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• Why do so many mathematicians format code so poorly? [closed]

  - by marcog

  I have done a fair amount of programming together with mathematicians. Now I am even teaching some high school kids coming from a mathematics background how to program. Most of these people format their code so hideously it's hard to believe. I've even worked with and taught mathematicians who will fight the auto-indenter! Why is this so common amongst mathematicians? BTW, this is one reason I have started teaching Python. Yet still they find ways other than indentation to produce whacked coding styles!

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• Django AND .htaccess rewrites/redirects, is this possible?

  - by Infinity

  Is it possible to have Apache htaccess rewrites take effect before it hits django? For example so I can redirect iPhone users to a completely different domain without even hitting django. We're using apache2 with mod_wsgi and the apache vhost looks like this: <VirtualHost *:80> DocumentRoot /usr/local/www/site/static Alias /css/ /usr/local/www/site/static/css/ Alias /js/ /usr/local/www/site/static/js/ Alias /img/ /usr/local/www/site/static/img/ Alias /flash/ /usr/local/www/site/static/flash/ <Directory /usr/local/www/site/static> AllowOverride All Order allow,deny Allow from all </Directory> WSGIDaemonProcess mallorca-site threads=15 WSGIScriptAlias / /usr/local/www/site/config/dev/dev.wsgi </VirtualHost> Thanks

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• How do I know if my PHP application is using too much memory?

  - by John

  I'm working on a PHP web application that let's users network with each other, book events, message etc... I launched it a few months ago and at the moment, there's only about 100 users. I set up the application on a VPS with ubuntu 9.10, apache 2, mysql 5 and php 5. I had 360 Mb of RAM, but upgraded to 720 MB a few minutes ago. Lately, my web application has been experiencing outages due to excessive memory usage. From what I can tell in error logs, it seems the server automatically kills apache processes that consume too much memory. As a result, I upgraded memory from 360 MB to 720 MB as a stop-gap measure. So my question is, how do I go about resolving these outage issues? How do I know if my website's need for more memory is due to poor code or if it's part of the website's natural growth? What's the most efficient way to determine which PHP scripts consume the most memory?

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• Web UI for inputting a function from the reals to the reals, such as a probability distribution.

  - by dreeves

  I would like a web interface for a user to describe a one-dimensional real-valued function. I'm imagining the user being presented with a blank pair of axes and they can click anywhere to create points that are thick and draggable. Double-clicking a point, let's say, makes it disappear. The actual function should be shown in real time as an interpolation of the user-supplied points. Here's what this looks like implemented in Mathematica (though of course I'm looking for something in javascript):

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• Convert Decimal number into Fraction

  - by alankrita

  I am trying to convert decimal number into its fraction. Decimal numbers will be having a maximum 4 digits after the decimal place. example:- 12.34 = 1234/100 12.3456 = 123456/10000 my code :- #include <stdio.h> int main(void) { double a=12.34; int c=10000; double b=(a-floor(a))*c; int d=(int)floor(a)*c+(int)b; while(1) { if(d%10==0) { d=d/10; c=c/10; } else break; } printf("%d/%d",d,c); return 0; } but I am not getting correct output, Decimal numbers will be of double precision only.Please guide me what I should do.

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• What are the core mathematical concepts a good developer should know?

  - by Jose B.

  Since Graduating from a very small school in 2006 with a badly shaped & outdated program (I'm a foreigner & didn't know any better school at the time) I've come to realize that I missed a lot of basic concepts from a mathematical & software perspective that are mostly the foundations of other higher concepts. I.e. I tried to listen/watch the open courseware from MIT on Introduction to Algorithms but quickly realized I was missing several mathematical concepts to better understand the course. So what are the core mathematical concepts a good software engineer should know? And what are the possible books/sites you will recommend me?

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• Modify a given number to find the required sum?

  - by Gaurav

  A friend of mine sent me this question. I haven't really been able to come up with any kind of algorithm to solve this problem. You are provided with a no. say 123456789 and two operators * and +. Now without changing the sequence of the provided no. and using these operators as many times as you wish, evaluate the given value: eg: given value 2097 Solution: 1+2+345*6+7+8+9 Any ideas on how to approach problems like these?

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• C++ greatest integer function

  - by Thomas

  I want to implement greatest integer function. int x = 5/3; My question is with greater numbers could there be a loss of precision as 5/3 would produce a double?

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• Dynamic creation of a pointer function in c++

  - by Liberalkid

  I was working on my advanced calculus homework today and we're doing some iteration methods along the lines of newton's method to find solutions to things like x^2=2. It got me thinking that I could write a function that would take two function pointers, one to the function itself and one to the derivative and automate the process. This wouldn't be too challenging, then I started thinking could I have the user input a function and parse that input (yes I can do that). But can I then dynamically create a pointer to a one-variable function in c++. For instance if x^2+x, can I make a function double function(double x){ return x*x+x;} during run-time. Is this remotely feasible, or is it along the lines of self-modifying code?

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• How do I determine when two moving points become visible to each other?

