Using arrays for indexing
It is possible to speed things up, using arrays for indexing, instead of looping through the elements an a matrix or vector. Test the following code segment and explain your results:
n = 100000;
a = randn(n, 1);
b = a;
c = a;
loops = 200;
a_save = a;
tic
for j = 1:loops
for k = 1:length(a)
if a_save(k) < 0
a(k) = 0;
end
end
end
toc
% Using logical indices instead (b < 0 is an array of 1 and 0, true
% and false).
tic
for j = 1:loops
b(a_save < 0) = 0;
end
toc
% In some applications it is more useful to get the actual indices.
tic;
for j = 1:loops
indx = find(a_save < 0);
c(indx) = 0;
end
toc
You may want to examine how this works when using matrices.