I have two arrays in matlab
a = [1 1.1 1.2 1.1 1.3]
b = [-2 0 1 2 -4]
For the negative values in array b, example -2 -4, I would like to convert array a into negative, -1 and -1.3.
I have used a 'for' loop, which takes too long for 700 000 columns(possibly hours) looking for a quicker ways to accomplish this. Thanks
a .* sign(b) works if b has no zero elements, a can have arbitrary elements.
Explanation:
multiply a itemwise with the sign of b.
Null fix:
To fix the null error you can write a function yourself like this (pseudocode):
function r = test_sign(b)
if b==0
r = 1;
else
r = sign(b);
end
You can use logical indexing.
(First I assume a and b have the same length or your question doesn't make sense).
To get a vector that is TRUE when b is negative and FALSE otherwise, you can just do:
b < 0 % depending on what you want, b <= 0
You can use this as an index into a to grab out those corresponding elements:
a( b < 0 )
Now that you've selected the right elements make them negative and assign them back:
a ( b < 0 ) = - a ( b < 0 );
In terms of efficiency, you may want to store the vector b < 0 to avoid re-calculating it (you'll have to try and see which on you prefer):
idx = b < 0;
a(idx) = -a(idx);
Matlab is a vectorised language so most times there is a for loop, you can avoid it. Read up about this sort of matrix indexing here.
