MATLAB: How to access specific elements of an array

array indexing

Suppose I have two matrices A and x as follows:
x =
     1     2
     2     3
A =
     1     1     1     1
     1     1     1     1
     1     1     1     1
     1     1     1     1
Each row of x contains the row and column indices of an element of A that I would like to access and change. In the above example, I want to change the (1,2) and (2,3) elements of A. Suppose I want to set the value of these elements to 5. The command 'A([1 2],[2 3]) = 5;' produces A =
     1     5     5     1
     1     5     5     1
     1     1     1     1
     1     1     1     1
However, what I want is the following:
A =
     1     5     1     1
     1     1     5     1
     1     1     1     1
     1     1     1     1
In my actual code, A and x may have thousands of elements each. Therefore, I am wondering whether it is possible to do this without using a for loop?

Best Answer

One approach:
ix = sub2ind(size(A), x(:,1), x(:,2));              % Change Subscripts To Linear Index Vector
A(ix) = 5;
A =
     1     5     1     1
     1     1     5     1
     1     1     1     1
     1     1     1     1
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