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• naive bayesian spam filter question

  - by Microkernel

  Hi guys, I am planning to implement spam filter using Naive Bayesian classification model. Online I see a lot of info on Naive Bayesian classification, but the problem is its a lot of mathematical stuff, than clearly stating how its done. And the problem is I am more of a programmer than a mathematician (yes I had learnt Probability and Bayesian theorem back in school, but out of touch for a long long time, and I don't have luxury of learning it now (Have nearly 3 weeks to come-up with a working prototype)). So if someone can explain or point me to location where its explained for programmers than a mathematician, it would be a great help. PS: By the way I have to implement it in C, if you want to know. :( Regards, Microkernel

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• Background processing in rails

  - by hashpipe

  Hi, This might seem like a FAQ on stackoverflow, but my requirements are a little different. While I have previously used BackgroundRB and DJ for running background processes in ruby, my requirement this time is to run some heavy analytics and mathematical computations on a huge set of data, and I need to do this only about the first 15 days of the month. Going by this, I am tempted to use cron and run a ruby script to accomplish this goal. What I would like to know / understand is: 1 - Is using cron a good idea (cause I'm not a system admin, and so while I have basic idea of cron, I'm not overly confident of doing it perfectly) 2 - Can we somehow modify DJ to run only on the first 15 days of the month (with / without using cron), and then just stop and exit once all the jobs in the queue for the day are over (don't want it to ping the DB every time for a new job...whatever the jobs will be in the queue when DJ starts, that will be all). I'm not sure if I have put the question in the right manner, but any help in this direction will be much appreciated. Thanks

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• Microsoft SQL 2005 - using the modulo operator

  - by cc0

  So I have a silly problem, I have not used much MSSQL before, or any SQL for that matter. I basically have a minor mathematical problem that I need solved, and I thought modulo would be good. I have a number of dates in the database, but I need them be rounded off to the closest [dynamic integer] (could be anything from 0 to 5000000) which will be input as a parameter each time this query is called. So I thought I'd use modulo to find the remainder, then subtract that remainder from the date. If there is a better way, or an integrated function, please let me know! What would be the syntax for that? I've tried a lot of things, but I keep getting error messages like integers/floats/decimals can't be used with the modulo operators. I tried casting to all kinds of numeric datatypes. Any help would be appreciated.

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• calculating the potential effect of inaccurate triangle vertex positions on the triangle edge lenght

  - by stingrey

  i'm not sure how to solve the following problem: i have a triangle with each of the three known vertex positions A,B,C being inaccurate, meaning they can each deviate up to certain known radii rA, rB, rC into arbitrary directions. given such a triangle, i want to calculate how much the difference of two specific edge lengths (for instance the difference between lengths of edge a and edge b) of the triangle may change in the worst case. is there any elegant mathematical solution to this problem? the naive way i thought of is calculating all 360^3 angle combinations and measuring the edge differences for each case, which is a rather high overhead.

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• Good Economics book for developers

  - by Rocket Surgeon

  Joel mentions in several of his blog posts that it is very important for a developer/software entrepreneur to have solid understanding of Economics. Yet the Fog Creek MBA book reading list does not include any Economics books. Is there any good material that people can recommend? Obviously, I am not as concerned about mathematical treatise as foundations and basic principles. For example, I was able to find a very good high-level read on Macroeconomics: Concise Guide to Macroeconomics but I am yet to find anything similar on Microeconomics. Any suggestions and reading pointers would be highly appreciated.

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• Quadratic bezier curve: Y coordinate for a given X ?

  - by stefan.at.wpf

  Hello, I have a quadratic bezier curve and I need the Y coordinate of a point on the bezier curve for a given X coordinate. I know that in pure maths this can be easily done, but I'm wondering is there's a simple / another way for this in C# / WPF? Is it possible to get the single points used by C# / WPF for drawing the bezier curve and then maybe just loop them and compare the X coordinate of each point with the given X coordinate? BTW for the mathematical way it would be good to know which step for the parameter t of the bezier curve has been choosen by C# / WPF? Any chance to find this out? Probably t is just scaled by / steps for t are 1/(distance of P0 and P2) ? Thank you very much for any hint!

