Wrong answer in modular calculator

Question

I created a modular calculator meant to go through a series of math executions and at the end take the mod of everything before. However, I keep on getting the wrong result.

#include <iostream>
using namespace std;
int main(){
    int num1, num2, ans = 0;
    char oper = ' ';
    cin >> num1>> oper >> num2;
    //The first time
    if (oper == '*')
        ans =num1*num2;
    else if (oper == '+')
        ans = num1+num2;
    //other times
    do {
        cin>> oper >> num2;
        if (oper == '*')
            ans =ans*num2;
        else if (oper == '+')
            ans = ans+num2;
    } while (oper!='%');
    if (oper == '%')
        ans = (ans % num2);
    cout<<ans;
}

Input:

4
* 8805
* 99
* 12
+ 6
+ 367
* 575
+ 66
+ 9
* 8
* 8
* 711
+ 130
* 5
+ 5
+ 1
+ 73
* 811
* 33
+ 56
+ 80
* 350
* 116
+ 179
* 383
* 12
+ 59
+ 5150
* 10
+ 5
+ 8783
* 48
* 84
* 7
+ 390
+ 7057
* 10
+ 8366
+ 8856
* 99
* 9
+ 3019
+ 228
* 334
+ 75
+ 6353
+ 7151
* 8
% 1408

Output: -1240 Expected: 808

Any explanations? (BigInt or double or unsigned int instead of int don't seem to fix anything; I already tried all of those).

Answer 1

You got wrong answer because of int overflow.

Lets give you an example

Lets say your input like below

4
* 432
* 422
* 432
% 8

your program will give wrong answer because

after first input, ans = 4 * 432 = 1728

after second input, ans = 1732 * 432 = 746496 (which is greater than int highest range, it occurs int overflow and gives you wrong answer)

For this reason, we can use below modular arithmetic formula so that we can leave int overflow.

1. (A + B) % R = (A % R + B % R) % R
2. (A * B) % R = (A % R * B % R) % R

For Your Information, (26+3*2)%3 = (26%3 + 3%3 * 2%3) % 3

Try this

#include <iostream>
#define MAX 100 // guess at most 100 lines input
using namespace std;
int main()
{
    int num1, count = 0, num2[MAX];
    char oper[MAX];
    cin >> num1;

    do{
       cin >> oper[count] >> num2[count];
       count = count + 1;
    }while(oper[count -1 ] != '%');

    int ans = num1, remainder = num2[count - 1]; // last number is remainder value

    for(int i = 0; i < count - 1; i++)
    {
       if(oper[i] == '+')
          ans = (ans % remainder + num2[i] % remainder) % remainder;
       if(oper[i] == '*')
         ans = (ans % remainder * num2[i] % remainder) % remainder;
    }

    cout << ans << endl;
 }