Prolog - List Access Predicates

Prolog provides various predicates to perform actions on the list. Following is the list of useful predicates to access elements of the List −

Example - member(Element, List)

member predicates succeeds if Element is a part of the List.

Output

| ?- member(b, [a, b, c]).  % true as b is member of the list.

true ? 

(15 ms) yes
| ?- member(d, [a, b, c]).  % no as d is not a member of the list. 

no
| ?- member(X, [a, b, c]).  % get all members of the List

X = a ? a

X = b

X = c

(16 ms) yes
| ?- 

Example - length(List, Length)

length predicate unifies the Length with the size of the list.

Output

| ?- length([a, b, c], L).   % gets length of List in L

L = 3

yes
| ?- length(X, 3).  % create list of three uninstantiated variables

X = [_,_,_]

yes
| ?- 

Example - select(Element, List, RemainingList)

select predicate succeeds if Element is removed from the List.

Output

| ?- select(2, [1, 2, 3, 2, 4], R).

R = [1,3,2,4] ? 

yes
| ?- 

Example - nth0(Index, List, Element)

nth0 predicate is used to access 0 index based element of the list.

Output

| ?- nth0(1, [a, b, c, d], E).

E = b

yes
| ?-

Example - nth1(Index, List, Element)

nth1 predicate is used to access 1 index based element of the list.

Output

| ?- nth1(1, [a, b, c, d], E).

E = a

yes
| ?-