DAG with edges labeled with numbers. Define the value of a path as the product of the labels. For each (source,sink)-pair I want to find the sum of the values of all the paths from source to sink. You can do this in polynomial time with dynamic programming, but there are still some choices that can be made in how you decompose the problem. In my case I have one DAG that has to be evaluated repeatedly with different labelings. My question is: for a given DAG, how can we pre-compute a good strategy for computing these values for different labelings repeatedly. It would be nice if there was an algorithm that finds an optimal way, for example a way that minimizes the number of multiplications. But perhaps this is too much to ask, I would be very happy with an algorithm that just gives a good decomposition.

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• Data tweaking code runs fine when executed directly - but never stops when used in trigger

  - by MBaas

  I have written some code to ensure that items on an order are all numbered (the "position number" or "item number" has been introduced only recently and we did not want to go and change all related code - as it is "asthetics only" and has no functional impact.) So, the idea is to go and check for an records that jave an itemno of NULL or 0 - and then compute one and assign it. When executing this code in a query window, it works fine. When putting it into an AFTER INSERT-trigger, it loops forever. So what is wrong here? /****** Objekt: Trigger [SetzePosNr] Skriptdatum: 02/28/2010 20:06:29 ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO CREATE TRIGGER [SetzePosNr] ON [dbo].[bestellpos] AFTER INSERT AS BEGIN DECLARE @idb int DECLARE @idp int DECLARE @pnr int SELECT @idp=id,@idb=id_bestellungen FROM bestellpos WHERE posnr IS NULL OR posnr=0 WHILE @idp IS NOT NULL BEGIN SELECT @pnr = 1+max(posnr) FROM bestellpos WHERE id_bestellungen = @idb print( 'idp=' + str(@idp) + ', idb=' + str(@idb) + ', posnr=' + str(@pnr)) UPDATE bestellpos SET posnr=@pnr WHERE id=@idp SELECT @idp=id,@idb=id_bestellungen FROM bestellpos WHERE posnr IS NULL OR posnr=0 END END

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• Best similarity metric for collaborative filtering?

  - by allclaws

  I'm trying to decide on the best similarity metric for a product recommendation system using item-based collaborative filtering. This is a shopping basket scenario where ratings are binary valued - the user has either purchased an item or not - there is no explicit rating system (eg, 5-stars). Step 1 is to compute item-to-item similarity, though I want to look at incorporating more features later on. Is the Tanimoto coefficient the best way to go for binary values? Or are there other metrics that are appropriate here? Thanks.

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• Haskell compile time function calculation

  - by egon

  I would like to precalculate values for a function at compile-time. Example (real function is more complex, didn't try compiling): base = 10 mymodulus n = n `mod` base -- or substitute with a function that takes -- too much to compute at runtime printmodules 0 = [mymodulus 0] printmodules z = (mymodulus z):(printmodules (z-1)) main = printmodules 64 I know that mymodulus n will be called only with n < 64 and I would like to precalculate mymodulus for n values of 0..64 at compile time. The reason is that mymodulus would be really expensive and will be reused multiple times.

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• Visual Studio error on valid SQL

  - by Sam

  I'm getting an annoying error in visual studio for SQL that executes fine. SELECT InvoiceLines.LineID, InvoiceLines.InvoiceID, InvoiceLines.Text, InvoiceLines.Rate, InvoiceLines.Count, InvoiceLines.Rate * InvoiceLines.Count AS LineTotal, ((InvoiceLines.Rate * InvoiceLines.Count) * (1 + Invoices.VatRate / 100)) * (1 - CAST(Invoices.Discount AS money) * InvoiceLines.ApplyDiscount / 100) AS LineTotalIncVat, InvoiceLines.ApplyDiscount FROM InvoiceLines LEFT JOIN Invoices ON Invoices.InvoiceID = InvoiceLines.InvoiceID What LineTotalIncVat is trying to do is compute the total for the invoice item while adding the vat and subtracting the discount, and yes, probably better to do this in code (would if I could) The error visual studio gives is: There was an error parsing the query [token line number =1, token line offset =14, token in error = InvoiceLines] Even though it will validate and execute without a problem in the query builder...

