SML in list parameter functions

Question

I had two parameters. The first one was list, and the other was integer n. Our function formed a new list which contained the first n elements of the parameter list and suppose that n

([1,2,3,4],2 )  
  [1,2]

How do I do that only using -> nil, ::, @ ?

pad · Accepted Answer · 2011-10-30 19:20:16Z

1

What you describe is List.take function in SML basis library:

List.take ([1,2,3,4],2 )
[1,2]

If you want to make the function yourself:

fun take ([], _) = []
  | take (_, 0)  = []
  | take (x::xs, n) = x::take(xs, n-1)

Or to demonstrate the use of -> nil, ::, @ (the use of @ is not recommended, just for illustration purpose):

fun take (nil, _) = nil
  | take (_, 0)  = nil
  | take (x::xs, n) = [x] @ take(xs, n-1)

edited Oct 30, 2011 at 19:20

answered Oct 30, 2011 at 18:34

pad

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7 Comments

Knaas Over a year ago

of course how i do that different way ? I thought your response. I'm looking at another solution can you help me?

Knaas Over a year ago

(nil, _) _ using of this. what does it mean ?

pad Over a year ago

That is wildcard pattern; it means you don't care about its value.

Knaas Over a year ago

_ instead of this . do i use another option ?

Jesper.Reenberg Over a year ago

@pad, You properly mean the sml basis library, and not basis sml library. Your use of append (@) in that example is very bad coding style (i know he almost asked for it), however there is no need to show bad habits.

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