CODEWITHSUNDEEP
prolog
Krab
Krab

Reputation: 6756

Prolog - is built-in command

I don't understand, why the second doesn't work. I thought in first example, the II variable isn't needed, but looks like it is, but i don't know why? If i call findex([1,2,5,4], 5, I), than first example returns correct index 3 and second returns false. Can somebody help me to understand it?

findex([X|_], X, 1).
findex([_|T], MAX, INDEX) :- findex(T, MAX, II), INDEX is II +1.

findex([X|_], X, 1).
findex([_|T], MAX, INDEX) :- findex(T, MAX, INDEX), INDEX is INDEX +1.

Upvotes: 0

Views: 50

Answers (1)

Sergii Dymchenko
Sergii Dymchenko

Reputation: 7209

The second example will not work.

The key thing to understand is that both INDEXs in INDEX is INDEX +1 are the same, they must have the same value. It's like variables in algebra - all Xs in an equation means the same value.

Another way to think of it is that all variables in Prolog are "assign once" variables: after a variable has its value it can't be changed without backtracking.

Upvotes: 2

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