Declare numerical-sequence set in AMPL .dat file

Question

Suppose I want an AMPL set to contain the integers 1 through 5. How can I declare this in the .dat file?

I can do it in the .mod file, like this:

things.mod:

set THINGS := 1..5;

Console:

ampl: model things.mod;
ampl: display THINGS;
set THINGS := 1 2 3 4 5;

ampl: display THINGS diff {2};
set THINGS diff  {2} := 1 3 4 5;

But now suppose I do this:

things.mod:

set THINGS;

things.dat:

set THINGS := 1..5;

Console:

ampl: model things.mod;
ampl: data things.dat;
ampl: display THINGS;
set THINGS := 1..5;

ampl: display THINGS diff {2};
set THINGS diff  {2} := 1..5;

AMPL seems to be treating 1..5 as a string rather than as an sequence of integers. Why, and how do I fix it?

Answer 1

The .. operator cannot be used in the data section (i.e. it won't be expanded, see the AMPL FAQ). What you can do is to declare the limits in the model, e.g.

param n;
set things := 1..n;

and define them in the data file

param n := 5;