How can make my Loop more efficient?

I have this Loop

m=0.05;
for
i=1:1000;
     r=  poissrnd(m);
    t(1,i)= r;
end
t

there is an orange tag to right of this line "" t(1,i)=[r]; "" says ( the variable appears to change size on every loop iteration. consider preallocating for speed)

What should I do to make my loop more efficient?

Answer 1

[Cam Salzberger]

If you preallocate the variable before the loop, the variable will not have to grow in size within the loop. Growing in size is an issue because it forces MATLAB to allocate a different memory block for the variable, copy over the data, and then delete the old memory every time. You can preallocate with:

t = zeros(1, 1000);

Of course, you could eliminate the "for" loop and just generate the array of Poisson random numbers directly:

t = poissrnd(m, 1, 1000);

-Cam

[zeezo]

Thank you Cam.

Do you mean I should write like this :

t = zeros(1, 1000);
m=0.05;
for
i=1:1000;
     r=  poissrnd(m);
    t(1,i)= r;
end
t

Am I understanding right?

Thanks

[Cam Salzberger]

Yes, that would take care of the preallocation. Really consider generating all the numbers in one go though, and avoid the loop entirely.

[zeezo]

Thank you very much.