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Marwan's Tutorial Blog

My blog for technical tutorials and related stuff (from beginner to more advanced.)

Tail Recursive Functions

05 Apr 2019

This is a back to basic post motivated by some comments I’ve recently read on-line about recursive functions and the tail recursion optimization. Tail recursion is a special case of recursive functions, for which efficient optimizations exist, but optimizations are no magic and you need to know when a function is tail recursive and how you can write one.

But, before explaining this form of optimization, let’s see what is a tail recursive function.

There and Back Again

Let’s use a toy useless function as example. Say, you want to compute addition of integers, but you’re language only know how to compute +1 and -1, and of course, we want to do it recursively. Here is a quick implementation in python:

def add(x, y):
    if y == 0:
        return x
    return 1 + add(x, y - 1)

Forget about negative numbers, or cases where x == 0, we can solve them separetly.

If we run my function with 3 and 2, here are the execution steps:

add(3, 2) -> 1 + add(3, 1)
    add(3, 1) -> 1 + add(3, 0)
        add(3, 0) -> 3
              -> 1 + 3
          -> 1 + 4
-> 5

You can see that once you get the result of a recursive call, you can compute the +1 and then return your current result. We’re going there until we reach the stop case, and then back to compute the result.

Now, let’s use magical math tricks, and write a similar function:

def add(x, y):
    if y == 0:
        return x
    return add(x + 1, y - 1)

It looks pretty similar, it has the same complexity (linear on y), is it different ?

add(3, 2) -> add(4, 1)
    add(4, 1) -> add(5, 0)
        add(5, 0) -> 5
              -> 5
          -> 5
-> 5

Yes ! Once we reach y == 0, we have the final result, the rest of the steps are just a cascade of returns without any computations. This version is a tail recursive function !

A tail recursive function, is a function that returns a value without recursive call or returns the result of a recursive call directly, and nothing happen after the recursive calls.

Optimizing tail recursive functions

The optimization is pretty obvious, once we reach a direct return, we want to skip every frames of recursion and directly return from the original call.

How do we do that ?

Of course, it really depends on the way you run your language, but let’s assume that we’re in rather classic context, with a stack used for saving caller context and return address. Our context is of course composed of local variables (including parameters), the address of the beginning of the frame for the current function (the top of stack when arrived in the function, the frame pointer) and our own return address. Saving context on the stack means that we push the frame pointer and the return address and we then we can push parameters and jump to the callee. When we return from a function, we restore the frame pointer and jump back on the return address. The details are not that important, it may even be partially done by processors instruction, we don’t care.

So how do we perform the tail recursion optimization ? After the initial call, we just don’t save the context ! That way, when we return, we return as if we were in the original call. It works because we are sure that nothing happens after the return and thus we don’t need our context anymore. If you want more details, open a compilation book, this is one of the classical optimization and it’s pretty easy to implement.

It has two main impacts: we do only half of the jump (only there and no back) and we use a constant space on the stack.

OK, this is highly theoretical, isn’t it ? Can we see this optimization in different way without relying on the compilation strategy ? Let’s try.

First, let’s take a more accurate example, binary search, in C this time:

int binary_search(int *begin, int *end, int x) {
    if (begin >= end) {
        return 0; // x is not in [begin, end[
    }
    int *mid = begin + (end - begin) / 2;
    if (x == *mid) {
        return 1; // we found it !
    }
    if (x < *mid) {
        return binary_search(begin, mid, x); // search in the first half
    } else {
        return binary_search(mid + 1, end, x); // second half
    }
}

OK, the idea is to rather than call the recursive function, replace begin or end, and then jump at the beginning of the function. Let’s use goto for that (yes, I’ve heard your screams):

int binary_search(int *begin, int *end, int x) {
  _start:
    if (begin >= end) {
        return 0; // x is not in [begin, end[
    }
    int *mid = begin + (end - begin) / 2;
    if (x == *mid) {
        return 1; // we found it !
    }
    if (x < *mid) {
        end = mid;
        goto _start;
    } else {
        begin = mid + 1;
        goto _start;
    }
}

