Études for Erlang

Étude 6-1: Recursive Iteration through a List

In a module named stats, write a function named minimum/1. It takes a list of numbers as its argument and returns the smallest value. This function already exists in the lists module (lists:min/1), but it’s a good exercise in learning about recursion.

Here’s the pseudocode.

• Call function minimum/2, which takes the list as its first argument and the “smallest number so far” (the current candidate) as its second argument. The starting value will be the head of the original number list passed to minimum/1.
• When the list passed to minimum/2 is empty, the final result is the current candidate. This stops the recursion.

• If the list passed to minimum/2 is not empty, then see if the head of the list is less than the current candidate.

  • If so, call minimum/2 with the tail of the list as the first argument and the list head (the new “smallest number”) as the second argument.
  • If not, call minimum/2 with the tail of the list as the first argument and the current candidate (still the “smallest number”) as the second argument.

Unlike most examples in Introducing Erlang, passing an empty list to this function will make it crash. That’s a reasonable thing to do, as an empty list can’t really be said to have a minimum value.

1> c(stats).
{ok,stats}
2> N = [4, 1, 7, -17, 8, 2, 5].
[4,1,7,-17,8,2,5]
3> stats:minimum(N).
-17
4> stats:minimum([]).
** exception error: bad argument
     in function  hd/1
        called as hd([])
 in call from stats:minimum/1 (stats.erl, ...