Updating values in an array with logical indexing with a non-constant value

Question

A common problem I encounter when I want to write concise/readable code:

I want to update all the values of a vector matching a logical expression with a value that depends on the previous value.

For example, double all even entries:

weights = [10 7 4 8 3];
weights(mod(weights,2)==0) = weights(mod(weights,2)==0) * 2;
% weights = [20 7 8 16 3]

Is it possible to write the second line in a more concise fashion (i.e. avoiding the double use of the logical expression, something like i+=3 for i=i+3 in other languages). If I often use this kind of vector operation in different contexts/variables, and I have long conditionals, I feel that my code is less concise and readable than it could be.

Thanks!

Answer 1

How about

ind = mod(weights,2)==0;
weights(ind) = weights(ind)*2;

This way you avoid calculating the indices twice and it's easy to read.

Answer 2

Starting your other comment to Wauzl, such powerful operation capabilities is the Fortran side. This is purely matlab's design that is quickly getting obsolete. Let's use this horribleness further:

for i=1:length(weights),if (mod(weights(i),2)==0)weights(i)=weights(i)*2;end,end

It is even slightly faster than your two liner because you are doing the conditional indexing twice on both sides. In general, consider switching to Python3.