I want to know the error in my code. This is to print sum of all even numbers till 1 to N-CodePudding

#include<iostream>
using namespace std;

int main(){
    int i = 1;
    int sum;
    int N;
    cout << "Enter a number N: ";
    cin >> N;
    while(i<=N)
    {
        if(i%2 == 0)
        {
            sum = sum   i;
        }
        else
        {
            i = i   1;
        }
    }
    cout << sum;
}

This is to print the sum of all even numbers till 1 to N.

As I try to run the code, I am being asked the value of N but nothing is being printed ahead.

CodePudding user response：

For starters the variable sum is not initialized.

Secondly you need to increase the variable i also when it is an even number. So the loop should look at least like

while(i<=N)
{
    if(i%2 == 0)
    {
        sum = sum   i;
    }
    i = i   1;
}

In general it is always better to declare variables in minimum scopes where they are used.

So instead of the while loop it is better to use a for loop as for example

for ( int i = 1; i   < N;   i )
{
    if ( i % 2 == 0 ) sum  = i;
}

CodePudding user response：

while(i<=N)
{
    if(i%2 == 0)
    {
        sum = sum   i;
    }
    else
    {
        i = i   1;
    }
}

Let's step through this. Imagine we're on the loop where i = 2 and you've entered N = 5. In that case...

while(i <= N)

2 <= 5 is true, so we loop

if(i%2 == 0)

2 % 2 == 0 is true, so we enter this branch

sum = sum   i;

Update sum, then head back to the top of the loop

while(i <= N)

Neither i nor N have changed, so 2 <= 5 is still true. We still loop

if(i%2 == 0)

2 % 2 == 0 is still true, so we enter this branch again...

Do you see what's happening here? Since neither i nor N are updated, you'll continue entering the same branch and looping indefinitely. Can you think of a way to prevent this? What would need to change?

Also note that int sum; means that sum will have a garbage value (it's uninitialized). If you want it to start at 0, you'll need to change that to

int sum = 0;

CodePudding user response：

You're looping infinitly when i is even because you don't increase it. Better option would be this if you want to use that while loop :

while(i<=N)
    {
       if(i%2 == 0)
            sum = sum   i;
       
       i=i 1;
    }
    cout << sum;

If you don't need to do anything when the condition is false, just don't use an else.

CodePudding user response：

No loops are necessary and sum can be evaluated at compile time if needed too

// use unsigned, the whole excercise is pointless for negative numbers
// use const parameter, is not intended to be changed
// constexpr is not needed, but allows for compile time evaluation (constexpr all the things)
// return type can be automatically deduced
constexpr auto sum_of_even_numbers_smaller_then(const unsigned int n)
{
    unsigned int m = (n / 2);
    return m * (m   1);
}

int main()
{
    // compile time checking of the function
    static_assert(sum_of_even_numbers_smaller_then(0) == 0);
    static_assert(sum_of_even_numbers_smaller_then(1) == 0);
    static_assert(sum_of_even_numbers_smaller_then(2) == 2);
    static_assert(sum_of_even_numbers_smaller_then(3) == 2);
    static_assert(sum_of_even_numbers_smaller_then(7) == 12);
    static_assert(sum_of_even_numbers_smaller_then(8) == 20);

    return 0;
}

CodePudding user response：

int main(){

int input;                  //stores the user entered number
int sum=0;                  //stroes the sum of all even numbers
repeat:
cout<<"Please enter any integer bigger than one: ";
cin>>input;
if(input<1)                 //this check the number to be bigger than one means must be positive integer.
    goto repeat;            // if the user enter the number less than one it is repeating the entry.
for(int i=input; i>0; i--){ // find all even number from your number till one and than totals it.
    if(i%2==0){
        sum=sum i;
        int j=0;
        j=j 1;
        cout<<"Number is: "<<i<<endl;
    }
}
cout<<endl<<"The sum of all even numbers is: "<<sum<<endl;}

Copy this C code and run it, it will solve your problem.

CodePudding user response：

There are 2 problems with your program.

Mistake 1

The variable sum has not been initialized. This means that it has(holds) a garbage value. And using this garbage value like you did when you wrote sum = sum i; is undefined behavior.

Undefined behavior means anything¹ can happen including but not limited to the program giving your expected output. But never rely on the output of a program that has undefined behavior.

This is why it is advised that:

always initialize built in types in local/block scope.

Mistake 2

The second problem is that you're not updating the value of variable i.

Solution

You can solve these problems as shown below:

int main(){
    int i = 1;
    int sum = 0; //INITIALIZE variable sum to 0
    int N;
    cout << "Enter a number N: ";
    cin >> N;
    while(i<=N)
    {
        if(i%2 == 0)
        {
            sum = sum   i;
        }
        
        i = i   1; //update(increase i)
       
    }
    cout << sum;
}

^{¹For more reading(technical definition of) on undefined behavior you can refer to undefined behavior's documentation which mentions that: there are no restrictions on the behavior of the program.}