Prolog cheat sheet

Idiom ID Imports Comments Links

The snippets are under the CC-BY-SA license.
Please consider keeping a bookmark
(instead of printing)

		Prolog
1	Print Hello World Print a literal string on standard output	`write('Hello, world!\n')` Alternative implementation: `format("Hello world~n")`
2	Print Hello 10 times Loop to execute some code a constant number of times	`:- initialization(main). main :- loop(10). loop(0). loop(N) :- N>0, write('Hello') , nl, N1 = N - 1, loop(N1).`
12	Check if list contains a value Check if the list contains the value x. list is an iterable finite container.	`member(X, [One]).` Doc
13	Iterate over map keys and values Access each key k with its value x from an associative array mymap, and print them.	`forall(get_dict(K, D, V), format("~w:~w~n", [K, V]))` Demo
50	Make an infinite loop Write a loop that has no end clause.	`repeat, write("hello\n").` This a bit strange, repeat is a predicate that will always succeed, that is always true. Doc Alternative implementation: `?- repeat, false. % repeat/0 is built-in, but could be defined like this: repeat. repeat :- repeat.` `repeat/0` succeeds an infinite number of times, and each time is immediately foiled by false/0. The default execution strategy just tries clauses in order from left to right so this induces an infinite loop. Alternative implementation: `loop :- loop. ?- loop.` loop succeeds if loop succeeds, so when we ask the top level if loop succeeds, it proceeds to ask if loop succeeds.
71	Echo program implementation Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline. The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.	`main(Argv) :- echo(Argv). echo([]) :- nl. echo([Last]) :- write(Last), echo([]). echo([H\|T]) :- write(H), write(' '), echo(T).`
112	Iterate over map entries, ordered by keys Print each key k with its value x from an associative array mymap, in ascending order of k.	`dict_pairs(D, _, Ps), forall(member(K-V, Ps), format("~w:~w~n", [K, V]))` Map a dict to an ordered list of pairs, and print them to standard output using forall Demo
113	Iterate over map entries, ordered by values Print each key k with its value x from an associative array mymap, in ascending order of x. Multiple entries may exist for the same value x.	`dict_pairs(D, _, Ps), transpose_pairs(Ps, TPs), keysort(TPs, Val_sorted), forall(member(V-K, Val_sorted), format("~w:~w~n", [V, K]))` Transpose the key-value pair list, sort by key (a stable sort), then print Demo
314	Fill array with value Set all the elements in the array x to the same value v	`set_all_elements_to_same_value(X, V) :- length(X, Length), length(FilledList, Length), maplist(=(V), FilledList, X).`