My c++ compiler is giving the correct output but the codeforces compiler for c++ is giving a wrong output

Question

Question - A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2k dollars for the second one and so on (in other words, he has to pay i·k dollars for the i-th banana).

He has n dollars. How many dollars does he have to borrow from his friend soldier to buy w bananas?

Input - The first line contains three positive integers k, n, w (1 ≤ k, w ≤ 1000, 0 ≤ n ≤ 109), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.

Output - Output one integer — the amount of dollars that the soldier must borrow from his friend. If he doesn't have to borrow money, output 0.

Test case input - 3 17 4 Test case output - 13

They have only given 1 example test case unfortunately. CodeForces link - https://codeforces.com/problemset/problem/546/A

My code -

int main()
{
    int k,n,w,s;
    cin >> k >> n >> w;
    for(int i=1;i<=w;i++){
        s = s+i*k;
    }
    cout << s-n; //line 11
    return 0;
}

The CodeForces compiler gives output = 543968169. On line 11, that compiler gives error - 'uninitialized value usage'. How do I fix this?

So you mean you want to fix the compiler that gives the wrong output "line 11"? — MikeCAT, Mar 18 '21 at 13:24
Saying it is an `int`, and giving that `int` a known value are two different things. [ProTip] If you want a variable to have a starting value, give it to it when you declare it. Assuming it will have an expected value leads to a lot of bugs. — NathanOliver, Mar 18 '21 at 13:24
@ProgrammerGuy when two compilers give different results, _always start by looking for Undefined Behavior_. In your code, reading from an uninitialized `s` is UB. — Drew Dormann, Mar 18 '21 at 13:27
Sometimes the "garbage" value is zero but don't count on it. Function local variables are not initialized by the language. — drescherjm, Mar 18 '21 at 13:42

score 2 · Accepted Answer · answered Mar 18 '21 at 13:21

2

You used the value of the variable s without initializing it.

s is non-static local variable, so it is default-initialized and it means no initialization will be performed for int.

Initialize it to proper value before the loop to fix the problem.

answered Mar 18 '21 at 13:21

MikeCAT

73,922
11
45
70

For example add `s=0;` before your cin command. – Veda Mar 18 '21 at 13:23
Yup, I got it, thanks. – ProgrammerGuy Mar 18 '21 at 13:27

My c++ compiler is giving the correct output but the codeforces compiler for c++ is giving a wrong output

1 Answers1