CODEWITHSUNDEEP
c++modulo
Ayush Yadav
Ayush Yadav

Reputation: 426

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

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 : enter image description here

Upvotes: 1

Views: 148

Answers (1)

Alessandro Teruzzi
Alessandro Teruzzi

Reputation: 3978

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).

Upvotes: 1

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