Questions about optimizing code for faster results, or how to do faster computations.

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33
votes
2answers
654 views

Why is the new PositionIndex horribly slow?

Disconcertingly I have discovered that the new (v10) PositionIndex is horribly slow. Using Szabolcs's clever GatherBy inversion ...
23
votes
4answers
1k views

What is the fastest way to count square-free words?

Background A word is a string of letters in an alphabet. A square-free word has no adjacent repeating substring. For example, (in the ternary alphabet of {0,1,2}) the words 00, 012121, and 0212012021 ...
21
votes
2answers
436 views

Why is Query so much slower than Part?

Time for another of these(1),(2) as yet another new-in-10 function appears to have poor performance compared to older alternatives. This time: Query appears to be ...
20
votes
2answers
954 views

How to quickly calculate intersections of filled curves?

I am trying to quickly calculate the intersection of polygons with more than 6,000 points. A compiled solution would be preferable. Here is one example of the problem: ...
19
votes
3answers
609 views

DeleteDuplicatesBy is not performing as I'd hoped. Am I missing something?

Déjà vu: a new-in-v10 function should provide a better solution to an old problem, but my enthusiasm is curbed when I run timings. This time the function is ...
17
votes
5answers
1k views

How to speed up my Project Euler code

The evaluation speed of Mathematica often depresses me. It did it again when I wrote a code to solve a Project Euler problem. https://projecteuler.net/problem=206 Here's my code: ...
15
votes
1answer
250 views

Speed up adding Poisson noise to an image

I'm simulating some images corrupted with Poisson noise, but I'm encountering a few problems with performance. According to the documentation on ImageEffect, one ...
15
votes
2answers
377 views

Is there a better way to add elements to a Graphics expression repeatedly?

Consider simple example where a set of points is generated and some are displayed. ...
14
votes
2answers
544 views

How can I speed up the classic GA for graph coloring?

I'm trying to compute the chromatic number of this graph (which is 28): g = Import@"http://www.info.univ-angers.fr/pub/porumbel/graphs/dsjc250.5.col"; My genetic ...
11
votes
3answers
2k views

Why is this code slow and how could I speed it up?

My goal is given an integer number to deduce if its digits increase (or remain the same) from left to right or not. Example: $1236, 123336$ are both considered numbers with increasing digits, but ...
11
votes
2answers
458 views

Fast Simulations with Compile

this post relates to another post that I didn't follow up propely. If I wanted to simulate a system of stochastic proesses like the following, and loop over this run many many times would writing the ...
10
votes
1answer
633 views

Is there a simple example using OpenCLLink that shows performance advantages?

OpenCLLink allows Mathematica to use the OpenCL parallel computing language. I have done some tests on simple examples, but so far, the CPU regularly outperforms ...
10
votes
0answers
114 views

Exp is slower for some number ranges

I get a hit on performance with the Exp function depending on the range of my inputs. for example, with: ...
9
votes
5answers
519 views

Fast way of finding position of Interval containing an Integer

I have a large list (about 2.2 million) of pairs of integers. Each pair defines an Interval[{}]. Given some integer x, I need to ...
9
votes
2answers
1k views

Convolution with interpolated function

I have some problems to calculate in reasonable speed the convolution of an interpolated function with a Gauss function. I have here (ExampleData.txt, alternate Pastebin link) data which I ...
9
votes
3answers
366 views

Why is AstronomicalData so slow?

I'm having trouble getting some pretty simple functions using AstronomicalData to perform at anywhere near the speed I need them to. For example, I have many ...
8
votes
4answers
260 views

Replacing matrix values

I want to find the n largest values in a matrix and replace all others with zero. The solution I found uses ReplaceAll and becomes very slow as the size of ...
8
votes
4answers
290 views

Fast method to select matrix elements based on a vector of positions

There is an operation for which I have long wanted to find a better solution. Let: a be a matrix of dimensions $m\times n$ ...
8
votes
1answer
165 views

Why is building a table of function values so much slower than just plotting the function?

Here is a function that take some time to evaluate: n = 1000; coeffs = RandomVariate[NormalDistribution[], n]; f[x_] := Sum[coeffs[[k]] Sin[k x]/k, {k, 1, n}]; ...
8
votes
2answers
140 views

Extracting values with multiple keys

I want to extract sorted data from a huge database based upon two (or more) keys in a very timely manner. Here is a reproducible toy example for two keys only: ...
8
votes
4answers
356 views

NIntegrate extremely piecewised functions

I often need to integrate extremely piecewised functions, like the following one (not extreme, but gives an idea): ...
8
votes
1answer
519 views

Integration strategies for oscillatory multidimensional function

I am seeking to integrate a highly oscillatory, multidimensional function. I am currently using NIntegrate's QuasiMonteCarlo approach. However, this is time-consuming and, given my current resources, ...
7
votes
3answers
342 views

Speed up ContourPlot3D

I have two issues with the following code. (1) It executes too slowly. If I use "Speed" instead of "Quality" for PerformanceGoal, I get the speed but an undesirable delay in proper rendering of the ...
7
votes
4answers
304 views

Slow plotting of trigonometric functions

I have two sets of solutions for a and b. I'm plotting solution only if it is positive for both ...
7
votes
5answers
357 views

Efficiently finding the positions of a large list of targets in another, even larger list

I have a big list. It contains nearly 60,000 sub-lists. It's structured like bigList = {{x1,y1,z1},{x2,y2,z2},{x3,y3,z3},.....,{x60000,y60000,z60000}}; I have ...
7
votes
4answers
398 views

Generating an Ulam spiral

An Ulam Spiral is quite an interesting construction, revealing unexpected features in the distribution of primes. Here is a related topic with one answer by Pinguin Dirk, who has provided one ...
7
votes
1answer
98 views

Why does Rescale performance vary significantly based on the real number values in a list?

