Replacing all values of a with b in a matrix?

Say I define that a and b are 3 and 4, respectively. I heva a matrix M that I want to swap all instances of a with b. How would I do that? I tried doing

res = M(M==a) = b;

it works if I don't have the res there but I need the res there to output my result for this specific issue. It keeps telling me it needs to be

res = M(M==a) == b; but that only gives me the 2x1 logical array that has 0 0 in it

[madhan ravi]

Show what’s M and how the resulting M should be with e a small example.

[JARED VAHRENBERG]

For example say I wanted to have matrix M= [2,5;2,8] and wanted to swap the 2's out for 1's to where then I get res = [1,5;1,8]. Why is it that my code works in the command window but as soon as I add in that res in the editor it gets angry?

Answer 1

[madhan ravi]

What do you expect == is a logical operator, when not used in symbolic calculations if you just want to assign it, then simply use

 res = M;
 res(res == a) = b

[JARED VAHRENBERG]

Ah that worked thanks a lot