How to Average in Prolog (2017)

Summary

If I had to average a list of numbers, I would probably do it like this: averagelist(List, Avg) :- length(List, N), sumlist(List, Sum), Avg is Sum / N. This resembles the actual mathematical definition. Then you could just make a list of numbers and average that. @lurker is right, this is a terrible way to go, but it would work: average(N, Avg) :- findall(I, between(1, N, I), Is), averagelist(Is, Avg). This is building up abstraction. But of course, this is for a class and the important thing is to not use Prolog or learn declarative programming or solve actual problems but rather to perform meaningless inductive calisthenics to prove you understand recursion. So a “better” (i.e. worse but likelier to be accepted by a clueless professor) solution is to take the procedural code: average(list) ::= sum := 0 count := 0 repeat with i ∈ list sum := sum + i count := count + 1 return sum / count and convert it into equivalent Prolog code: average(List, Result) :- average(List, 0, 0, Result). average([], Sum, Count, Result) :- Result is Sum / Count. average([X|Xs], Sum, Count, Result) :- Sum1 is Sum + X, succ(Count, Count1), average(Xs, Sum1, Count1, Result). The list result of my findall/3 must be delicately hand-assembled using only tools available in the 18th century lest anyone develop a sense that Prolog can be used effectively in fewer than 40 lines of code: iota(N, Result) :- iota(1, N, Result). iota(X, Y, [X|Result]) :- X < Y, succ(X,X1), iota(X1, Y, Result). iota(X, X, [X]). Then you could build averagelist/2 without the taint of library code (of course, you’ll have to write length/2 and sumlist/2, and probably member/2 even though it isn’t used, but just because it’s clever and useful and it sort of seems like it should be in the source file next to all this other stuff we might need), but it would look generally like this: average(N, Avg) :- iota(N, List), averagelist(List, Avg). Now, of course, it will be pointed out that the introduction of additional predicates...