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• Microsoft SQL Server 2005 - using the modulo operator

  - by cc0

  So I have a silly problem, I have not used much SQL Server before, or any SQL for that matter. I basically have a minor mathematical problem that I need solved, and I thought modulo would be good. I have a number of dates in the database, but I need them be rounded off to the closest [dynamic integer] (could be anything from 0 to 5000000) which will be input as a parameter each time this query is called. So I thought I'd use modulo to find the remainder, then subtract that remainder from the date. If there is a better way, or an integrated function, please let me know! What would be the syntax for that? I've tried a lot of things, but I keep getting error messages like integers/floats/decimals can't be used with the modulo operators. I tried casting to all kinds of numeric datatypes. Any help would be appreciated.

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• Finding zoom level from window object properties

  - by Mihai Fonoage

  Hi, I want to find out the zoom level of what is being displayed in a browser window based on the javascripts' window object properties (http://www.javascriptkit.com/jsref/window.shtml) to which I have access. I just can't seem to find the right mathematical formula for the zoom based on the inner width, page offset, etc. I found a solution, but that uses the document.body.getBoundingClientRect call which does not return anything in my case and for which I can't tell if there's a suitable replacement from the window properties. I am using Safari. Thank you, Mihai

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• Computing unique index for every poker starting hand

  - by Aly

  As there are 52 cards in a deck we know there are 52 choose 2 = 1326 distinct matchups, however in preflop poker this can be bucketed into 169 different hands such as AK offsuit and AK suited as whether it is A hearts K hearts or A spade K spades it makes no difference preflop. My question is, is there a nice mathematical property in which I can uniquely index each of these 169 hands (from 0 to 168 preferably). I am trying to create a look up table as a double[][] = new double [169][169] but have no way of changing a hand representation such as AKs (an Ace and a King of the same suit) to a unique index in this array.

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• Union of complex polygons

  - by grenade

  Given two polygons: POLYGON((1 0, 1 8, 6 4, 1 0)) POLYGON((4 1, 3 5, 4 9, 9 5, 4 1),(4 5, 5 7, 6 7, 4 4, 4 5)) How can I calculate the union (combined polygon)? Dave's example uses SQL server to produce the union, but I need to accomplish the same in code. I'm looking for a mathematical formula or code example in any language that exposes the actual math. I am attempting to produce maps that combine countries dynamically into regions. I asked a related question here: http://stackoverflow.com/questions/2653812/grouping-geographical-shapes

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• Things to do with supercomputer.

  - by BigBoss

  HI, Guys I wanted to create SuperComputer out of my 3 old pcs soon. There are bunch of techniquies available on net for doing this and I will be using one of them. What I would like to get suggestion is what is next !! Many of them suggest some mathematical calculation or some graphic image rendering, which still is to me so boring and not inspiring at all. Please try to suggest something out of box. I would post results of it when I am done. Cheers !

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• what is order notation f(n)=O(g(n))?

  - by Lopa

  2 questions: question 1: under what circumstances would this[O(f(n))=O(k.f(n))] be the most appropriate form of time-complexity analysis? question 2: working from mathematical definition of O notation, show that O(f(n))=O(k.f(n)), for positive constant k.? My view: For the first one I think it is average case and worst case form of time-complexity. am i right? and what else do i write in that? for the second one I think we need to define the function mathematically, so is the answer something like because the multiplication by a constant just corresponds to a readjustment of value of the arbitrary constant 'k' in definition of O.

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• How do I programmatically determine the current zoom level of a browser window?

  - by Mihai Fonoage

  Hi, I want to find out the zoom level of what is being displayed in a browser window based on the javascripts' window object properties (http://www.javascriptkit.com/jsref/window.shtml) to which I have access. I just can't seem to find the right mathematical formula for the zoom based on the inner width, page offset, etc. I found a solution, but that uses the document.body.getBoundingClientRect call which does not return anything in my case and for which I can't tell if there's a suitable replacement from the window properties. I am using Safari. Thank you, Mihai

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• Check if BigDecimal is integer value

  - by Adamski

  Can anyone recommend an efficient way of determining whether a BigDecimal is an integer value in the mathematical sense? At present I have the following code: private boolean isIntegerValue(BigDecimal bd) { boolean ret; try { bd.toBigIntegerExact(); ret = true; } catch (ArithmeticException ex) { ret = false; } return ret; } ... but would like to avoid the object creation overhead if necessary. Previously I was using bd.longValueExact() which would avoid creating an object if the BigDecimal was using its compact representation internally, but obviously would fail if the value was too big to fit into a long. Any help appreciated.

