Set number of columns in array
Call mxSetN to set the number of columns in the specified mxArray. The term columns always means the second dimension of a matrix. Calling mxSetN forces an mxArray to have two dimensions. For example, if pm points to an mxArray having three dimensions, calling mxSetN reduces the mxArray to two dimensions.
You typically use mxSetN to change the shape of an existing mxArray. The mxSetN function does not allocate or deallocate any space for the pr, pi, ir, or jc arrays. Consequently, if your calls to mxSetN and mxSetM increase the number of elements in the mxArray, enlarge the pr, pi, ir, and/or jc arrays.
If your calls to mxSetM and mxSetN end up reducing the number of elements in the mxArray, you might want to reduce the sizes of the pr, pi, ir, and/or jc arrays in order to use heap space more efficiently. However, reducing the size is not mandatory.
See the following examples in matlabroot/extern/examples/mx.
See the following examples in matlabroot/extern/examples/refbook.