Question

I’ve heard that any recursive algorithm can always be expressed by using a stack. Recently, I’ve been working on programs in an environment with a prohibitively small available call stack size.

I need to do some deep recursion, so I was wondering how you could rework any recursive algorithm to use an explicit stack.

For example, let’s suppose I have a recursive function like this

function f(n, i) {
  if n <= i return n
  if n % i = 0 return f(n / i, i)
  return f(n, i + 1)
}

how could I write it with a stack instead? Is there a simple process I can follow to convert any recursive function into a stack-based one?

Answer 1

If you understand how a function call affects the process stack, you can understand how to do it yourself.

When you call a function, some data are written on the stack including the arguments. The function reads these arguments, does whatever with them and places the result on the stack. You can do the exact same thing. Your example in particular doesn’t need a stack so if I convert that to one that uses stack it may look a bit silly, so I’m going to give you the fibonacci example:

fib(n)
    if n < 2 return n
    return fib(n-1) + fib(n-2)

function fib(n, i)
    stack.empty()
    stack.push(<is_arg, n>)
    while (!stack.size() > 2 || stack.top().is_arg)
        <isarg, argn> = stack.pop()
        if (isarg)
            if (argn < 2)
                stack.push(<is_result, argn>)
            else
                stack.push(<is_arg, argn-1>)
                stack.push(<is_arg, argn-2>)
        else
            <isarg_prev, argn_prev> = stack.pop()
            if (isarg_prev)
                stack.push(<is_result, argn>)
                stack.push(<is_arg, argn_prev>)
            else
                stack.push(<is_result, argn+argn_prev>)
     return stack.top().argn

Explanation: every time you take an item from the stack, you need to check whether it needs to be expanded or not. If so, push appropriate arguments on the stack, if not, let it merge with previous results. In the case of fibonacci, once fib(n-2) is computed (and is available at top of stack), n-1 is retrieved (one after top of stack), result of fib(n-2) is pushed under it, and then fib(n-1) is expanded and computed. If the top two elements of the stack were both results, of course, you just add them and push to stack.

If you’d like to see how your own function would look like, here it is:

function f(n, i)
    stack.empty()
    stack.push(n)
    stack.push(i)
    while (!stack.is_empty())
        argi = stack.pop()
        argn = stack.pop()
        if argn <= argi
            result = argn
        else if n % i = 0
            stack.push(n / i)
            stack.push(i)
        else
            stack.push(n)
            stack.push(i + 1)
    return result