I am using modulo operator, but it still giving me a negative number-CodePudding

I am trying to solve a programming problem in c (version : (MinGW.org GCC Build-2) 9.2.0)
I am using modulo operator to give answer in int range but for 6 ,it is giving me -ve answer
why is this happening??
my code :

#include <cmath>
#include <iostream>

using namespace std;

int balancedBTs(int h) {
    if (h <= 1) return 1;
    
    int x = balancedBTs(h - 1);
    int y = balancedBTs(h - 2);
    int mod = (int)(pow(10, 9)   7);
    
    int temp1 = (int)(((long)(x) * x) % mod);
    int temp2 = (int)((2 * (long)(x) * y) % mod);

    int ans = (temp1   temp2) % mod;
    
    return ans;
}
int main()
{
    int h;
    cin >> h;
    cout << balancedBTs(h) << endl;
    return 0;
}

output :

CodePudding user response：

The code makes two implicit assumptions:

int is at least 32 bit (otherwise the 1,000,000,007 for mod will not fit)
long is bigger than int (to avoid overflows in the multiplication)

Neither of these assumptions are guarantee by the standard https://en.cppreference.com/w/cpp/language/types

I don't have access to the same platform in the question, but I can reproduce the output exactly if I remove the cast to long in the assignment of temp1 and temp2 (effectively simulating a platform were sizeof int and long is both 4).

You can verify if the second assumptions hold in your platform checking the sizeof(int) and sizeof(long).

CodePudding user response：

if you try int ans = (temp1 temp2 mod) % mod; this will give you the positive congruent to any negative answer in modulo mod and will not affect the positive answers

as a counter example : -2 ≡ (-2 5) ≡ 3 (modulo 5) , 3 ≡ (3 5) ≡ 3 (modulo 5)