Set a single value in a character-valued array attribute.
A non-zero return value indicates that a problem occurred while setting the attribute. Refer to the Error Code table for a list of possible return values. Details on the error can be obtained by calling GRBgeterrormsg.
model: A loaded optimization model, typically created by routine GRBnewmodel or GRBreadmodel.
attrname: The name of a character-valued array attribute. Available attributes are listed and described in the Attributes section of this document.
element: The index of the array element to be changed.
newvalue: The value to which the attribute element should be set.
error = GRBsetcharattrelement(model, "VType", 0, GRB_BINARY);