Number of digits in a number

Question

I am trying to write a function in SML that takes an integer and outputs the number of digits int he number. For example, user inputs 122, I want to output 3. I thought this functions would do the trick but I am getting some errors in regards to result constraints of clauses not agreeing. And right-hand-side doesn't agree and operator and operand don't agree. I am very new to ML and I do not know what I am doing wrong! Thanks in advance.

fun num_digits nil = nil
| num_digits 0 = 1
| num_digits n = 
    let 
        val x = 1
        fun f(z, a) =
            let
                val c = z+1
                val m = a div 10
            in
                if m >= 1
                then
                    f(c, m)
                else z
            end;
    in
        f(x, n)
    end;

Peter Pei Guo Peter Pei Guo · Accepted Answer · 2014-10-01T05:12:13

nil is [] (an empty list).

If you define a function like:

fun f nil = nil
| f x = 3;

That causes syntax error, since int and list are mixed up.

However the following examples are fine, since types matched perfectly:

fun f nil = 2
|f x = 3;

Or

fun f nil = nil
| f x = [3];

Number of digits in a number

3 Answers