MATLAB^®

mxGetField (C and Fortran)

Get field value, given field name and index into structure array

C Syntax

#include "matrix.h"
mxArray *mxGetField(const mxArray *pm, mwIndex index,
         const char *fieldname);

Fortran Syntax

mwPointer mxGetField(pm, index, fieldname)
mwPointer pm
mwIndex index
character*(*) fieldname

Arguments

pm

Pointer to a structure mxArray

index

Index of the desired element.

In C, 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 mxArray.

In Fortran, the first element of an mxArray has an index of 1, the second element has an index of 2, and the last element has an index of N, where N is the total number of elements in the mxArray.

fieldname

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

Returns

A pointer to the mxArray in the specified field at the specified fieldname, on success. Returns NULL in C (0 in Fortran) if passed an invalid argument or if there is no value assigned to the specified field. Common causes of failure include:

Specifying an array pointer pm that does not point to a structure mxArray. To determine whether pm points to a structure mxArray, call mxIsStruct.
Specifying an index to an element outside the bounds of the mxArray. For example, given a structure mxArray that contains 10 elements, you cannot specify an index greater than 9 in C (10 in Fortran).
Specifying a nonexistent fieldname. 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:

pm[index].fieldname

mxGetFieldByNumber is similar to mxGetField. Both functions return the same value. The only difference is in the way you specify the field. mxGetFieldByNumber takes a field number as its third argument, and mxGetField takes a 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 a MATLAB^® argument causes unpredictable results.

In C, calling:

mxGetField(pa, index, "field_name");

is equivalent to calling:

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

where index is 0 if you have a 1-by-1 structure.

In Fortran, calling:

mxGetField(pm, index, 'fieldname')