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mxSetField (C and Fortran)

Set field value in structure array, given index and field name

C Syntax

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

Fortran Syntax

#include "fintrf.h"
subroutine mxSetField(pm, index, fieldname, pvalue)
mwPointer pm, pvalue
mwIndex index
character*(*) fieldname



Pointer to a structure mxArray. Call mxIsStruct to determine whether pm points to a structure mxArray.


Index of an element in the array.

In C, the first element of an mxArray has an index of 0. The index of the last element is N-1, where N is the number of elements in the array. In Fortran, the first element of an mxArray has an index of 1. The index of the last element is N, where N is the number of elements in the array.

See mxCalcSingleSubscript for details on calculating an index.


Name of a field in the structure. The field must exist in the structure. Call mxGetFieldNameByNumber or mxGetFieldNumber to determine existing field names.


Pointer to an mxArray containing the data you want to assign to fieldname.


Use mxSetField to assign the contents of pvalue to the field fieldname of element index.

If you want to replace the contents of fieldname, first free the memory of the existing data. Use the mxGetField function to get a pointer to the field, call mxDestroyArray on the pointer, then call mxSetField to assign the new value.

You cannot assign pvalue to more than one field in a structure or to more than one element in the mxArray. If you want to assign the contents of pvalue to multiple fields, use the mxDuplicateArray function to make copies of the data then call mxSetField on each copy.

To free memory for structures created using this function, call mxDestroyArray only on the structure array. Do not call mxDestroyArray on the array pvalue points to. If you do, MATLAB® attempts to free the same memory twice, which can corrupt memory.


Inputs to a MEX-file are constant read-only mxArrays. Do not modify the inputs. Using mxSetCell* or mxSetField* functions to modify the cells or fields of a MATLAB argument causes unpredictable results.


See the following examples in matlabroot/extern/examples/mx.


C Language

In C, you can replace the statements:

field_num = mxGetFieldNumber(pa, "fieldname");
mxSetFieldByNumber(pa, index, field_num, new_value_pa);

with a call to mxSetField:

mxSetField(pa, index, "fieldname", new_value_pa);

Fortran Language

In Fortran, you can replace the statements:

fieldnum = mxGetFieldNumber(pm, 'fieldname')
mxSetFieldByNumber(pm, index, fieldnum, newvalue)

with a call to mxSetField:

mxSetField(pm, index, 'fieldname', newvalue)

Introduced before R2006a

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