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• Different results when applying function to equal values

  - by Johannes Stiehler

  I'm just digging a bit into Haskell and I started by trying to compute the Phi-Coefficient of two words in a text. However, I ran into some very strange behaviour that I cannot explain. After stripping everything down, I ended up with this code to reproduce the problem: let sumTup = (sumTuples°concat) frequencyLists let sumFixTup = (138, 136, 17, 204) putStrLn (show ((138, 136, 17, 204) == sumTup)) putStrLn (show (phi sumTup)) putStrLn (show (phi sumFixTup)) This outputs: True NaN 0.4574206676616167 So although the sumTupand sumFixTup show as equal, they behave differently when passed to phi. The definition of phi is: phi (a, b, c, d) = let dividend = fromIntegral(a * d - b * c) divisor = sqrt(fromIntegral((a + b) * (c + d) * (a + c) * (b + d))) in dividend / divisor

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• Can anyone tell me what the authors mean on this line?

  - by Anirudh Goel

  i was going through this link: FAT16 Basics to Assemble Clusters. I have read the structures involved in defining a directory entry in FAT. Now when giving the example for a FAT16 File, it says the data cluster is 0x03 for the example file MyFile.txt. Which means if we logically compute the Data Cluster we will be able to reach to the first node which happens to be cluster no 3. But what I fail to understand is what the author is trying to say in the next line where it says What we can see in the File Allocation Table at this moment? How suddenly we reach to the File Allocation Table? Weren't we already there when we were going through the information of Myfile.txt? I couldn't find any reason how suddenly the author jumped to an offset location of 00000200 and is identifying the emptiness of the clusters. It will be great if someone can help me understand. Thanks

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• GeoDjango: is there an out-of-the-box way to generate clusters of points?

  - by vaughnkoch

  Hi, I'm trying to compute clusters on a set of points in Python, using GeoDjango. The problem: Given a set of points, output a set of clusters of those points. (i'm fine specifying # of clusters/cluster size/distance in advance to simplify) There are a few solutions on the web to do clustering, so it's a well known problem. I thought that GeoDjango would handle these types of problems out of the box, but it's not clear how - I've searched the GeoDjango documentation, Google, and a few other places, but couldn't find anything. Before I roll my own clustering solution, I thought I'd ask to see if there's a straightforward way to do this using GEOS or another package within GeoDjango.

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• Can games be considered real-time systems?

  - by harry

  I've been reading up on real-time systems and how they work etc. I was looking at the wikipedia article as well that said a game of Chess with a timer per move can be considered a real-time system because the program MUST compute a move in that time. What about other games? As we know, games generally try and run at 25+ FPS, could it be considered a soft real-time system since if it falls under 25 (I'm using 25 as a pre-defined threshold btw) it's not the end of the world, just a hit to the performance that we wanted? Also - games have events they must handle as well. The user uses the keyboard/mouse and the system must answer those events accordingly within (again) a pre-defined time, before the game is considered to have "failed". Oh, and I'm talking single-player for now to keep things simple. It sounds like games fit the soft real-time system criteria, but I'd like to know if I'm missing anything... thanks.

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• Simple NLP: How to use ngram to do word similarity?

  - by sadawd

  Dear Everyone, I Hear that google uses up to 7-grams for their own data. I am interested in finding words that are similar in context (i.e. cat and dog) and I was wondering how do I compute the similarity of two words on a n-gram model given that n 2. Given a sample set like this forexample: (I, love cats), (cats, loves, dogs), (dogs, hate, human) What is a good way to compare the similarity of this pair (I, cats)? Also does anyone know of anyway to do levels for NLP? like: Army-Military-Solider ?

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• C# Changing class method at run-time

  - by Flavio

  Hi, I need to extend the behavior of an instance, but I don't have access to the original source code of that instance. For example: /* I don't have the source code for this class, only the runtime instance */ Class AB { public void execute(); } in my code I would to intercept every call to execute, compute some sutff and then call the original execute, something like /* This is how I would like to modify the method invokation */ SomeType m_OrgExecute; { AB a = new AB(); m_OrgExecute = GetByReflection( a.execute ); a.execute = MyExecute; } void MyExecute() { System.Console.Writeln( "In MyExecute" ); m_OrgExecute(); } Is that possible? Does anyone have a solution for this problem?

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• Automatic e-mail processing

  - by Jon Harrop

  I'd like to write a .NET application in F# to automate some of the processing of my e-mails. For example, when an order comes in my program might compute a new htpasswd from the e-mail's contents, upload it to our web server and reply to the customer with login details. How do people do this? I've tried Outlook 2007 automation but it just prompts the user for security and my attempts to get it to stop doing this have failed so I cannot automate anything with it. Is there a .NET-friendly e-mail client I can use more easily? This has been so tedious that I'm seriously considering writing my own .NET-friendly e-mail client...

