Question

Given an arbitrary number, how can I process each digit of the number individually?

Edit
I’ve added a basic example of the kind of thing Foo might do.

For example, in C# I might do something like this:

static void Main(string[] args)
{
    int number = 1234567890;
    string numberAsString = number.ToString();

    foreach(char x in numberAsString)
    {
        string y = x.ToString();
        int z = int.Parse(y);
        Foo(z);
    }
}

void Foo(int n)
{
    Console.WriteLine(n*n);
}

Answer 1

Have you heard of div and mod?

You’ll probably want to reverse the list of numbers if you want to treat the most significant digit first. Converting the number into a string is an impaired way of doing things.

135 `div` 10 = 13
135 `mod` 10 = 5

Generalize into a function:

digs :: Integral x => x -> [x]
digs 0 = []
digs x = digs (x `div` 10) ++ [x `mod` 10]

Or in reverse:

digs :: Integral x => x -> [x]
digs 0 = []
digs x = x `mod` 10 : digs (x `div` 10)

This treats 0 as having no digits. A simple wrapper function can deal with that special case if you want to.

Note that this solution does not work for negative numbers (the input x must be integral, i.e. a whole number).