Segmentation Fault C++ (array too large?)

Question

I'm working on the Project Euler Problem 14 , where I need to find the longest collatz sequence under 1,000,000. I've come up with an algorithm that works for smaller numbers (say, 100) that stores each collatz number from 1 - 100 into an array and uses that array as a reference to speed up the computations for higher numbers. My code is as follows:

#include <iostream>
using namespace std;

long even(long n){ //the even-collatz function
    n=n/2;
    return n;
}

long odd(long n){ //the odd collatz function
    n=3*n+1;
    return n;
}

int main(){
    long  x, c=0, y[1000000]; // x= the number we are finding the collatz number of, c a counter that keeps track of how many steps we've taken in the sequence, y is an array to store the collatz numbers.

    for (x=1; x<1000000; x++){ //iterates from x=1 to 1 million
            long a = x;     //sets a number a equal to the number we are currently trying to find the collatz number of
            long b = a;     
            c=0;                    //intializes counter at 0 
            while (a!=0){           //loops infinitely; the only way to exit is through a break.
                    if (a%2==0){    // detects if the number is even
                            a=even(a);      //applies the even-collatz function if so; sets x=x/2
                            c=c+1;
                            if (y[a]!=0){   // checks if the collatz number of x is already discovered
                                    y[b]=c+y[a]; //adds the current number of steps to the collatz number of x and 
                                    break;  //exits the while loop
                            }

                    }
                    else if (a==1){         //checks if the new x is equal to one and
                            y[b]=c;         //if it is, it writes the current value of c to y[b] and
                            break;          // exits the loop
                    }
                    else if (a%2==1){       //same as the "even" block, except for odd numbers 

                            a=odd(a);
                            c=c+1;
                            if( y[a]!=0){
                                    y[b]=c+y[a];
                                    break;
                            }

                    }
            //this is the end of the while loop; we've applied the collatz function as many times as we've needed to to x, and incremented the counter each time
            }
  }

    long z;
    for (int n=0;n!=100;n++){
            if (y[n+1]>y[n]){
                    z=y[n+1];
            }
    }
    cout << z << "\n";


}

The issue I'm having is that I get a segfault after x=1818 in the for loop. Through debugging, I've found that how quickly the segfault occurs depends on the size of array y, so I'm assuming that the array is just too big. From my (basic) understanding of segfaults, I think I'm just accessing memory that I'm "not allowed". Is there any way for me to circumvent this, or should I just start working towards another solution to this problem? I'm compiling using g++ on Ubuntu studio.

Answer 1

This array is probably too big for your system's default stack size; the simplest fix is to change its definition to:

std::vector<long> y(1000000);

and everything else can stay the same. You could use y.size() instead of the magic number 1000000 later in your loop.

Answer 2

For a starting number under N collatz sequence can go way beyond N . For N == 1000000 consider x == 333335 .

I would suggest you to make y a vector<int> and expand it dynamically, or just make it unordered_map<int, int> .

Answer 3

If y was too big for your stack, you would get a stack overflow exception as soon as main tried to run.

Your problem is more likely that a gets bigger than the size of y . When I ran it through the debugger, a was 1417174 when x was 4255, so you might have a problem with your algorithm.

That said, you should either allocate it yourself, or make it static, as there is no guarantee that whatever compiler Project Euler uses will allow such a large stack size.