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• How to create a container that holds different types of function pointers in C++?

  - by Alex

  I'm doing a linear genetic programming project, where programs are bred and evolved by means of natural evolution mechanisms. Their "DNA" is basically a container (I've used arrays and vectors successfully) which contain function pointers to a set of functions available. Now, for simple problems, such as mathematical problems, I could use one type-defined function pointer which could point to functions that all return a double and all take as parameters two doubles. Unfortunately this is not very practical. I need to be able to have a container which can have different sorts of function pointers, say a function pointer to a function which takes no arguments, or a function which takes one argument, or a function which returns something, etc (you get the idea)... Is there any way to do this using any kind of container ? Could I do that using a container which contains polymorphic classes, which in their turn have various kinds of function pointers? I hope someone can direct me towards a solution because redesigning everything I've done so far is going to be painful.

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• Ruby RegEx not matching valid expression

  - by Matthew Carriere

  I have the following expression: ^\w(\s(+|-|\/|*)\s\w)*$ This simply looks to match a mathematical expression, where a user is prompted for terms separated by basic operators (ex: price + tax) The user may enter more than just 2 terms and one operator (ex: price + tax + moretax) I tested this expression in Rubular http://rubular.com/ With the terms: a + a (MATCH) a + a + a (MATCH) a + a + a + a a a + a a Everything works, but when I use it in Ruby it does not work! expression =~ /^\w(\s(+|-|\/|*)\s\w)*$/ I started picking the expression apart and noticed that if I remove the start of line caret it finds matches but isn't correct. a + a (MATCH) a a (MATCH) <-- this is not correct Why is this expression not working in Ruby code? (I am using Ruby 1.8.7 p174)

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• Inaccurate Logarithm in Python

  - by Avihu Turzion

  I work daily with Python 2.4 at my company. I used the versatile logarithm function 'log' from the standard math library, and when I entered log(2**31, 2) it returned 31.000000000000004, which struck me as a bit odd. I did the same thing with other powers of 2, and it worked perfectly. I ran 'log10(2**31) / log10(2)' and I got a round 31.0 I tried running the same original function in Python 3.0.1, assuming that it was fixed in a more advanced version. Why does this happen? Is it possible that there are some inaccuracies in mathematical functions in Python?

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• Code golf: Reverse Polish notation (postfix) evaluator

  - by Dario

  After having had code-golf contests on ordinary mathematical expressions, I just thought it would also be quite interesting how short an evaluation function for postfix notation (RPN) can be. Examples for RPN: 1 2 + == 3 1 2 + 3 * == 9 0 1 2 3 + + - 2 6 * + 3 / 1 - == 1 3 2 / 2.0 + == 3.5 To make things shorter, only +, -, * and /, all just in their binary version, must be supported. Operators/Operands are delimited by one space, divsions by zero don't have to be handled. The resulting code should be a function that takes the postfix string as input and returns the resulting number.

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• posmax: like argmax but gives the position(s) of the element x for which f[x] is maximal

  - by dreeves

  Mathematica has a built-in function ArgMax for functions over infinite domains, based on the standard mathematical definition. The analog for finite domains is a handy utility function. Given a function and a list (call it the domain of the function), return the element(s) of the list that maximize the function. Here's an example of finite argmax in action: http://stackoverflow.com/questions/471029/canonicalize-nfl-team-names/472213#472213 And here's my implementation of it (along with argmin for good measure): (* argmax[f, domain] returns the element of domain for which f of that element is maximal -- breaks ties in favor of first occurrence. *) SetAttributes[{argmax, argmin}, HoldFirst]; argmax[f_, dom_List] := Fold[If[f[#1]>=f[#2], #1, #2]&, First[dom], Rest[dom]] argmin[f_, dom_List] := argmax[-f[#]&, dom] First, is that the most efficient way to implement argmax? What if you want the list of all maximal elements instead of just the first one? Second, how about the related function posmax that, instead of returning the maximal element(s), returns the position(s) of the maximal elements?

