Kotlin tail recursive function causing stack overflow - recursion

I was working on this easy problem to practice basic Kotlin, and I ran into a stack overflow with the following code on the recursive return line:
class Solution {
fun isPalindrome(s: String): Boolean {
val cleaned = s.toLowerCase().replace(Regex("[^a-z0-9]"), "")
tailrec fun isPalindrome(start: Int, end: Int): Boolean {
if (start >= end) return true
return cleaned[start] == cleaned[end] && isPalindrome(start+1, end-1)
}
return isPalindrome(0, cleaned.length-1)
}
}
My understanding of tailrec is that it's supposed to convert my recursive function into an iterative one, which wouldn't be susceptible to this sort of crash. If I didn't implement tail recursion correctly, the compiler is supposed to issue an error.
Can someone explain to me why this crashes on large inputs, just like a standard recursive call would?

This behavior looks like a missing optimization of tail calls in short circuiting operators, where the fact that the last operand is being evaluated means that the expression result doesn't depend anymore on the previous operands.
Meanwhile you can rewrite your return statement as
return if (cleaned[start] != cleaned[end]) false else isPalindrome(start+1, end-1)
to get the same result + tail call optimization.

Related

In functional programming, is "saving the state" of an algorithm at the recursive function argument cheating?

for instance, lets suppose we had to write an algorithm to get the max value of an array of integers, could we still call the code functional if we make the recursive function return various information that simulates an assignment to a global object? an exemple:
function getMax(array, props={}) {
const {index = 0, actualMax = array[0]}= props ///initial props
const arrayNotEnded = array[index + 1] !== undefined
if (arrayNotEnded) {
const maxOf= (a, b) => a > b ? a : b
const newMax = maxOf(actualMax, array[index+1])
const nextIndex = index+1
return getMax(array, {index:nextIndex, actualMax:newMax} )
}else return actualMax
}
a funny thing about that is, in Haskell, we cannot have optional arguments, so this logic would not be something cool to work with, since we would have to pass the initial props every time we would need to call this function.
Yes, you could consider it cheating, but this is a well-known technique in functional programming, the accumulator argument [1][2][3]. Remember: code doesn't become functional by not having state, functional programming is all about making state explicit. There's no better way of doing that than by making it a parameter of your function.
Your code has some other problems, though. Most prominently, the state should be internal to your function, only being passed to a helper function (that might be locally declared or separate) but not as part of your function's public interface. This also prevents confusing your helper function by passing invalid state (e.g. out-of-bound indices). And yes, also the optional parameter smells - not because you think this is not possible in Haskell (it is, using Maybe), but because it can be forgotten or passed mistakenly. Instead, the helper function should have a required state parameter, and getMax should have none.
Last but not least, you should avoid out-of-bounds indexed access on arrays - check the length to know where the end is, don't compare to undefined. This includes unconditionally accessing array[0] - that makes it very easy to overlook that your function can return undefined. Make this error condition explicit as well.
Here's how I'd write it:
function getMax(array) {
if (!array.length)
throw new Error("array must be non-empty");
else
return maxFrom(1, array[0]);
function maxFrom(index, max) {
if (index < array.length)
return maxFrom(index+1, array[index] > max ? array[index] : max);
else
return actualMax
}
}
Even better than throwing exceptions would be if you'd had an algebraic data type at hand that you could return to represent the error-or-result.

