Counting
All Xpress NonLinear entities are numbered from 1. The 0th item is defined, and is an empty entity of the appropriate type. Therefore, whenever an Xpress NonLinear function returns a zero value, it means that there is no data of that type.
In parsed and unparsed function arrays, the indices always count from 1. This includes types XSLP_VAR and XSLP_CONSTRAINT: the index is the matrix column or row index +1.
Note that for input of function arrays, types XSLP_COL and XSLP_ROW can be used, but will be converted into standard XSLP_VAR or XSLP_CONSTRAINT references. When a function array is returned from Xpress NonLinear, the XSLP_VAR or XSLP_CONSTRAINT type will always be used.