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• Code computing the cross-product

  - by WizardOfOdds

  This is not a dupe of my question: http://stackoverflow.com/questions/2532810/detecting-one-points-location-compared-to-two-other-points If I have the following piece of pseudo-C/Java/C# code: int a[]= { 30, 20 }; int b[] = { 40, 50 }; int c[] = {12, 12}; How do I compute the sign of the cross-product ABxAC? I'm only interested in the sign, so I have: boolean signABxAC = ? Now concretely what do I write to get the sign of the cross-product ABxAC?

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• How do I get the position of a result in the list after an order_by?

  - by Bob Bob

  I'm trying to find an efficient way to find the rank of an object in the database related to it's score. My naive solution looks like this: rank = 0 for q in Model.objects.all().order_by('score'): if q.name == 'searching_for_this' return rank rank += 1 It should be possible to get the database to do the filtering, using order_by: Model.objects.all().order_by('score').filter(name='searching_for_this') But there doesn't seem to be a way to retrieve the index for the order_by step after the filter. Is there a better way to do this? (Using python/django and/or raw SQL.) My next thought is to pre-compute ranks on insert but that seems messy.

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• Projection matrix + world plane ~> Homography from image plane to world plane

  - by B3ret

  I think I have my wires crossed on this, it should be quite easy. I have a projection matrix from world coordinates to image coordinates (4D homogeneous to 3D homgeneous), and therefore I also have the inverse projection matrix from image coordinates to world "rays". I want to project points of the image back onto a plane within the world (which is given of course as 4D homogeneous vector). The needed homography should be uniquely identified, yet I can not figure out how to compute it. Of course I could also intersect the back-projected rays with the world plane, but this seems not a good way, knowing that there MUST be a homography doing this for me. Thanks in advance, Ben

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• How to refer to a previously computed value in SQL Query statement

  - by Mort

  I am trying to add a CASE statement to the end of my SQL query to compute a value depending upon another table value and a previously computed value in the SELECT. The error is returned that DelivCount is an invalid column name. Is there a better way to do this or am I missign something? SELECT jd.FullJobNumber, jd.ProjectTitle, jd.ClientName, jd.JobManager, jd.ProjectDirector, jd.ServiceGroup, jd.Status, jd.HasDeliverables, jd.SchedOutsideJFlo, jd.ReqCompleteDate,(SELECT COUNT(*)FROM DeliverablesSchedule ds WHERE jd.FullJobNumber = ds.FullJobNumber) as DelivCount, SchedType = CASE WHEN (jd.SchedOutsideJFlo = 'Yes') THEN 'outside' WHEN (jd.HasDeliverables = 'No ') THEN 'none' WHEN (DelivCount > 0) THEN 'has' WHEN (jd.HasDeliverables = 'Yes' AND DelivCount = 0) THEN 'missing' ELSE 'unknown' END FROM JobDetail jd

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• filter that uses elements from two arrays at the same time

  - by Gacek

  Let's assume we have two arrays of the same size - A and B. Now, we need a filter that, for a given mask size, selects elements from A, but removes the central element of the mask, and inserts there corresponding element from B. So the 3x3 "pseudo mask" will look similar to this: A A A A B A A A A Doing something like this for averaging filter is quite simple. We can compute the mean value for elements from A without the central element, and then combine it with a proper proportion with elements from B: h = ones(3,3); h(2,2) =0; h = h/sum(h(:)); A_ave = filter2(h, A); C = (8/9) * A_ave + (1/9) * B; But how to do something similar for median filter (medfilt2 or even better for ordfilt2)

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• How to specify execution time of x86 and PowerPC instructions?

  - by Goofy

  Hello! I have to approximate execution time of PowerPC and x86 assembler code.I understand that I cannot compute exact it dependson many problems (current processor state - x86 processor dicides internal instructions in microinstructions, memory access time obtainig code from cache of from slower memory etc.). I found some information in Intel Optimizaction reference (APPENDIX C), but it does not provide information about all general purpose instructions. Is there any complete reference about it? What about PowerPC processors? Where can I fund such information?

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• Can you figure out the password hashing scheme?

  - by Adal

  I have two passwords and two resulting hashes. I can't figure out how the hash is derived from the password. I don't know if salting is used. I don't know if the password is hashed as a integer value or as a string (possibly Unicode). Password: 6770 Hash: c12114b91a3841c143bbeb121693e80b Password: 9591 Hash: 25238d578b6a61c2c54bfe55742984c1 The hash length seems to suggest MD5. Anybody has any ideas what I could try? Note: This is not for hacking purposes. I'm trying to access a service through an API instead of it's desktop client, and I can't figure out how to compute the password hash. Currently instead of using my real password I'm sending directly the hash.

