Odtnhj

I tried to write code to find the minimum of an array (having > 1 element) in Prolog.

min([H,T|], H) :- H < T.
min([H,T|], T) :- H >= T.
min([H|T], M) :- min(T, N), min([H|N], M).

But when I try to run it for :

min([1,2,3], K)

It goes into the recursion where single array is involved. But my 1st statement is is to prevent that from happening. I am assuming rules defined first will get the higher priority when prolog is looking to reduce.

Can somebody help me with what the mistake might be ?

Link to code : https://swish.swi-prolog.org/p/min-array.pl

It is easy to see mistake if you go step by step. But you probably already tried this so I will not try to repeat what you already did.

But yes rules have priority but no this priority is not how you think it is. I try very hard to understand what those first two lines do in your head but this is too difficult for me. But if you just run the program or trace you can see what it does on computer and try to compare and contrast to what it does in your head.

But to solve the problem you must have two different predicates and not just one predicate.

One predicate is to get min of two things.

Second predicate is to reduce list to min of many things by applying first predicate to first and second, then min of min of first and second and third, .... like this:

min([x0, x1, x2, ...) = min(min(min....(x0, x1), x2), ...)

But first min is not second min. In Prolog first min is arithmetic function.

?- X is min(1, 2).
X = 1.

And min([1,2,3], X) will be success if X is min(min(1, 2), 3). And probably a failure if not.

How to write min to make list into min(min(min(...)))? is a good question. I find in SWI very nice foldl predicate so I don't reduce myself but you could write reduce if you don't like to reduce with foldl.

min([H|T], Min) :- foldl(min, T, H, Min).
min(A, B, Min) :- Min is min(A, B).

So many mins... maybe too many mins? I don't know. You should know better in your head how many mins are too many mins. Maybe too many in this code?