# GLPK/Using GMPL (MathProg)

This page gives examples and tips about the syntax and usage of GMPL (MathProg).

## Introduction

editThe first thing to note is that MathProg is not a procedural language. For example, unlike most programming languages, MathProg has no assignment statement. Rather MathProg is a functional language. MathProg is modeled on the AMPL language and provides a subset of that language. And like AMPL, its syntax is tailored to the expression of mathematical optimization problems.

## MathProg objects

edit### Sets

editWithin MathProg, sets are **unordered** collections of elements (unlike arrays
in programming languages), so the element positions are indeterminate.

## MathProg syntax

edit### MathProg suffixes

editAs from GLPK 4.44 released 03 June 2010, MathProg supports suffixes for constraints and variables. The use of suffixes is covered in the official documentation under appendix A of `doc/gmpl.pdf`. The following table provides a summary:

Suffix | Purpose |
---|---|

.lb |
lower bound |

.ub |
upper bound |

.status |
status in the solution |

.val |
primal value |

.dual |
dual value |

The MathProg `printf` statement can be used to create customized output. In the following case, the dual value of constraint `c1` will be printed to the terminal:

... solve; printf "dual value of constraint c1: %g\n", c1.dual;

Terminal output can also be duplicated to a text file with the GLPSOL command-line option `--log filename.log`.

### Set statement

editThe `data` parameter of the `set` statement is (as of May 2011) undocumented, but the idea is quite simple. Imagine that you declare a plain set `S` consisting of 5-tuples `(i,j,k,l,m)`:

set S, dimen 5;

which will be provided with data in the data section.
But what you actually want is an array of sets `T{j,m}`, where `T[j,m]` is a set of triplets `(l,i,k)` from `S`. Using the `data` attribute, additionally declare a set `T` as follows:

set T{j,m}, data S[2,5,4,1,3];

This statement has the same effect as providing the corresponding data for `T` itself from the data section, where `[2,5,4,1,3]` is a permutation of the component indices: the 2nd and 5th components of every 5-tuple in `S` gives a pair `(j,m)` used to index the elements of `T`, and the 4th, 1st, and 3rd components of the 5-tuple are used to build the `(l,i,k)` triplets, which are then included in the set `T[j,m]`.
Finally, only plain sets are supported by the `data` attribute.

## Limitations

edit### Ordered sets

editOrdered sets are not supported in MathProg. The discussions here and here provide some background as to why not. This 2006 thread describes one particular work-around.