Multiple input in for loop

Question

I have to write a programme in c++ where user enters a number N, then on a second line he enters as many numbers as N, no more. The ouput shoud be the sum of all positive numbers among the entered numbers. I have to use for loop. Also we have not covered much so far, only if statements.

The code I have tried gives the sum of positive numbers only, but I can't make the programme use N inputs and stop. It either calculates only one or continues as long as user enters numbers.

 #include <iostream>
using namespace std;

int main ()
{
    int n, sum=0;
    cin>> n;
    cout<<endl;
    cout<<"Enter numbers"<<endl;
    for (int i=1; i<=n; i++)
    {
        cin>>i;
        if(i>0)
    {sum=sum+i;
    }
 cout<<sum<<endl;
    }


    return 0;
}

Why are you using the same variable, as the loop counter in cin>>i;? Why don't you use different variable for read value? — Algirdas Preidžius
– Algirdas Preidžius, Commented Dec 13, 2019 at 19:15
Code works just fine here: wandbox.org/permlink/naZfBzsuB3Hj3itO — NathanOliver
– NathanOliver, Commented Dec 13, 2019 at 19:15
Unrelated: Prefer for (int i=0; i<n; i++) to for (int i=1; i<=n; i++) because everything in C++ starts at 0 and runs to n-1. Forcing origin 1 indexing will cause a great many problems. — user4581301
– user4581301, Commented Dec 13, 2019 at 19:22

Ted Lyngmo · Accepted Answer · 2019-12-13 19:24:58Z

4

The problem is that you're using the same variable (i) for looping and input.

for (int i=1; i<=n; i++)
{
        cin>>i;

Whatever gets entered in that cin>>i ruins the logic of your program. Add one separate input variable and keep your i for the loop.

Example:

#include <iostream>

int main() {
    int n, sum = 0;
    std::cout << "How many numbers do you want to enter? \n";
    std::cin >> n;
    std::cout << std::endl;

    std::cout << "Enter numbers: \n";
    for(int i = 1; i <= n; i++) {
        std::cout << i << ": ";

        int input;
        if(std::cin >> input) {
            if(input > 0) {
                sum = sum + input;
            }
            std::cout << sum << std::endl;
        } else
            break; // user failed to enter a number
    }
}

answered Dec 13, 2019 at 19:24

Ted Lyngmo

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3 Comments

Ted Lyngmo Over a year ago

When I said "Whatever gets entered ..." I meant "there is only one number that would not ruin the logic of your program and it has the value i in every iteration" - I hope that makes sense.

Imemi Over a year ago

Thanks! I think I got it. I am a beginner, we study this at school, maths high school, and this stuff is not easy for 5th grade. we only have two hous a week, not enough.

Ted Lyngmo Over a year ago

@Imemi You're welcome! It's great that you study this in 5:th grade! It never stops being fun, promise! (... and as always, a disclaimer: there are rumours about people who liked programming and now don't - rumours)

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