MATLAB: How to make a matrix mirror another

array

How do I do this?
X=[4 7 1 0;
   3 0 6 8] 
Y=[32 6 4 21;
   77 89 0 2]
X(X==0)=a;
after the alteration:
X=[4 7 1 a;
   3 a 6 8]    
Y=[ 32 6 4 a;
   77  a  0  2];

Best Answer

Try this: 
a = 42;
X=[4 7 1 0;
   3 0 6 8] 
Y=[32 6 4 21;
   77 89 0 2]
Idx = X==0
X(Idx) = a
Y(Idx) = a                         % Use The Same Index On Both Matrices,
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