The GLPK wiki discusses sorting - and I believe there is a typo in it.
Should the line:
set ind{k in 1..card(I)} := setof{i in I: pos[i] = k} i;
really be:
set ind{k in 1..card(I)} := setof{i in I: pos[i] = k-1} i;
A copy/paste of the article is below:
# sorting_symbolic.mod - how to sort arrays in MathProg
# based on code by Andrew Makhorin
# Sometimes it is necessary to print parameters or variables in the
# order of ascending or descending their values. Suppose, for example,
# that we have the following subscripted parameter:
set I;
param a{i in I} := Uniform(2, 7);
# If we print all its members:
printf{i in I} "a[%2s] = %g\n", i, a[i];
# the output may look like follows:
#
# a[a] = 2.64156
# a[b] = 2.04798
# a[c] = 2.14843
# a[d] = 4.76896
# a[e] = 6.09132
# a[f] = 3.27780
# a[g] = 4.06113
# a[h] = 4.05898
# a[i] = 6.63120
# a[j] = 6.50318
# a[k] = 3.46065
# a[l] = 4.69845
#
# However, we would like the parameter members to appear in the order
# of ascending values.
#
# Introduce the following auxiliary parameter:
param pos{i in I} :=
card({j in I: a[j] < a[i] or a[j] = a[i] and j < i});
# where pos[i] = k - 1 means that in the sorted list member a[i] has
# k-th position, 1 <= k <= |I|. Then introduce another auxiliary
# parameter:
set ind{k in 1..card(I)} := setof{i in I: pos[i] = k} i;
# where ind[k] = {i} iff pos[k] = i.
#
# Now, the following statement:
printf "\n";
printf{k in 1..card(I), l in ind[k]} "a[%2s] = %g\n", l, a[l];
# prints the parameter members in the desired order:
#
# a[b] = 2.04798
# a[c] = 2.14843
# a[a] = 2.64156
# a[f] = 3.27780
# a[k] = 3.46065
# a[h] = 4.05898
# a[g] = 4.06113
# a[l] = 4.69845
# a[d] = 4.76896
# a[e] = 6.09132
# a[j] = 6.50318
# a[i] = 6.63120
solve;
data;
set I := a b c d e f g h i j k l;
#set I := 1 2 3 4 5 6 7 8 9 10 11 12;
end;
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