Let’s refactor the first if a bit:

int binary_search(int *begin, int *end, int x) {
  _start:
    if (begin < end) {
        int *mid = begin + (end - begin) / 2;
        if (x == *mid) {
            return 1; // we found it !
        }
        if (x < *mid) {
            end = mid;
            goto _start;
        } else {
            begin = mid + 1;
            goto _start;
        }
    }
    return 0; // x is not in [begin, end[
}

Hey, it’s looks like a while loop ! Yes, this is exactly what this optimization is about, transforming tail recursive functions into a loop !

int binary_search(int *begin, int *end, int x) {
    while (begin < end) {
        int *mid = begin + (end - begin) / 2;
        if (x == *mid) {
            return 1; // we found it !
        }
        if (x < *mid) {
            end = mid;
        } else {
            begin = mid + 1;
        }
    }
    return 0;
}

Cool, we no longer need to bother, do we ?

Remember the first function ? You can’t directly optimize it, you need to first transform it into at taill recursive one, and that’s not always obvious. The good news is that, it’s theoretically always possible, the bad news is that it’s not always that simple. For simple, single call functions, it’s a matter of adding some parameters in order to carry more information. But even in thoses cases, you need to be sure that you can reorder the operations. Let’s see a classic trap:

def list_of_int(n):
    if n == 0:
        return []
    r = list_of_int(n-1)
    r.append(n)
    return r

We can pass the result list as a parameter and transform that into a tail rec version:

def list_of_int2(n, r):
    if n == 0:
        return r
    r.append(n)
    return list_of_int2(n-1, r)

Now, we run both:

print(list_of_int(5))
print(list_of_int2(5, []))

We get:

[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1]

I let you correct it …

More calls, more problems

First, start with an easy one, Fibonacci. For that one, we need to add parameters and inverse the order of computation:

def fibo(n):
    if n < 2:
        return 1
    return fibo(n-1) + fibo(n-2)

# Becomes

def _fib(n, i, f_i, f_i1):
    if i == n:
        return f_i
    return _fib(n, i+1, f_i + f_i1, f_i)

def fibo(n):
    if n < 1: return 1
    return _fib(n, 1, 1, 1)

Note that while the function computes the same values, it is not a transformation of the first one but a different algorithm, a kind of recursive version of the loop based one.

Now, say I have a binary tree and I want to compute its size:

class Tree:
    def __init(self, key, left=None, right=None):
        self.key = key
        self.left = left
        self.right = right

def size(t):
    if t == None:
        return 0
    return 1 + size(t.left) + size(t.right)

It becomes a bit more tricky no ? Unlike Fibonacci, we can’t avoid the call on each node of the tree, we need to find a way to do them. For that, we can use a stack !

I’ll give you directly the loop version, it makes more sense:

def size(t):
    if t == None:
        return 0
    sz = 0
    stack = [t]
    while len(stack) > 0:
        n = stack.pop()
        sz += 1
        if n.right:
            stack.append(n.right)
        if n.left:
            stack.append(n.left)
    return sz

Note the order of the push, to be sure that we traverse the tree in the same order.

OK, but now, what if I want to print my tree in order (the root in the middle):

def in_order(t):
    if t == None:
        return
    in_order(t.left)
    print(t.key)
    in_order(t.right)

The trick of stack is not enough, we need to simulate the there and back again, so we really need to see each node at least twice !

def in_order(t):
    if t == None:
        return
    stack = [(t, True)]
    while len(stack) > 0:
        n, cont = stack.pop()
        if cont:
            stack.append((n, False))
            stack.append((n.left, True)) if n.left
        else:
            print(t.key)
            stack.append((n.right, True)) if n.right

And if you need to do something in-order and post-order (so between the children and after the children), you will need to push each node three times in the stack. Of course, with general trees, or graph, this becomes even more complex.

Does it really matters ?

So, yes, it matters. Of course, with binary trees, maximum size of the stack is linear in the depth of the tree which is most of the time not that big even for big trees. But what about graph ?

