MATLAB: Simple Question (I think) about finding zeros(and other number) in an array

Question

I have the following array.

R = [ 1 2 0 1 0 2]

if values in R = 0 I want to replace it with 3, so I did

If R == 0
R = 3
end

So it should end up

R = [ 1 2 3 1 3 2]

However as you probably know, this does not work. How do I make this work? Thanks! Same problem with the following:

P = [ 1 2 3 4 5]
Q = [ 5 4 3 2 5]

I want to find positions where P == Q so I tried

if P == Q
Q = 6
end

ie all positions where P = Q change to 6 so it should end up with

Q = [ 5 4 6 2 6]

Answer 1

TRY THIS:

    R = [ 1 2 0 1 0 2]
    R(R==0)=3
    P = [ 1 2 3 4 5]
    Q = [ 5 4 3 2 5]
    Q(Q==P)=6