mxGetField (External Interfaces/API Reference)

Get a field value, given a field name and an index in a structure array

C Syntax

#include "matrix.h"
mxArray *mxGetField(const mxArray *array_ptr, int index, 
         const char *field_name);

Arguments
array_ptr
Pointer to a structure mxArray.

index

The desired element. The first element of an mxArray has an index of 0, the second element has an index of 1, and the last element has an index of N-1, where N is the total number of elements in the structure mxArray.

field_name

The name of the field whose value you want to extract.

Returns

A pointer to the mxArray in the specified field at the specified field_name, on success, and NULL otherwise. One possibility is that there is no value assigned to the specified field. Another possibility is that there is a value, but the call failed. Common causes of failure include:

Specifying an array_ptr that does not point to a structure mxArray. To determine if array_ptr points to a structure mxArray, call mxIsStruct.

Specifying an out-of-range index to an element past the end of the mxArray. For example, given a structure mxArray that contains 10 elements, you cannot specify an index greater than 9.

Specifying a nonexistent field_name. Call mxGetFieldNameByNumber or mxGetFieldNumber to get existing field names.

Insufficient heap space to hold the returned mxArray.

Description

Call mxGetField to get the value held in the specified element of the specified field. In pseudo-C terminology, mxGetField returns the value at

array_ptr[index].field_name

mxGetFieldByIndex is similar to mxGetField. Both functions return the same value. The only difference is in the way you specify the field. mxGetFieldByIndex takes field_num as its third argument, and mxGetField takes field_name as its third argument.

Note Inputs to a MEX-file are constant read-only mxArrays and should not be modified. Using mxSetCell* or mxSetField* to modify the cells or fields of an argument passed from MATLAB causes unpredictable results.

Calling

mxGetField(pa, index, "field_name");

is equivalent to calling

field_num = mxGetFieldNumber(pa, "field_name");
mxGetFieldByNumber(pa, index, field_num);

where index is zero if you have a one-by-one structure.

mxGetEps mxGetFieldByNumber