  - by Devin Jeanpierre

  Suppose I have two points, Point1 and Point2. At any given time, these points may be at different positions-- they are not necessarily static. Point1 is located at some position at time t, and its position is defined by the continuous functions x1(t) and y1(t) giving the x and y coordinates at time t. These functions are not differentiable, they are constructed piecewise from line segments. Point2 is the same, with x2(t) and y2(t), each function having the same properties. The obstacles that might prevent visibility are simple (and immobile) polygons. How can I find the boundary points for visibility? i.e. there are two kinds of boundaries: where the points become visible, and become invisible. For a become-visible boundary i, there exists some ?0, such that for any real number a, a ? (i-?, i) , Point1 and Point2 are not visible (i.e. the line segment that connects (x1(a), y1(a)) to (x2(a), y2(x)) crosses some obstacles). For b ? (i, i+?) they are visible. And it is the other way around for becomes-invisible. But can I find such a precise boundary, and if so, how?

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• m-estimate for continuous values

  - by Null

  I'm building a custom regression tree and want to use m-estimate for pruning. Does anyone know how to calculate that. http://www.ailab.si/blaz/predavanja/UISP/slides/uisp07-RegTrees.ppt might help (slide 12, how should Em look like?)

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• How do I find the millionth number in the series: 2 3 4 6 9 13 19 28 42 63 ... ?

  - by HH

  It takes about minute to achieve 3000 in my comp but I need to know the millionth number in the series. The definition is recursive so I cannot see any shortcuts except to calculate everything before the millionth number. How can you fast calculate millionth number in the series? Series Def n_{i+1} = \floor{ 3/2 * n_{i} } and n_{0}=2. Interestingly, only one site list the series according to Google: this one. Too slow Bash code #!/bin/bash function series { n=$( echo "3/2*$n" | bc -l | tr '\n' ' ' | sed -e 's@\\@@g' -e 's@ @@g' ); # bc gives \ at very large numbers, sed-tr for it n=$( echo $n/1 | bc ) #DUMMY FLOOR func } n=2 nth=1 while [ true ]; #$nth -lt 500 ]; do series $n # n gets new value in the function through global value echo $nth $n nth=$( echo $nth + 1 | bc ) #n++ done

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• How do I find the next multiple of 10 of any integer?

  - by Tommy

  Dynamic integer will be any number from 0 to 150. i.e. - number returns 41, need to return 50. If number is 10 need to return 10. Number is 1 need to return 10. Was thinking I could use the ceiling function if I modify the integer as a decimal...? then use ceiling function, and put back to decimal? Only thing is would also have to know if the number is 1, 2 or 3 digits (i.e. - 7 vs 94 vs 136) Is there a better way to achieve this? Thank You,

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• Distributing players to tables

  - by IVlad

  Consider N = 4k players, k tables and a number of clans such that each member can belong to one clan. A clan can contain at most k players. We want to organize 3 rounds of a game such that, for each table that seats exactly 4 players, no 2 players sitting there are part of the same clan, and, for the later rounds, no 2 players sitting there have sat at the same table before. All players play all rounds. How can we do this efficiently if N can be about ~80 large? I thought of this: for each table T: repeat until 4 players have been seated at T: pick a random player X that is not currently seated anywhere if X has not sat at the same table as anyone currently at T AND X is not from the same clan as anyone currently at T seat X at T break I am not sure if this will always finish or if it can get stuck even if there is a valid assignment. Even if this works, is there a better way to do it?

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• Separating merged array of arithmetic and geometric series

  - by user1814037

  My friend asked me an interseting question. Given an array of positive integers in increasing order. Seperate them in two series, an arithmetic sequence and geometric sequence. The given array is such that a solution do exist. The union of numbers of the two sequence must be the given array. Both series can have common elements i.e. series need not to be disjoint. The ratio of the geometric series can be fractional. Example: Given series : 2,4,6,8,10,12,25 AP: 2,4,6,8,10,12 GP: 4,10,25 I tried taking few examples but could not reach a general way. Even tried some graph implementation by introducing edges if they follow a particular sequence but could not reach solution.

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• Uniform distance between points

  - by Reonarudo

  Hello, How could I, having a path defined by several points that are not in a uniform distance from each other, redefine along the same path the same number of points but with a uniform distance. I'm trying to do this in Objective-C with NSArrays of CGPoints but so far I haven't had any luck with this. Thank you for any help. EDIT I was wondering if it would help to reduce the number of points, like when detecting if 3 points are collinear we could remove the middle one, but I'm not sure that would help.

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• Complexity of subset product

  - by threenplusone

  I have a set of numbers produced using the following formula with integers 0 < x < a. f(x) = f(x-1)^2 % a For example starting at 2 with a = 649. {2, 4, 16, 256, 636, 169, 5, 25, 649, 576, 137, ...} I am after a subset of these numbers that when multiplied together equals 1 mod N. I believe this problem by itself to be NP-complete (based on similaries to Subset-Sum problem). However starting with any integer (x) gives the same solution pattern. Eg. a = 649 {2, 4, 16, 256, 636, 169, 5, 25, 649, 576, 137, ...} = 16 * 5 * 576 = 1 % 649 {3, 9, 81, 71, 498, 86, 257, 500, 135, 53, 213, ...} = 81 * 257 * 53 = 1 % 649 {4, 16, 256, 636, 169, 5, 25, 649, 576, 137, 597, ...} = 256 * 25 * 137 = 1 % 649 I am wondering if this additional fact makes this problem solvable faster? Or if anyone has run into this problem previously or has any advice?

