Using the C Math Library

Optional Output Arguments

MATLAB functions may also have optional or multiple output arguments. For example, you invoke the find function, which locates nonzero entries in arrays, with one, two, or three output arguments.

k = find(X);
[i,j] = find(X);
[i,j,v] = find(X);

In interpreted MATLAB, find returns one, two, or three values. In C, a function cannot return more than one value. Therefore, the additional arrays must be passed to find in the argument list. They are passed as pointers to mxArray pointers (mxArray** variables).

Output arguments always appear before input arguments in the parameter list. In order to accommodate all the combinations of output arguments, the MATLAB C Math Library mlfFind() function takes three arguments, the first two of which are mxArray** parameters corresponding to output values.

Using the MATLAB C Math Library, you call mlfFind like this

mlfAssign(&k, mlfFind(NULL,NULL,X));
mlfAssign(&i, mlfFind(&j,NULL,X));
mlfAssign(&i, mlfFind(&j,&v,X));

where i, j, k, v, and X are mxArray* variables. i, j, k, and v are initialized to NULL.

The general rule for multiple output arguments is that the function return value, an mxArray*, corresponds to the first output argument. All additional output arguments are passed into the function as mxArray** parameters.

Optional Input Arguments

Optional Input and Output Arguments