problem looping through arrays

This is the question: Let M be a matrix of positive integers. Write a function with header [Q] =myTrigOddEven(M), where Q(i ,j ) = sin (M(i,j)) if M (i , j ) is even, and Q(i ,j ) =cos (M (i , j )) if M (i , j ) is odd.

This is the function I came up with:

function [Q] = myTrigOddEven(M)

for i = 1:(length(M)-1)

for j = 1:(length(M)-1)

if mod(Q(i,j),2) == 0

Q(i,j) = sin (M (i , j ));

else

Q(i,j) = cos (M (i , j ));

end

end

end

end

I'm getting an error with the statement in the if and the else. At the parts Q(i,j) = sin (M (i , j )); and Q(i,j) = cos (M (i , j ));, it says that Q changes size on every loop interation, and I'm not sure how to fix that.

Answer 1

[the cyclist]

I think you intended

if mod(M(i,j),2) == 0

rather than

if mod(Q(i,j),2) == 0

Regarding the warning about Q changing size, you want to preallocate that array. For example, you could put

Q = zeros(size(M));

at the beginning of the function.

You can read about why preallocation is important in this documentation.