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• Choice of programming language for learning data structures and algorithms

  - by bguiz

  Which programming language would you recommend to learn about data structures and algorithms in? Considering the follwing: Personal experience Language features (pointers, OO, etc) Suitability for learning DS & A concepts I ask because there are some books out there that are programming language-agnostic (written from a Mathematical perspective, and use pseudocode). If I learn from one of these I would like to work out the algorithms in a chosen language. Then, there are other books which introduce DS & A concepts with examples in a particular programming laguage - and I would follow these examples as well. Either way, I have to choose a language, and I would like to stick to one throughout. Which one best fits the bill.

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• Statement hierarchy in programming languages

  - by sudo

  I quickly wrote an interpreter for some sort of experimental programing language i came up with, in PHP (yes, in PHP). The language itself doesn't have anything really special, I just wanted to give it a try. I got the basic things working (Hello World, input to output, string manipulation, arithmetics) but I'm getting stuck with the management of blocks and grouped statements. What I mean is: PHP and most other languages let you do this: ((2+2)*(8+2)+2), of course not only with mathematical computations. My program structure currently consists of a multidimensional array built like this: ID => Type (Identifier, String, Int, Newline, EOF, Comma, ...) Contents (If identifier, int or string) How could I allow statements to be executed in a defined order like in the PHP example above?

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• maths: motion depending on time

  - by clamp

  hello, i have a mathematical problem: i have a function where the only parameter is the current time. the return should be a position which is used to place an object on a certain place. int position(int time) { int x = 0; //TODO implement x depending on time return x; } so basically the function is called every frame to put the object in motion. the motion should look like this (this is the actual question): a linear motion for time A, the object moves at constant speed no motion for time B, the object is stopped repeat at 1. thanks!

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• Math/numerical formula every computer programmer should know

  - by aaa

  This is a follow-up question to What should every programmer know and Is mathematics necessary. So the question is, as a computer programmer, what is the most important/useful mathematical or numerical formula that you use? By Formula I mean anything that involves less obvious manipulations, whenever binomial coefficients or bit hacks. I work with multidimensional arrays and various matrix representations. So for me most commonly used formulas are: A(i,j,k,..) = a[i + j*Dim0 + k*Dim0*Dim1 + ... to map indexes to one dimension ( which is basic address calculation which many people do not seem to know). And triangular number T(i) = (i*i + i)/2 which is related to binomial coefficients, used to calculate address in triangular matrixes and many other things. What is your workhorse formula that you think programmer should know?

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• TypeError: 'NoneType' object does not support item assignment

  - by R S John

  I am trying to do some mathematical calculation according to the values at particular index of a NumPy array with the following code X = np.arange(9).reshape(3,3) temp = X.copy().fill(5.446361E-01) ind = np.where(X < 4.0) temp[ind] = 0.5*X[ind]**2 - 1.0 ind = np.where(X >= 4.0 and X < 9.0) temp[ind] = (5.699327E-1*(X[ind]-1)**4)/(X[ind]**4) print temp But I am getting the following error Traceback (most recent call last): File "test.py", line 7, in <module> temp[ind] = 0.5*X[ind]**2 - 1.0 TypeError: 'NoneType' object does not support item assignment Would you please help me in solving this? Thanks

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• Flipping issue when interpolating Rotations using Quaternions

  - by uhuu

  I use slerp to interpolate between two quaternions representing rotations. The resulting rotation is then extracted as Euler angles to be fed into a graphics lib. This kind of works, but I have the following problem; when rotating around two (one works just fine) axes in the direction of the green arrow as shown in the left frame here the rotation soon jumps around to rotate from the opposite site to the opposite visual direction, as indicated by the red arrow in the right frame. This may be logical from a mathematical perspective (although not to me), but it is undesired. How could I achieve an interpolation with no visual flipping and changing of directions when rotating around more than one axis, following the green arrow at all times until the interpolation is complete? Thanks in advance.

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