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• Small Python optional arguments question

  - by ooboo

  I have two functions: def f(a,b,c=g(b)): blabla def g(n): blabla c is an optional argument in function f. If the user does not specify its value, the program should compute g(b) and that would be the value of c. But the code does not compile - it says name 'b' is not defined. How to fix that? Someone suggested: def g(b): blabla def f(a,b,c=None): if c is None: c = g(b) blabla But this doesn't work, because maybe the user intended c to be None and then c will have another value.

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• How to make double[,] x_List in C#3.0?

  - by Newbie

  I ned to implement the multi-linear regression in C#(3.0) by using the LinESt function of Excel. Basically I am trying to achieve =LINEST(ACL_returns!I2:I10,ACL_returns!J2:K10,FALSE,TRUE) So I have the data as below double[] x1 = new double[] { 0.0330, -0.6463, 0.1226, -0.3304, 0.4764, -0.4159, 0.4209, -0.4070, -0.2090 }; double[] x2 = new double[] { -0.2718, -0.2240, -0.1275, -0.0810, 0.0349, -0.5067, 0.0094, -0.4404, -0.1212 }; double[] y = new double[] { 0.4807, -3.7070, -4.5582, -11.2126, -0.7733, 3.7269, 2.7672, 8.3333, 4.7023 }; I have to write a function whose signature will be Compute(double[,] x_List, double[] y_List) { LinEst(x_List,y_List, true, true); < - This is the excel function that I will call. } My question is how by using double[] x1 and double[] x2 I will make double[,] x_List ? I am using C#3.0 and framework 3.5. Thanks in advance

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• Can SHA-1 algorithm be computed on a stream? With low memory footprint?

  - by raoulsson

  I am looking for a way to compute SHA-1 checksums of very large files without having to fully load them into memory at once. I don't know the details of the SHA-1 implementation and therefore would like to know if it is even possible to do that. If you know the SAX XML parser, then what I look for would be something similar: Computing the SHA-1 checksum by only always loading a small part into memory at a time. All the examples I found, at least in Java, always depend on fully loading the file/byte array/string into memory. If you even know implementations (any language), then please let me know!

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• Stata output files in surveys

  - by William Shakespeare

  I have some survey data which I'm using Stata to analyze. I want to compute means of one variable by group and save those means to a Stata file. My code looks like this: svyset [iw=wtsupp], sdrweight(repwtp1-repwtp160) vce(sdr) svy: mean x I tried svy: by grp: mean x but that did not work. I could save each mean to a separate file by simply saying svy: mean x if grp==1 but that's inefficient. Is there a better way? Saving results to a file like one can use SAS ODS to capture results is also a need. I am not talking about the log here. I need the means and the associated group. I'm thinking estimates save [path],replace but I'm not sure if that will give me a Stata file or the group if I can figure out how to use by processing.

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• Java's TreeSet equivalent in Python?

  - by viksit

  I recently came across some Java code that simply put some strings into a Java TreeSet, implemented a distance based comparator for it, and then made its merry way into the sunset to compute a given score to solve the given problem. My questions, Is there an equivalent data structure available for Python? The Java treeset looks basically to be an ordered dictionary that can use a comparator of some sort to achieve this ordering. I see there's a PEP for Py3K for an OrderedDict, but I'm using 2.6.x. There are a bunch of ordered dict implementations out there - anyone in particular that can be recommended? Thanks.

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• Initial conditions with a non-linear ODE in Mathematica

  - by buggy

  Hi, I'm trying to use Mathematica's NDSolve[] to compute a geodesic along a sphere using the coupled ODE: x" - (x" . x) x = 0 The problem is that I can only enter initial conditions for x(0) and x'(0) and the solver is happy with the solution where x" = 0. The problem is that my geodesic on the sphere has the initial condition that x"(0) = -x(0), which I have no idea how to tell mathematica. If I add this as a condition, it says I'm adding True to the list of conditions. Here is my code: s1 = NDSolve[{x1''[t] - (x1[t] * x1''[t] + x2[t] * x2''[t] + x3[t]*x3''[t]) * x1[t] == 0, x2''[t] - (x1[t] * x1''[t] + x2[t] * x2''[t] + x3[t]*x3''[t]) * x2[t] == 0, x3''[t] - (x1[t] * x1''[t] + x2[t] * x2''[t] + x3[t]*x3''[t]) * x3[t] == 0, x1[0] == 1, x2[0] == 0, x3[0] == 0, x1'[0] == 0, x2'[0] == 0, x3'[0] == 1} , { x1, x2, x3}, {t, -1, 1}][[1]] I would like to modify this so that the initial acceleration is not zero but -x(0). Thanks

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