Question

I’ve been using Python the past couple of months and now am trying to give F# a whirl. Only…I don’t really get it. I’ve been reading documentation for the past few days and still don’t completely understand how to accomplish basic tasks.

I’ve been following the tutorials on tryfsharp.org and fsharp.net.

For instance, how would I accomplish this basic task written in Python, in F# instead?

unsorted = [82, 9, 15, 8, 21, 33, 4, 89, 71, 7]
sorted = []
for n in range(1,len(unsorted)):
    lowest = 0
    for i in range(0,len(unsorted)-1):
        if unsorted[i] < unsorted[lowest]:
            lowest = i
    sorted.append(unsorted[lowest])
    del unsorted[lowest]
print sorted

Answer 1

When porting code from an imperative language to a functional language, you should try to convert the algorithm that is used in the code, rather than the code itself IMHO.

The code is doing a selection sort so you want to ask yourself, what does the selection sort do?

• find the minimum
• put it in the front of the sorted list.
• sort the rest of the items placing the results after the minimum.

So what would the code look like? This would certainly work:

let rec selection_sort = function
    | [] -> []
    | l -> let min = List.min l in                         (* find the minimum *)
           let rest = List.filter (fun i -> i <> min) l in (* find the rest *)
           let sorted_rest = selection_sort rest in        (* sort the rest *)
           min :: sorted_rest                              (* put everything together *)