While loop does not terminate when condition is met (c++)

Question

This is a problem for an online judge. It takes two inputs, i and j where the largest 3n+1 cycle length has to be found between the two inputs.

The program should terminate when x becomes 1. But when it is 1, the code does not terminate but continues to loop 1. Thanks in advance for any help.

#include <iostream>

using namespace std;

int main(){
    int i, j, temp_i, temp_j, counter, max;

    max = 0;

    cin >> i >> j;

    temp_i = i;
    temp_j = j;

    for(int x = i; x < j; x++){
        counter = 1;
        while(x != 1){
            if(x % 2 == 0){
                x = x/2;
                // cout << x << endl;
            }
            else{
                x = 3*x + 1;
            }
            counter++;
        }
        if(counter > max){
            max = counter;
        }
    }

    cout << temp_i << " " << temp_j << " " << max << endl;

    return 0;
}

Are you sure that x becomes 1 at any time? It looks like it can possibly diverges to infinity. — Yeldar Kurmangaliyev
– Yeldar Kurmangaliyev, Commented Jun 17, 2015 at 4:46

Some programmer dude · Accepted Answer · 2015-06-17 04:46:39Z

1

You have a logical error regarding your loops. Think about this: When x == 1 and the inner loop ends, then x is increased to 2 and the inner loop runs again until x == 1 when x is increased to 2 and the inner loop runs again... And so on and so on.

You need to use a second variable for the outer loop counting. Something like

for (int y = i; y < j; ++y)
{
    int x = y;
    while (x != 1)
    {
        ...
    }
}

answered Jun 17, 2015 at 4:46

Some programmer dude

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