CODEWITHSUNDEEP
listocaml
송재민
송재민

Reputation: 63

I'm in trouble with my nth-element function in OCaml. I can't find what's wrong

the function nth must work like this :

# nth [1;2;3;4;5] 0;;
- : int = 1
# nth [1;2;3;4;5] 1;;
- : int = 2
# nth [1;2;3;4;5] 2;;
- : int = 3
# nth [1;2;3;4;5] 3;;
- : int = 4

I wrote this function named nth like this : 

let rec nth l n =
    match l with
    |[] -> raise (Failure "list is too short")
    |hd::tl -> 
    if n > 0 then nth tl n-1 else hd

So I typed nth [1;2;3;4;5] 3 and the expected output was 4 but the console said "list is too short"

My expected algorithm is this : nth [1;2;3;4;5] 3

1::[2;3;4;5] 3 > 0 is true, so nth [2;3;4;5] 2 is called.
2::[3;4;5] 2 > 0 is true, so nth [3;4;5] 1 is called.
3::[4,5] 1 > 0 is true, so nth [4;5] 0 is called
4::[5] 0 > 0 is false, so return 4
so nth [1;2;3;4;5] 3 = 4

What is wrong with my code?

Upvotes: 1

Views: 811

Answers (1)

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66823

This is a precedence problem. The expression nth tl n-1 is parsed like this: (nth tl n) - 1. You need parentheses like this: nth tl (n - 1).

If you try your current code with a list of strings (say), you'll get a possibly more helpful error message from the toplevel.

Other than this one problem your code looks excellent btw.

Upvotes: 3

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