Why does this Standard ML bubblesort's loop only execute once?

Question

So I tried to implement bubblesort using ML's reference types. I compiled the code in Poly/ML and it seems that the "while(!flag)" loop only executes once for any input.

For example: [2,3,1] gets "sorted" to [2,1,3], i.e. the first loop worked but the second never ran.

"Flag up" got printed once.

What's wrong?

Thanks.

fun bubbleSort l =        (* l being a list of references *)
let
    val it = ref 0        (* iterator variable *)
    val len = length l
    val flag = ref true   (* to be set to true when a swap is made *)
in
    while (!flag) do
    (
        flag := false;
        while ((!it) < (len-1)) do
        (
            if (get l (!it)) > (get l ((!it)+1))
            then (swap_next l (!it); flag := true; TextIO.print "Flag up\n")
            else ();
            it := (!it) + 1
        )
    )
end;

Full code of the module I wrote, if needed, can be found here.

Answer 1

Bubblesort repeatedly scans over the same array. Your inner loop scans over it just once but never resets the counter it. Before the line

while ((!it) < (len-1)) do

You should put the line

it := 0;