Here is a classic algorithm that finds the cut-points of a connected undirected graph:

def _cut_points(g, v, parent, c, pre, cuts):
    c[0] += 1
    pre[v] = c[0]
    high = c[0]
    children = 0
    for succ in g[v]:
        if pre[succ] == None:
            children += 1
            r = _cut_points(g, succ, v, c, pre, cuts)
            if parent != None and r >= pre[v]:
                cuts.add(v)
            high = min(high, r)
        elif succ != parent:
            high = min(high, pre[succ])
    if parent == None and children > 1:
        cuts.add(v)
    return high

def cut_point(g):
    cuts = set()
    c = [0]
    pre = [None] * len(g)
    _cut_points(g, 0, None, c, pre, cuts)
    return cuts

OK, it’s a DFS (depth first traversal) of an undirected graph, with pre-order counting (the array pre using c to store the counter and simulate a reference). It works with a similar idea as Tarjan’s algorithm for finding strongly connected components, but translated to undirected graphs.

The graph is represented as adjacency list: it’s an array of lists, vertices are numbered from 0 to the order of the graph and g[v] is the list of successors of v.

You can extend it to compute cut-edges and build 2-connected components. Open a good algorithms text book if you want to know more about it.

What we see here is that we have multiple recursive calls (in a loop) and we perform operation in pre-order, after each call and in post-order. So we need to meet each vertex a first time, then do something after each of it’s successors and then a last time. It’s easy to see that removing recursion won’t be easy …

Why this example ? Some years ago, for some experiments with huge graphs, I needed the cut-points and I implemented the very same algorithm (in C++). On huge graphs, highly connected, the recursion depth was pretty high and I ended up with a lot of stackoverflow. The only way to get it to work, was to derecursify it. It’s doable, it’s finally not that ugly (a big thanks to C++ iterators) and it saved my experiments !

Let’s see our python version:

def cut_points_iter(g, v = 0):
    cuts = set()
    high = [None] * len(g)
    parent_it = [0] * len(g)
    pre = [None] * len(g)
    parents = [None] * len(g)
    root_children = 0
    c = 1
    pre[v] = c
    parents[v] = v
    high[v] = c
    stack = [ (v, -1, 0) ]
    while len(stack) > 0:
        v, ret, it = stack.pop()
        if ret != -1:
            if parents[v] == v:
                root_children += 1
            else:
                if ret >= pre[v]:
                    cuts.add(v)
                high[v] = min(high[v], ret)
            it += 1
        while it < len(g[v]) and pre[g[v][it]] != None:
            if g[v][it] != parents[v]:
                high[v] = min(high[v], pre[g[v][it]])
            it += 1
        if it < len(g[v]):
            if it + 1 < len(g[v]):
                stack.append( (v, -1, it + 1) )
            succ = g[v][it]
            stack.append( (succ, -1, 0) )
            c += 1
            pre[succ] = c
            high[succ] = pre[succ]
            parents[succ] = v
            parent_it[succ] = it
        else:
            if parents[v] == v:
                if root_children > 1:
                    cuts.add(v)
                return cuts
            stack.append( (parents[v], high[v], parent_it[v]) )
    return cuts

Looks a little bit more complex, isn’t it ?

How it works ? In the original version, for each vertex, we have several steps:

  • we need to iterate on each successor:
    • if the successor has not been seen yet, we continue the traversal on it
    • when returning from a recursive call, we have some checks to do with the result
    • if the the successor is marked (already seen) we have another check
  • we have a special last step for the root
  • we have a result to return

So, the idea is to schedule as much context as possible, first vertices are clearly identified by an id, we can thus store some contextual information in arrays (high, parent_it, parents), we can then build the information we want to schedule for the next iteration: the idea is to schedule the parent and the position in of the target vertex in the its parent successors list, thus v is the parent and g[v][it] represents the current vertex. And last element, since we need to handle the return, we also had the return value to the context (ret).

With that in mind, you should be able to reconnect the different steps of the recursive version. Note that the first vertex (the root) has some special treatement that are more explicit in the iterative version (which can be misleading). It’s not an easy task if you don’t understand the original algorithm, so spend some time on it first.

Conclusion

To connect with my introduction, the first thing to remember is, yes, compilers are able to optimize tail recursive functions, yes it makes some algorithms more tractable (a bit faster but mostly it avoids stackoverflows) but, no, it’s not magical, you need to do the hardest part, make your Function tail recursive in the first place !