Download MATLAB C Math Library 2.0 User's Guide

Using mlfIndexRef( ) for Logical Indexing
Using mlfIndexRef( ) for Logical Indexing
This section describes how to use:
• A logical index as a one-dimensional subscript
• Two logical vectors as indices in a two-dimensional subscript
• A colon index and a logical vector as a two-dimensional subscript
• A logical index to select elements from a row or column
The examples work with matrix A and the logical array B.
A
1 4 7
2 5 8
3 6 9
B
1 0 1
0 1 0
1 0 1
‘‘Assumptions for the Code Examples’’ on page 5-16 explains the conventions
used in the examples.
Overview
Logical indexing is a special case of n-dimensional indexing. A logical index is
a vector or a matrix that consists entirely of ones and zeros. Applying a logical
subscript to a matrix selects the elements of the matrix that correspond to the
nonzero elements in the subscript.
Logical indices are generated by the relational operator functions (mlfLt(),
mlfGt(), mlfLe(), mlfGe(), mlfEq(), mlfNeq()) and by the function
mlfLogical(). Because these functions attach a logical flag to a logical matrix,
you cannot create a logical index simply by assigning ones and zeros to a vector
or matrix.
You can form an n-dimensional logical subscript by combining a logical index
with scalar, vector, matrix, or colon indices.
5-31