Using the C++ Math Library

Dimensions and Subscripts in the MATLAB C++ Math Library

The MATLAB C++ Math Library supports N-dimensional standard, cell array, and structure indexing. You use:

• The indexing operator () for standard indexing
• mwArray::cell() for cell array indexing
• mwArray::field() for structure indexing

The indexing operator () and mwArray::cell() take numeric arguments, one index for each dimension of the array being indexed. mwArray::field() takes the name of the structure field as an argument.

Note    You cannot index into an array with more dimensions than the array has, although you can use fewer dimensions.

Applying a subscript to an array allows you to:

• Access
• Modify
• Delete

elements of an array. For example, the two-dimensional indexing expression

A(3,1) 

applies the subscript (3,1) to A and returns the element at row 3, column 1.  A(9), a one-dimensional indexing expression, returns the ninth element of array A.

Note    The indexing functions follow the MATLAB convention for array indices: indices begin at one rather than zero.

An index can be a scalar, a vector, a matrix, or a call to the special function colon().

• A scalar subscript selects a scalar value.
• A subscript with vector or matrix indices selects a vector or matrix of values.
• The colon() index, which loosely interpreted means ``all,'' selects, for example, all the columns in a row or all the rows in a column.

If you provide arguments to colon(), the subscript specifies a vector. For example, colon(1,10) specifies the vector [ 1 2 3 4 5 6 7 8 9 10 ].

Tip    for-loops provide an easy model for thinking about indexing. A one-dimensional index is equivalent to a single for-loop; a two-dimensional index is equivalent to two nested for-loops. The size of the subscript determines the number of iterations of the for-loop. The value of the subscript determines the values of the loop iteration variables.

The MATLAB C++ Math Library implements indexing via the interaction of three classes: mwArray, mwIndex, and mwSubArray. mwArray represents the array itself. mwIndex represents an index. mwSubArray represents the result of an index operation. The indexing routines themselves create mwSubArray objects when an indexing expression appears as the target of an assignment operation (on the left-hand side of an assignment operator). The library handles mwSubArray objects for you; you do not need to create them.

Dimensions and Subscripts in MATLAB

Array Storage