What happens after non-tail recursion bottoms out

I am new to recursion, and have been looking at non-tail recursion. I believe this is where the recursive call is not at the end of the function. I was looking at some recursive code examples, and I am confused about when and how the code after the recursive call is executed. To be more specific, when the current line of the program reaches the recursive call, is that last bit of code executed before actually executing the entire function again, or does the current line of execution immediately jump back to the beginning of the function, only to execute that remaining bit of code after the entire recursion bottoms out? Can anybody please clarify how this works for me?
Imagine this code here: (I'm using JS here, but it applies to all eager languages)
function add(a, b) {
return a + b;
}
function sub(a, b) {
return a - b;
}
function fib(n) {
if (n < 2) {
return n;
} else {
return add(
fib(sub(n, 2)),
fib(sub(n, 1)),
);
}
}
console.log(fib(10));
Now when n is 2 or more we we add the result of calling fib on the two predecessors to n. The order between some of the calls are JS engines choice, but sub needs to be called and return before fib can get a value and add needs to be called when both it's arguments have results. This can be rewritten as tail calls using continuation passing style:
function cadd(a, b, k) {
k(a + b);
}
function csub(a, b, k) {
k(a - b);
}
function cfib(n, k) {
if (n < 2) {
k(n);
} else {
csub(n, 2, n2 =>
cfib(n2, rn2 =>
csub(n, 1, n1 =>
cfib(n1, rn1 =>
cadd(rn2, rn1, k)))));
}
}
cfib(10, console.log);
Here you see the order which was engines choice is explicit left to right. Each call does one thing then the continuation is called with that result. Notice also that this never returns anything. "return" is only done by continuation, yet it does the exact same thing as the previous code.
In many languages they would compile to the same.

tail rec kotlin list

I'm trying to do some operations that would cause a StackOverflow in Kotlin just now.
Knowing that, I remembered that Kotlin has support for tailrec functions, so I tried to do:
private tailrec fun Turn.debuffPhase(): List<Turn> {
val turns = listOf(this)
if (facts.debuff == 0 || knight.damage == 0) {
return turns
}
// Recursively find all possible thresholds of debuffing
return turns + debuff(debuffsForNextThreshold()).debuffPhase()
}
Upon my surprise that IDEA didn't recognize it as a tailrec, I tried to unmake it a extension function and make it a normal function:
private tailrec fun debuffPhase(turn: Turn): List<Turn> {
val turns = listOf(turn)
if (turn.facts.debuff == 0 || turn.knight.damage == 0) {
return turns
}
// Recursively find all possible thresholds of debuffing
val newTurn = turn.debuff(turn.debuffsForNextThreshold())
return turns + debuffPhase(newTurn)
}
Even so it isn't accepted. The important isn't that the last function call is to the same function? I know that the + is a sign to the List plus function, but should it make a difference? All the examples I see on the internet for tail call for another languages allow those kind of actions.
I tried to do that with Int too, that seemed to be something more commonly used than addition to lists, but had the same result:
private tailrec fun discoverBuffsNeeded(dragon: RPGChar): Int {
val buffedDragon = dragon.buff(buff)
if (dragon.turnsToKill(initKnight) < 1 + buffedDragon.turnsToKill(initKnight)) {
return 0
}
return 1 + discoverBuffsNeeded(buffedDragon)
}
Shouldn't all those implementations allow for tail call? I thought of some other ways to solve that(Like passing the list as a MutableList on the parameters too), but when possible I try to avoid sending collections to be changed inside the function and this seems a case that this should be possible.
PS: About the question program, I'm implementing a solution to this problem.
None of your examples are tail-recursive.
A tail call is the last call in a subroutine. A recursive call is a call of a subroutine to itself. A tail-recursive call is a tail call of a subroutine to itself.
In all of your examples, the tail call is to +, not to the subroutine. So, all of those are recursive (because they call themselves), and all of those have tail calls (because every subroutine always has a "last call"), but none of them is tail-recursive (because the recursive call isn't the last call).
Infix notation can sometimes obscure what the tail call is, it is easier to see when you write every operation in prefix form or as a method call:
return plus(turns, debuff(debuffsForNextThreshold()).debuffPhase())
// or
return turns.plus(debuff(debuffsForNextThreshold()).debuffPhase())
Now it becomes much easier to see that the call to debuffPhase is not in tail position, but rather it is the call to plus (i.e. +) which is in tail position. If Kotlin had general tail calls, then that call to plus would indeed be eliminated, but AFAIK, Kotlin only has tail-recursion (like Scala), so it won't.
Without giving away an answer to your puzzle, here's a non-tail-recursive function.
fun fac(n: Int): Int =
if (n <= 1) 1 else n * fac(n - 1)
It is not tail recursive because the recursive call is not in a tail position, as noted by Jörg's answer.
It can be transformed into a tail-recursive function using CPS,
tailrec fun fac2(n: Int, k: Int = 1): Int =
if (n <= 1) k else fac2(n - 1, n * k)
although a better interface would likely hide the continuation in a private helper function.
fun fac3(n: Int): Int {
tailrec fun fac_continue(n: Int, k: Int): Int =
if (n <= 1) k else fac_continue(n - 1, n * k)
return fac_continue(n, 1)
}

How can I avoid using goto for a series of prompts?

