mxSetField (External Interfaces/API Reference)

Set a field value of a structure array, given a field name and an index

C Syntax

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

Arguments

array_ptr
Pointer to a structure mxArray. Call mxIsStruct to determine if array_ptr points 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. See mxCalcSingleSubscript for details on calculating an index.

field_name
The name of the field whose value you are assigning. Call mxGetFieldNameByNumber or mxGetFieldNumber to determine existing field names.

value
Pointer to the mxArray you are assigning.

Description

Use mxSetField to assign a value to the specified element of the specified field. In pseudo-C terminology, mxSetField performs the assignment

```
array_ptr[index].field_name = value;
```

If there is already a value at the given position, the value pointer you specified overwrites the old value pointer. However, mxSetField does not free the dynamic memory that the old value pointer pointed to. Consequently, you should free this old mxArray immediately before or after calling mxSetField.

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

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

is equivalent to calling

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

Examples

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

mxSetDimensions mxSetFieldByNumber