Indexing problem - how to obtain row and column and use later

If I have a matrix A = [ 1 2 3; 2 3 4; 2 0 8; 4 8 0; 1 2 0; 0 0 0] and I wanted to store the places where the zeros exist and then use this later to form a new matrix B which equals A but all the zero values are replaced with 4, how would I do this?

Answer 1

[Iain]

 whereAequalszero = A ==0;
 B = A;
 B(whereAequalszero) = 4;