saving multiplications with zeros in a loop

%if true
for i iterations
Matrix = Matrix.*phase %Matrix has lot of zeros I want to skip multiplications with zero. Is it possible to do it in a single line without using if else or any loops
%end

Please dont use sparse, both matrix and phase are two-dimensional array

Answer 1

[Walter Roberson]

 Mask = Matrix ~= 0;

for i iterations 
  Matrix(mask) = Matrix(mask) * phase ;
end

This assumes that phase is a scalar nonzero value. It does not apply if phase is a vector or 2d array.

If phase is a 2d array then

phases = sparse(phase) ;
Matrix = sparse(Matrix) ;
for i iterations
  Matrix = Matrix * phases;
end

Or better yet,

Matrix = sparse(Matrix) * (phase^iterations) ;

with no loop.

[Walter Roberson]

In your revised specifications of 2D matrices and using .* then

mask = Matrix ~= 0; Matrix(mask) = Matrix(mask) .* (phase(mask).^iterations);

This does more multiplications than are strictly necessary if entries in phase can be 0. This code uses two statements; optimizing the multiplications for phase potentially being 0 would require three statements.

[Walter Roberson]

There are also optimizations available to reduce the multiplications cost of phase.^iterations but at the potential cost of more memory. These are easiest to code if iterations is a constant. The requirement to use only a single line of code acts against the requirement to reduce multiplications. As I indicated before, if efficiency is the top priority then you should give up on trying to force everything into one line.

[D_coder]

actually this code is slow as compared to sparse. And sparse is slower as compared to the normal multiplication.Here is my detailed question. https://www.mathworks.com/matlabcentral/answers/418975-sparse-and-binary-mask

[D_coder]

Hey Walter my top priority is speed and saving multiplications.