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• What is the smallest amount of bits you can write twin-prime calculation?

  - by HH

  A succinct example in Python, its source. Explanation about the syntactic sugar here. s=p=1;exec"if s%p*s%~-~p:print`p`+','+`p+2`\ns*=p*p;p+=2\n"*999 The smallest amount of bits is defined by the smallest amount of 4pcs of things you can see with hexdump, it is not that precise measure but well-enough until an ambiguity. $ echo 's=p=1;exec"if s%p*s%~-~p:print`p`+','+`p+2`\ns*=p*p;p+=2\n"*999' > .test $ hexdump .test | wc 5 36 200 $ hexdump .test 0000000 3d73 3d70 3b31 7865 6365 6922 2066 2573 0000010 2a70 2573 2d7e 707e 703a 6972 746e 7060 0000020 2b60 2b2c 7060 322b 5c60 736e 3d2a 2a70 0000030 3b70 2b70 323d 6e5c 2a22 3939 0a39 000003e so in this case it is 31 because the initial parts are removed.

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• Draw arrow on line

  - by Pete

  Hi, I have this code: CGPoint arrowMiddle = CGPointMake((arrowOne.x + arrowTo.x)/2, (arrowOne.y + arrowTo.y)/2); CGPoint arrowLeft = CGPointMake(arrowMiddle.x-40, arrowMiddle.y); CGPoint arrowRight = CGPointMake(arrowMiddle.x, arrowMiddle.y + 40); [arrowPath addLineToScreenPoint:arrowLeft]; [arrowPath addLineToScreenPoint:arrowMiddle]; [arrowPath addLineToScreenPoint:arrowRight]; [[mapContents overlay] addSublayer:arrowPath]; [arrowPath release]; with this output: http://yfrog.com/edschermafbeelding2010032p What have i to add to get the left and right the at same degree of the line + 30°. If someone has the algorithm of drawing an arrow on a line, pleas give it. It doesn't matter what programming language it is... Thanks

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• Getting Factors of a Number

  - by Dave

  Hi Problem: I'm trying to refactor this algorithm to make it faster. What would be the first refactoring here for speed? public int GetHowManyFactors(int numberToCheck) { // we know 1 is a factor and the numberToCheck int factorCount = 2; // start from 2 as we know 1 is a factor, and less than as numberToCheck is a factor for (int i = 2; i < numberToCheck; i++) { if (numberToCheck % i == 0) factorCount++; } return factorCount; }

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• Vector Calculations in LISP

  - by abidikgubidik

  How can I perform vector calculations in lisp, such as magnitude of a vector, norm of a vector, distance (between two points), dot product, cross product, etc. Thanks.

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• How do I calculate the average direction of two vectors

  - by Mike Broughton

  Hi, I am writing and opengl based iphone app and would like to allow a user to translate around a view based on the direction that they move two fingers on the screen. For one finger I know I could just calculate the vector from the start position to the current position of the users finger and then find the unit vector of this to get just the direction, but I don't know how I would do this for two fingers, I don't think adding the components of the vectors and calculating the average would work so I'm pretty much stuck... thanks in advance

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• How do I get points on a curve in PHP with log()?

  - by Erick

  I have a graph I am trying to replicate: I have the following PHP code: $sale_price = 25000; $future_val = 5000; $term = 60; $x = $sale_price / $future_val; $pts = array(); $pts[] = array($x,0); for ($i=1; $i<=$term; $i++) { $y = log($x+0.4)+2.5; $pts[] = array($i,$y); echo $y . " <br>\n"; } How do I make the code work to give me the points along the lower line (between the yellow and blue areas)? It doesn't need to be exact, just somewhat close. The formula is: -ln(x+.4)+2.5 I got that by using the Online Function Grapher at http://www.livephysics.com/ Thanks in advance!!

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• Find current value of a variable that depreciates linearly over time

  - by Hugi

  Let's say we have an empty bathtub. We've lost the plug, so once water is added it will drain away at a constant rate of 2 liters pr. minute. We add water to the tub in increments. 60 liters at 10:51, 30 liters at 11:54 and 50 liters at 13:18. So, the question is: How can I find out how much water is in the bathtub at any given time?

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• Python/Django: log to console under runserver, log to file under Apache

  - by Justin Grant

  How can I send trace messages to the console (like print) when I'm running my Django app under manage.py runserver, but have those messages sent to a log file when I'm running the app under Apache? I reviewed Django logging and although I was impressed with its flexibility and configurability for advanced uses, I'm still stumped with how to handle my simple use-case. My apologies for not being able to find the answer elsewhere-- this is a newbie question I know.

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