External Interfaces/API Reference

mxCalcSingleSubscript

Return the offset (index) from the first element to the desired element

C Syntax

• #include <matrix.h>
  int mxCalcSingleSubscript(const mxArray *array_ptr, int nsubs, 
      int *subs);
  

Arguments

array_ptr
   Pointer to an mxArray.

nsubs
   The number of elements in the subs array. Typically, you set nsubs equal to the number of dimensions in the mxArray that array_ptr points to.

subs
   An array of integers. Each value in the array should specify that dimension's subscript. The value in subs[0] specifies the row subscript, and the value in subs[1] specifies the column subscript. Note that mxCalcSingleSubscript views 0 as the first element of an mxArray, but MATLAB sees 1 as the first element of an mxArray. For example, in MATLAB, (1,1) denotes the starting element of a two-dimensional mxArray; however, to express the starting element of a two-dimensional mxArray in subs, you must set subs[0] to 0 and subs[1] to 0.

Returns

The number of elements between the start of the mxArray and the specified subscript. This returned number is called an "index"; many mx routines (for example, mxGetField) require an index as an argument.

If subs describes the starting element of an mxArray, mxCalcSingleSubscript returns 0. If subs describes the final element of an mxArray, then mxCalcSingleSubscript returns N-1 (where N is the total number of elements).

Description

Call mxCalcSingleSubscript to determine how many elements there are between the beginning of the mxArray and a given element of that mxArray. For example, given a subscript like (5,7), mxCalcSingleSubscript returns the distance from the (0,0) element of the array to the (5,7) element. Remember that the mxArray data type internally represents all data elements in a one-dimensional array no matter how many dimensions the MATLAB mxArray appears to have.

MATLAB uses a column-major numbering scheme to represent data elements internally. That means that MATLAB internally stores data elements from the first column first, then data elements from the second column second, and so on through the last column. For example, suppose you create a 4-by-2 variable. It is helpful to visualize the data as

A	E
B	F
C	G
D	H

shown below.

Although in fact, MATLAB internally represents the data as the following:

A	B	C	D	E	F	G	H
Index 0	Index 1	Index 2	Index 3	Index 4	Index 5	Index 6	Index 7

If an mxArray is N-dimensional, then MATLAB represents the data in N-major order. For example, consider a three-dimensional array having dimensions 4-by-2-by-3. Although you can visualize the data as

MATLAB internally represents the data for this three-dimensional array in the order shown below:

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23

Avoid using mxCalcSingleSubscript to traverse the elements of an array. It is more efficient to do this by finding the array's starting address and then using pointer auto-incrementing to access successive elements. For example, to find the starting address of a numerical array, call mxGetPr or mxGetPi.

Examples

See mxcalcsinglesubscript.c in the mx subdirectory of the examples directory.

mxAssertS

mxCalloc