MATLAB^®

mxGetFieldByNumber (C and Fortran) - Get field value, given field number and index into structure array

C Syntax

#include "matrix.h"
mxArray *mxGetFieldByNumber(const mxArray *pm, mwIndex index, 
         int fieldnumber);

Fortran Syntax

mwPointer mxGetFieldByNumber(pm, index, fieldnumber)
mwPointer pm
mwIndex index
integer*4 fieldnumber

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.

See mxCalcSingleSubscript for more details on calculating an index.

fieldnumber

The position of the field whose value you want to extract

In C, the first field within each element has a field number of 0, the second field has a field number of 1, and so on. The last field has a field number of N-1, where N is the number of fields.

In Fortran, the first field within each element has a field number of 1, the second field has a field number of 2, and so on. The last field has a field number of N, where N is the number of fields.

Returns

A pointer to the mxArray in the specified field for the desired element, 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. Call mxIsStruct to determine whether pm points to a structure mxArray.
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 field number. Call mxGetFieldNumber to determine the field number that corresponds to a given field name.

Description

Call mxGetFieldByNumber to get the value held in the specified fieldnumber at the indexed element.

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')

is equivalent to calling:

fieldnum = mxGetFieldNumber(pm, 'fieldname')
mxGetFieldByNumber(pm, index, fieldnum)

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

C Examples

See phonebook.c in the refbook subdirectory of the examples directory.

Additional examples:

mxisclass.c in the mx subdirectory of the examples directory
explore.c in the mex subdirectory of the examples directory