Why do I need to initialize an int variable to 0?

Question

I just made this program which asks to enter number between 5 and 10 and then it counts the sum of the numbers which are entered here is the code

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
    int a,i,c;
    cout << "Enter the number between 5 and 10" << endl;
    cin >> a;
    if (a < 5 || a > 10)
    {
        cout << "Wrong number" << endl;
        system("PAUSE");
        return 0;
    }
    for(i=1; i<=a; i++)
    {
        c=c+i;
    }
    cout << "The sum of the first " << a << " numbers are " << c << endl;
    system("PAUSE");
    return 0;
}

if i enter number 5 it should display

The sum of the first 5 numbers are 15

but it displays

The sum of the first 5 numbers are 2293687

but when i set c to 0

it works corectly

So what is the difference ?

Answer 1

Because C++ doesn't automatically set it zero for you. So, you should initialize it yourself:

int c = 0;

An uninitialized variable has a random number such as 2293687, -21, 99999, ... (If it doesn't invoke undefined behavior when reading it)

Also, static variables will be set to their default value. In this case 0.

Answer 2

If you don't set c to 0, it can take any value (technically, an indeterminate value). If you then do this

c = c + i;

then you are adding the value of i to something that could be anything. Technically, this is undefined behaviour. What happens in practice is that you cannot rely on the result of that calculation.

In C++, non-static or global built-in types have no initialization performed when "default initialized". In order to zero-initialize an int, you need to be explicit:

int i = 0;

or you can use value initialization:

int i{};
int j = int();