Nonnegative deviation variables in AMPL

Question

I'm using AMPL and need to input my model that has nonnegative deviation variables (s+ - s-).

An example constraint is: (x - 5) = (s+ - s-)

Answer 1

This is the way to do it:

var x;
var sp >= 0;
var sm >= 0;

s.t.

cons1:
  (x - 5) = (sp - sm);

FYI, the AMPL book can be downloaded for free.