74 lines
1.1 KiB
Markdown
74 lines
1.1 KiB
Markdown
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## Chapter 5: Arithmetic in Prolog
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Prolog provides basic arithmetic operators.
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Ex.
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```
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?- 8 is 6+2.
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yes
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?- 12 is 6*2.
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yes
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?- -2 is 6-8.
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yes
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?- 3 is 6/2.
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yes
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?- 1 is mod(7,2).
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yes
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?- X is 12/4.
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X = 3.
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```
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The operators don't actually do arithmetic:
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```
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?- X = 2 + 3.
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X = 2+3.
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```
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The default is just to do unification; `is` must be used. The arithmetic
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expression must be on the RHS. This part of Prolog is a black box that
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handles this, and isn't part of the normal KB and unification parts.
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### Arithmetic and lists
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A recursive list length calculator:
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```
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len([], 0).
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len([_|T], N) :-
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len(T, X),
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N is X+1.
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```
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A tail-recursive length calculator:
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```
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alen_([], A, A).
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alen_([_|H], A, L) :-
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A2 is A+1,
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alen_(H, A2, L).
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alen(X, L) :- alen_(X, 0, L).
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```
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Standard notes about tail recursion efficiency apply here.
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### Comparing integers
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* *x < y* → `X < Y.`
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* *x ≤ y* → `X =< Y.`
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* *x = y* → `X =:= Y.`
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* *x ≠ y* → `X =\= Y.`
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* *x ≥ y* → `X >= Y`
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* *x > y* → `X > Y`
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Note the difference between `=` and `=:=`.
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Let's write a `max` function:
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```
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