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prolog

How do I get the index of a number in a list in Prolog?

Just heads I'm fairly new to prolog so this might be a bit of a dumb question, but, Basically what I'm trying to accomplish is to get the index from a natural number, except 0, e.g.

list = [3,0,0,0,0,6]

3 has an index of 1 and 6 an index of 6

Thanks :D

Solution

  • You can use the built-in predicate nth1/3, as follows:

    ?- nth1(1, [one, two, three], Element).
Element = one.

?- nth1(Index, [one, two, three], two).
Index = 2 ;
false.

?- nth1(Index, [one, two, three, one], one).
Index = 1 ;
Index = 4.

?- nth1(Index, [one, two, three], Element).
Index = 1,
Element = one ;
Index = 2,
Element = two ;
Index = 3,
Element = three.

    To index from 0 (instead of 1), use nth0/3.