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Vector initialization (randn) with loop 'for'

Asked by Viacheslav Klimentyev on 13 Jan 2015
Latest activity Commented on by Viacheslav Klimentyev on 13 Jan 2015
I am initializing vector by randn function (two ways).
1.
clc
clear all
N = 1e6;
x = zeros(N, 1);
tic
for k = 1 : N
x(k) = randn;
end
toc
Elapsed time is 0.054535 seconds.
2.
clc
clear all
N = 1e6;
x = zeros(N, 1);
tic
for k = 1 : N
x(k) = randn(1,1);
end
toc
Elapsed time is 1.491623 seconds.
In both cases function randn returns one element (array 1x1), but elapsed time differs by ~30 times. Why?

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1 Answer

Answer by Iain
on 13 Jan 2015

The time difference is due to what gets done in the loop.
randn - This checks the number of inputs (0), generates a number and spits it out.
randn(x,y) - This checks the number of inputs (2), checks to see if it includes instructions to reset the seed, checks to see if they're actual valid numbers, etc., then goes about generating a number to spit it out.

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