Using the C Math Library

Indexing Functions

An array subscript consists of one or more indices passed as mxArray * arguments to one of the indexing functions. For example, the two-dimensional indexing expression

mlfIndexRef(A, "(?,?)", mlfScalar(3), mlfScalar(1)) 

applies the subscript (3,1) to A and returns the element at row three, column one.  mlfIndexRef(A, "(?)", mlfScalar(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.

Both standard indexing and cell array indexing take numeric arguments, one argument for each dimension of the array being indexed into. Structure indexing uses only the name of the structure field.

Note    Standard indexing can be used with all three types of data Cell array indexing can only be used on cell arrays and structure indexing only on structures. You can combine, for example, standard indexing and structure indexing on a structure.

The indexing functions in the MATLAB C Math Library support N-dimensional standard, cell array, and structure indexing.

An index mxArray argument can contain a scalar, vector, matrix, or the result from a call to the special function mlfCreateColonIndex().

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

You can also use the mlfColon() function, which is patterned after the MATLAB colon operator, to specify a vector subscript. For example, mlfColon(mlfSclar(1), mlfScalar(10), NULL) specifies the vector
[ 1 2 3 4 5 6 7 8 9 10 ].

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

Note    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.

Terminology

Array Storage