Find the smallest subsequence of k distinct elements in a list

Question

I am super new to sml. I am trying to write a simple code that takes an array of 5 positions with certain numbers and returns the length of the smallest subarray that contains all numbers. However I am getting many error messages that I cannot find in Google. Can anyone help me? The code is the following

 fun Min x y = if x>y then return y else return x
    local
        val a = Array.array (3,0)
        val cordela = Array.array(5,0)
        val k=0
        val front=0
        val tail=0
        val min=5
        update(cordela,0,1)
        update(cordela,1,3)
        update(cordela,2,3)
        update(cordela,3,2)
        update(cordela,4,1)
    in   
        fun loop front = 
            case k>3 of
                if sub(a,sub(cordela,front)-1) = 0 then k=k+1 else()
                update(a,sub(cordela,front)-1),sub(a,sub(cordela,front)-1)+1)
                front = front +1
            | 
                min= Min (front-tail) min
                if sub(a,sub(cordela,front)-1) = 0 then k=k-1 else()
                update(a,sub(cordela,front)-1),sub(a,sub(cordela,front)-1)-1)
                tail=tail+1

            if 5>front then loop front+1 else min
    end 

The error messages that I get are:

pl2.sml:16.13-16.15 Error: syntax error: replacing  OF with  LBRACKET
pl2.sml:18.36 Error: syntax error: inserting  LPAREN
pl2.sml:20.4 Error: syntax error: replacing  BAR with  EQUALOP
pl2.sml:22.5 Error: syntax error: inserting  LPAREN
pl2.sml:26.4 Error: syntax error: inserting  LPAREN
pl2.sml:27.2 Error: syntax error found at END

Edit: I am trying to write this code in sml. It is written in c++

while(front < N){
        if( k < K ){
            if ( e[cordela[front]-1] == 0 ) k += 1;
            e[cordela[front]-1] +=1;
            front++ ;
        }
        else{
            min = MIN(front - tail ,min);
            if ( e[cordela[tail]-1] ==1 ) k -= 1;
            e[cordela[tail]-1] -= 1;
            tail++;
        }
    }

Answer 1

As John Coleman says, SML/NJ will not give very helpful error messages. You could try and install Moscow ML instead, since it gives better error messages. Unfortunately, there are some things wrong with this code at a syntactic level that makes it difficult for the compiler to give a meaningful error. Here are some hints to get the syntax right so that you can focus on the algorithm problems:

• Don't use local, use let.
• Match each ( with a ); you have too many )s.
• Declare fun loop ... = ... inside let and in.

• Once you've done that, a template for the function that solves your problem could look like:

  fun smallest_subarray (needles : Array.array, haystack : Array.array) =
      let
        val ... = ...
        fun loop ... = ...
      in
        if Array.length needles > Array.length haystack
        then ...
        else loop ...
      end
  

What if there's no solution to the problem, what will the function return? ~1? NONE?

If you're trying to convert a C++ program to SML, try and include the function part in such a way that it's obvious what identifiers are arguments to the function, and try to name them logically; I have no idea what cordela, e and k are, or if N is a function of the size of the input array, or a constant.

Since an idiomatic solution in SML uses recursion (the function calling itself) rather than iteration (while), you are dealing with both a non-trivial algorithm problem and another paradigm. Try instead to solve a similar, but simpler problem where the algorithm is more trivial and apply the recursion paradigm.

For example, try and write a function that finds the position of an element in a sorted array using binary search:

fun find x arr =
    let
      fun loop ... = ...
    in
      loop ...
    end

The loop function would take the search bounds (e.g. i and j) as argument and return either SOME i if x is found at position i, or NONE. You could extend this problem in the direction of your original problem by then trying to write a function that determines if an input array, needles, occurs in another input array, haystack, in the order given in needles. You could first assume that needles and haystack are sorted, and then assume that they're not.