Exclude matrix diagonal in for loop

Hi,

Let's say I have a 20x20 matrix for which I wish to run some formulas over a loop. But I wish to exclude the main diagonal (1,1 to 20,20) from the for loop.

I came up with:

for j=2:20
    for i:(j-1)

Any thoughts please?

Thanks

Answer 1

[Guillaume]

Your second for is not going to do much. This is one way to do it:

for row = 1:size(matrix, 1)  %use better names than i or j and don't hardcode sizes
   for col = 1:size(matrix, 2)
      if col ~= row      %if not on diagonal
         %do calculation
      end
   end
end

However, if you can vectorise your calculation and forego the loops entirely that would be even better.

Answer 2

[Michael Haderlein]

In case you cannot vectorize the problem, why don't you simply check for equality?

for cnt1 = 1:20
   for cnt2=1:20
      if cnt1~=cnt2
        %calculations
      end
   end
end