prev (scheduling)

OPL function (scheduling) to sequence interval variables

Purpose

OPL function (scheduling) to sequence interval variables. For the function regarding element position in a set, see prev.

type 
boolean

Syntax

prev(p, a, b)

Where:

dvar interval a;
dvar interval b;
dvar sequence p;

Description

States that if intervals a and b are present, a appears just before b in sequence p; no other intervals can be ordered between a and b in sequence p. Compare prev with before, which allows other intervals to be ordered between the two.

Notes

  • Unless a noOverlap constraint is applied to the sequence variable, the order of intervals in a sequence does not necessarily correspond to the order of the intervals in time.

  • The prev scheduling constraint cannot be used in a meta-constraint.

Example

A set of 2*n activities to be sequenced on a unary resource such that A[2*i-1] is constrained to be executed immediately prior to A[2*i].

dvar interval A[i in 1..2*n] size d[i];
dvar sequence p in A;

subject to {
  noOverlap(p);
  forall (i in 1..n)
    prev(p, A[2*i-1], A[2*i]);
};