Assigning value to array by logical indexing doesn't work

Question

In Matlab, I want to replace a certain value with some other value.

I know that I can do this:

X(X==0) = -1 

If I want to replace all

Answer 1

Why your approach was not working is indeed a little curios. The reason is that the left hand side of your equation is completely logical, the right hand side is casted logical as well and

logical(-1) = 1

Therefore:

Y = (X == 5)    %// Y is logical
Y(Y == 0) = -1  %// Y is logical, Y == 0 is logical, 
                %// -1 is casted to logical and logical(-1) = 1

So transform your first logical array to double, and it works.

Y = (X == 5)    %// Y is logical
Y = double(Y)   %// Y is double
Y(Y == 0) = -1  %// Y is double, Y == 0 is logical

---

Example:

X = randi(9,20,1);
Y = (X == 5)   
Y = double(Y)   
Y(Y == 0) = -1  
out = [X Y]

---

out =

     7    -1
     3    -1
     5     1
     7    -1
     9    -1
     9    -1
     5     1
     2    -1
     2    -1
     3    -1
     8    -1
     3    -1
     8    -1
     3    -1
     9    -1
     4    -1
     2    -1
     3    -1
     6    -1
     5     1