I'm writing a console based application that prompts a user for a series of questions. E.g:
"Enter a record to open:"
"Do you want to do X?"
"Do you want to do Y?"
"Are you sure you want to continue?"
If the user enters nothing at any prompt I want to go up one level. This is easy using goto. The only other way I can think of to do it is nested for loops which looks far uglier and gets pretty unwieldy for more than a couple of prompts. There must be an easy way to do this though, I just can't think of it.
You're basically writing a very simple state machine - use functions to represent every state: (I'll use random pseudocode, since you didn't specify a language)
get_information():
return get_record()
ask_record():
record = read_line()
return () if !record
return ask_y(record)
ask_x(record):
x = read_line()
return ask_record() if !x
return ask_y(record, x)
ask_y(record, x):
y = read_line()
return ask_x(record) if !y
return ask_continue(record, x, y)
ask_continue(record, x, y)
continue = read_line()
return ask_y(record, x) if !continue
return (record, x, y)
this is a trivial approach. In some languages the call stack will grow, in others it won't. If you have a language which causes the stack to grow, you can use trampolines to prevent it, by rewriting get_information to do:
x = get_information
while x is function:
x=x()
return x
ask_x(record):
x = read_line()
return (lambda: ask_record()) if !x
return (lambda: ask_y(record, x))
Or even abstracting the question and memory address of result in some question structure:
struct question {
question *next, *prev;
char prompt[];
char **result;
}
and then running in a loop, calling question* run_question(question*), going to ->next, or ->prev depending on the answer, until the result is NULL (as a stop condition, when results are filled in and no questions are left).
The last solution is probably the most "normal one" if you use an imperative language with direct pointer access.
Write it recursively instead of iteratively.
write it as a simple state machine.
while(running)
{
if (state == INIT)
{
out("enter record");
state = DO_X;
}
else if (state == DO_X)
{
do whatever for x.
state = WHATEVER_NEXT_STATE_IS;
}
}

Base case in a recursive method

A theoretical question here about the base or halting case in a recursive method, what's its standards?
I mean, is it normal not to have body in it, just a return statement?
Is it always like the following:
if (input operation value)
return sth;
Do you have different thoughts about it?
The pattern for recursive functions is that they look something like this:
f( value )
if ( test value )
return value
else
return f( simplify value )
I don't think you can say much more than that about general cases.
The base case is to terminate the loop (avoid becoming an infinite recursion). There's no standard in the base case, any input that is simple enough to be solved exactly can be chosen as one.
For example, this is perfectly valid:
int factorial (int n) {
if (n <= 5) {
// Not just a return statement
int x = 1;
while (n > 0) {
x *= n;
-- n;
}
return x;
} else {
return n * factorial(n-1);
}
}
In some cases, your base case is
return literal
In some cases, your base case is not simply "return a literal".
There cannot be a "standard" -- it depends on your function.
The "Syracuse Function" http://en.wikipedia.org/wiki/Collatz_conjecture for example,
doesn't have a trivial base case or a trivial literal value.
"Do you have different thoughts about it??" Isn't really a sensible question.
The recursion has to terminate, that's all. A trivial tail recursion may have a "base case" that returns a literal, or it may be a calculation. A more complex recursion may not have a trivial "base case".
It depends entirely on the particular recursive function. In general, it can't be an empty return; statement, though - for any recursive function that returns a value, the base case should also return a value of that type, since func(base) is also a perfectly valid call. For example, a recursive factorial function would return a 1 as the base value.

Resources