How can I make my code faster?

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Delshad Ayoubi
Delshad Ayoubi on 4 May 2018
Edited: per isakson on 5 May 2018
clear, clc, close all
N = 10^3;
M = 400;
radie = 50;
x_t = zeros(M, N+1);
y_t = zeros(M, N+1);
Nbins = 10;
D = 2*radie;
d = D/Nbins;
xc = -D/z + d/z:d:D/z - d/z;
yc = xc;
ctrs = {xc, yc};
for n = 1:N
for m = 1:M
distance = radie + 1/N;
while distance > radie
x_t(m, n+1) = x_t(m, n) + (-1).^randi(2);
y_t(m, n+1) = y_t(m, n) + (-1).^randi(2);
distance = hypot(x_t(m, n+1), y_t(m, n+1));
plot(x_t(:, 1:n+1)', y_t(:, 1:n+1).','.', 'MarkerSize', 5);
caption = sprintf('Steg %d av %d', n, N);
title(caption, 'FontSize', 25, 'FontWeight', 'bold');
hold on
axis equal
hold off
X = [x_t(:,n) y_t(:,n)];
N_i = hist3(X, ctrs);
N_p = M;
N_i = N_i(N_i > 0);
p_i = N_i/N_p;
S = 0;
for i = 1:length(p_i)
S = S - p_i(i)*log(p_i(i))
B(:,i) = S;
hist3(X, ctrs);
grid on
I think I have to somehow remove the for loops but I don't know how. This is a simulation for entropy.
Guillaume on 4 May 2018
In addition to Stephen's links, some comments explaining what the code do wouldn't go amiss. For example, it looks like you're moving some coordinates randomly in a loop until they're close enough to something. Why? Wouldn't it be faster to simply generate the coordinates within your allowed circle?

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Answers (1)

per isakson
per isakson on 4 May 2018
Firstly, run profile on your code to find the bottlenecks. See profile, Profile execution time for functions. I converted your script to a function and run profile. Plot and drawnow dominated totally. (I have an old graphic card and run Software OpenGL.)
The old trick to make fast graphic
  1. First, create all graphic objects needed ('visible','off')
  2. Then use set to set the attributes of the objects
  3. Avoid calls to drawnow and make the diagram visible at the end.
per isakson
per isakson on 5 May 2018
Edited: per isakson on 5 May 2018
"What else would the problem be?" The problem is that the code creates and destroys zillions of handle graphic objects without any real purpose. Excerpt from plot.m
% If [...] HOLD is off, PLOT resets all axes properties, except Position, to their
% default values, deletes all axes children (line, patch, text, surface, and image
% objects) [...]
Thus, instead of keeping the objects and modify property values the code destroys the objects and creates new ones.
Examples of time thieves:
  • the red circle is drawn and erased one thousand times.
  • the value of radie, which is constant, controls the axes of the diagram. Despite that the axes, which the user sees, are constant, the limits of the axes are changed on thousand times.
  • et cetera.
The poor performance of your code has very little to do with the for-loops.

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