Problem Statement

Sort an array of composite structures by a key.

For example, if you define a composite structure that presents a name-value pair (in pseudo-code): and an array of such pairs: then define a sort routine that sorts the array x by the key name. This task can always be accomplished with Sorting Using a Custom Comparator. If your language is not listed here, please see the other article.

Let's start with the solution:

MODE SORTSTRUCT = PERSON;
OP < = (PERSON a,b)BOOL: age OF a < age OF b;
PR READ "prelude/sort.a68" PR;

MODE PERSON = STRUCT (STRING name, INT age);
FORMAT person repr = $"Name: "g", Age: "g(0)l$;

[]SORTSTRUCT person = (("joe", 120), ("foo", 31), ("bar", 51));
printf((person repr, shell sort(person), $l$))