Using the C Math Library

Using a Scalar and a Logical Vector

Let matrix X be a 4-by-4 magic square.

X = magic(4);

16   2   3  13
 5  11  10   8
 9   7   6  12
 4  14  15   1

Let B be a logical matrix that indicates which elements in row two of matrix X are greater than 9. B is the result of the greater than (>) operation

mlfAssign(&target_row,
          mlfIndexRef(X, "(?,?)", mlfScalar(2), 
                     mlfCreateColonIndex()));
mlfAssign(&B, mlfGt(target_row, mlfScalar(9)));

and contains the vector

0 1 1 0

In MATLAB, B = (X(2,:) > 9) performs the same operation.

Use B as a logical index that selects those elements from matrix X.

mlfAssign(&C, mlfIndexRef(X, "(?,?)", mlfScalar(2), B));

selects these elements:

11 10

Using One Colon Index and One Logical Vector as Indices

Extending Logical Indexing to N Dimensions