Question

I’m trying to solve the 3n+1 problem using VS2010 c++,in small inputs it works well,but when it reaches 113383 it overflows.

Here is the problem link.

This is the code I’m using to solve this problem :

#include <iostream>
using namespace std;
int main(void) {
    while (!cin.eof()) {
        int i, j, maxCycle = 0, tmaxCycle = 0;
        cin >> i >> j;
        for (int x = i; x <= j; x++) {
            int n = x;
            tmaxCycle = 0;
            while (n != 1) {
                if ((float)(n/2) != (n/2.0)) {
                    n = 3*n + 1;
                }
                else {
                    n /= 2;
                }
                tmaxCycle += 1;
                if (n < 0) {
                    int blah = 0; //just for the breakpoint
                } 
            }
            tmaxCycle += 1;
            if (tmaxCycle > maxCycle) {
                maxCycle = tmaxCycle;
            }
        }
        cout << i << "\t" << j << "\t" << maxCycle << endl;
    }
    system("pause");
}

I made a breakpoint at line 15,and in this point values overflows

n=-1812855948 

Answer 1

Use 64-bit unsigned integers. If those overflow, use a bignum library like the GNU Multiple Precision Library. Bignums give you unlimited precision and size.