algorithm,matlab,time,big-o,nested-loops , Why are my nested for loops taking so long to compute?

Why are my nested for loops taking so long to compute?


Tag: algorithm,matlab,time,big-o,nested-loops

I have a code that generates all of the possible combinations of 4 integers between 0 and 36.

This will be 37^4 numbers = 1874161.

My code is written in MATLAB:

for a = 0:36
    for b= 0:36
        for c = 0:36
            for d = 0:36
                combination(i,:) = [a,b,c,d];             

I've tested this with using the number 3 instead of the number 36 and it worked fine.

If there are 1874161 combinations, and with An overly cautions guess of 100 clock cycles to do the additions and write the values, then if I have a 2.3GHz PC, this is:

1874161 * (1/2300000000) * 100 = 0.08148526086

A fraction of a second. But It has been running for about half an hour so far.

I did receive a warning that combination changes size every loop iteration, consider predefining its size for speed, but this can't effect it that much can it?


As @horchler suggested you need to preallocate the target array

The only solution to this without preallocation is to store the result in linked list

If you need even more speed then there are ways (probably not for Matlab):


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