SWI-Prolog -- sub

Availability:built-in

[ISO]sub_atom(+Atom, ?Before, ?Length, ?After, ?SubAtom)

ISO predicate for breaking atoms. It maintains the following relation: SubAtom is a sub-atom of Atom that starts at (0-based index) Before, has Length characters, and Atom contains After characters after the match. The implementation minimises non-determinism and creation of atoms. This is a flexible predicate that can do search, prefix- and suffix-matching, etc. Scenarios that use this predicate often generate atoms that with a short lifetime; in such cases sub_string/5 may be a better alternative. Examples:

Pick out a sub-atom of length 3 starting a 0-based index 2:

?- sub_atom(aaxyzbbb, 2, 3, After, SubAtom).
After = 3,
SubAtom = xyz.

The following example splits a string of the form <name>=<value> into the name part (an atom) and the value (a string).

name_value(String, Name, Value) :-
    sub_atom(String, Before, _, After, "="),
    !,
    sub_atom(String, 0, Before, _, Name),
    sub_atom(String, _, After, 0, Value).

This example defines a predicate that inserts a value at a position. Note that sub_string/5 is used here instead of sub_atom/5 to avoid the overhead of creating atoms for the intermediate results.

atom_insert(Str, Val, At, NewStr) :-
    sub_string(Str, 0, At, A1, S1),
    sub_string(Str, At, A1, _, S2),
    atomic_list_concat([S1,Val,S2], NewStr).

On backtracking, matches are delivered in order left-to-right (i.e. Before increases monotonically):

?- sub_atom('xATGATGAxATGAxATGAx', Before, Length, After, 'ATGA').
Before = 1, Length = 4, After = 14 ;
Before = Length, Length = 4, After = 11 ;
Before = 9, Length = 4, After = 6 ;
Before = 14, Length = 4, After = 1 ;
false.

See also sub_string/5, the corresponding predicate for SWI-Prolog strings.