I have a list of the form: testList = Table[{Flatten[RandomReal[{-200, 200}, {1, 2}], 1], RandomReal[{-200, 200}, {69, 2}]}, {i, 1, 100}]; And I would like to ...
7
votes
3answers
420 views

Suppress connection to Wolfram|Alpha when making unit conversions

I want to speed up calculations with unit conversions. I suspect primarily that some units evaluate from Wolfram|Alpha. Any way to prevent this automatically, so that Mathematica throws ...
7
votes
1answer
215 views

Porting code from MATLAB

I'd like to switch from MATLAB to Mathematica for my research in digital signal processing. I used Mathematica a lot during the '90s but I haven't used it since then, so I might as well be a complete ...
7
votes
2answers
173 views

Slow work of SQLInsert with SQLite

I want to store my data in SQLite database, but writing large numbers of data its too slow. For example: ...
7
votes
3answers
241 views

How can I efficiently check if a graph is unit distance?

A unit distance graph is a graph that is embeddable in the plane so that every edge has length 1. ...
6
votes
3answers
190 views

Speed of ConvexHullMesh

Pick $N$ random points on the surface of the sphere, then use ConvexHullMesh[] to compute their convex hull. Empirically, this takes time quadratic time in $N,$ ...
6
votes
1answer
99 views

How do I speed up a plotting of NIntegrate when repeated multiple times inside Plot?

I am studying a set of functions (many of which I know only as a definite integral) and I have assembled into a list. Here is a sample: ...
6
votes
2answers
292 views

Use of Cellular Automaton or Discrete Convolution to calculate Potts Energy of a 2D Matrix

In a project of Monte Carlo Modelling of Grain Growth (Link) one of the most costly subroutines is the calculation of the Potts-Energy of an Monte Carlo Grid cell by comparing the center cell value ...
6
votes
1answer
148 views

Why is AllTrue much slower than VectorQ?

This seems to be a theme with V10, a new function dedicated to a specific task doesn't live up to expectation performance-wise. Mr. Wizard has already uncovered 2 such functions here and here. So how ...
6
votes
1answer
115 views

Why LinearModelFit is slower than NonlinearModelFit?

Why LinearModelFit is slower than NonlinearModelFit? ...
6
votes
1answer
105 views

Garbage collection for memoized functions on subkernels

I have a top-level function which operates on some data: findBestIntegers[data_]:=ParallelMap[optimize,data] which makes parallelized calls to an optimization ...
5
votes
4answers
224 views

Speed up Schwarz-Christoffel mapping

I would like to ask your advice how to speed up Contour and Parametric plots in the following example. Let as start by defining a function ...
5
votes
3answers
225 views

Split list of states by run lengths and discontinuous changes in state

I was asked to look into writing some simulation code for a user, where the simulation output (x) would oscillate between three possible states: ...
5
votes
2answers
161 views

How does the speed of WriteString compare to other languages?

I'm doing something where the most sensible approach seems to be to open a file and write to it as I process data from some large tables. Old school. While testing the program, I remembered the low ...
5
votes
2answers
153 views

how to improve efficiency when I want to classify a list according to certain laws

I have a function that generates lists: f[n_] := Join @@ Permutations /@ PadLeft@IntegerPartitions[n]; For example f[3] ...
5
votes
2answers
171 views

How to optimize computing a product?

Problem description Write a function to caculate the formula My solutions Solution 1: ...
5
votes
1answer
435 views

symbolic integration of a definite integral, how to speed up

I am wondering what are the general tips & hints for speeding up symbolic integration. As far as I understand, it solely depends on the form a function is represented before it is processed by ...
5
votes
1answer
134 views

Is there a faster way to Map an Association?

Consider mapping an existing Association in a manner such as this: ...
5
votes
2answers
195 views

Parallelizing or optimizing a script for calculating the size of all pairwise intersections between collections of sets

I need to speed up or parallelize an operation that looks essentially like this: ...
5
votes
1answer
131 views

Strange phenomenon with Outer

...
5
votes
1answer
328 views

Random number generation with specific distribution

I am writing a program for solving the shortest path in travelling salesman problem, with a twist that there are multiple salesmen who partition the cities among themselves, thus creating two part ...
5
votes
0answers
200 views

Problems extending NelderMeadMinimize into 2D

Update The eventual route I followed was to adapt a version of the NM-simplex written in C and link to it with MathLink. Once I've tested this out fully I'll post it here. I've been playing around ...
5
votes
0answers
84 views

Is there something like MaxProcessorUsed?

For standard procedures it is easy to test if the implementation is better or worse than other by using Timing etc. It does not help us with creating ...
4
votes
5answers
321 views

How to generalize and speed up this program?

There is a vector whose length is $n*m$, we can part it into $n$ sections in order. I want to define a function to compare the $i$-th and $i+1$-th section and find the minimum. The program for $n=m=3$ ...