Question

Hi all im new to programming and im doing a problem for learning and enjoyment. Im a bit stuck at this point.. The problem is from Introduction to Programming using Sml 5.9

I want to split a list of [x1, x2, x3, ... ,xn] = ([x1, x3,....], [x2, x4,...])

This is what I have made so far:

fun split [] =  []
  | split (x1::x2::x3::x4::xs) = ([x1, x3], [x2, x4])::split xs



val test1split = split [1, 1, 2, 3];

From this I get:

[([1, 2], [1, 3])]…. (I want a tuple with splitting list and not this obviously)

If there are more than 4 elements then the function doesn’t work. Maybe I need a helper function to sort even and odd elements in a list first? I hope someone can help me with tracking my mind in the correct direction, until then I keep trying.

Answer 1

I’ll try not to give too much away, but here are some tips:

• You need two base cases – one for [], one for [x].
• Your general case only needs to deal with two elements, not four (putting one in the first list, and one in the second)
• At the moment, you’ve got split returning a list, rather than a tuple. The result of your first base case should be ([],[]).
• In the general case, the recursive split xs will return a tuple (ys,zs). You need to extract these values, and build the resulting tuple in terms of ys, zs, x1 and x2.

(Edit) A couple of points on your revised solution:

• You only need to deal with two elements at a time – the general case should be split x1::x2::xs
• split [x,y] is handled by the general case – no need for another base case.
• You’re missing the recursive call! Elements are ending up in both lists because you’re putting xs directly into both halves of your output – you need to split it first. Start with
  let (ys, zs) = split xs in ...