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I tried a recursive factorial algorithm in Rust. I use this version of the compiler:
rustc 1.12.0 (3191fbae9 2016-09-23)
cargo 0.13.0-nightly (109cb7c 2016-08-19)
Code:
extern crate num_bigint;
extern crate num_traits;
use num_bigint::{BigUint, ToBigUint};
use num_traits::One;
fn factorial(num: u64) -> BigUint {
let current: BigUint = num.to_biguint().unwrap();
if num <= 1 {
return One::one();
}
return current * factorial(num - 1);
}
fn main() {
let num: u64 = 100000;
println!("Factorial {}! = {}", num, factorial(num))
}
I got this error:
$ cargo run
thread 'main' has overflowed its stack
fatal runtime error: stack overflow
error: Process didn't exit successfully
How to fix that? And why do I see this error when using Rust?
592
The most-common cause of stack overflow is excessively deep or infinite recursion, in which a function calls itself so many times that the space needed to store the variables and information associated with each call is more than can fit on the stack.
A general method for avoiding a stack overflow is to include what's called a "bootstrap condition" within the recursion. It's some condition that gets hit every time the function calls itself. You set the condition to something that causes the function to return when some state is reached, thereby unwinding the stack.
The factorial function can be written as a recursive function call. Recall that factorial(n) = n × (n – 1) × (n – 2) × … × 2 × 1. The factorial function can be rewritten recursively as factorial(n) = n × factorial(n – 1).
Recursive functions use something called “the call stack.” When a program calls a function, that function goes on top of the call stack. This is similar to a stack of books. You add things one at a time. Then, when you are ready to take something off, you always take off the top item.
Rust doesn't have tail call elimination, so your recursion is limited by your stack size. It may be a feature for Rust in the future (you can read more about it at the Rust FAQ), but in the meantime you will have to either not recurse so deep or use loops.
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