MATLAB Answers

How can I examine the Runtime of a preallocated loop vs non preallocated?

1 view (last 30 days)
Catayoun Lissa Eleonore Azarm
Answered: Swetha Polemoni on 30 Mar 2021
Hey guys, i need your help.
I want to compare the runtime of pre-allocation and built-in matrix/vector operations in comparison to using loops and no pre-allocation.
% Examine the efficiency of pre-allocation and built-in matrix/vector operations
% in comparison to using loops and no pre-allocation. For this, measure the
% runtime of the calculation of mean, std, and z-scoring for a random
% matrix three times:
% 1) As a simple built-in matrix/vector multiplication
% 2) in a loop using preallocated result variables, iterating over n, but
% not increasing the size of the result variables each iteration
% 3) in a loop without preallocation
% Plot the nine values in a bar plot. Don't create new random matrices each
% iteration but use the one that is created here.
mu = 2;
SD = 3;
m = 1000;
n = 100000;
random_matrix = randn(m,n)*SD + mu;
means = mean(random_matrix);
SDs = std(random_matrix);
zscored = (random_matrix - means) ./ SDs;
The build in matrix multiplication 1) was easy, i am struggeling at 2) and 3), do you know how to do it? thanks

Answers (1)

Swetha Polemoni
Swetha Polemoni on 30 Mar 2021
Hi
It is my understanding that you want to calculate mean of a matrix without using inbuilt function.
m=10;
n=10;
x=randn(m,n)
mean=zeros(1,n)
tic
for i=1:n
interm=0;
for j=1:m
interm=x(j,i)+interm;
end
mean(1,i)=interm/m;
end
toc
tic and toc are used to calculate the execution time. Similarly you can try for standard deviation.

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!