Question

calculate and display the sum of first n positive integers where n is a number entered by the user from the key board

Answer 1

To calculate and display you can use loop. 

for e.g.
int n,sum = 0,c,value;
  printf("How many numbers you want to add?\n");
  scanf("%d", &n);
  printf("Enter %d integers\n", n);
  for (c = 1; c <= n; c++)
  {
    scanf("%d", &value);
    sum = sum + value;
  }

Comments

if you are begginer than it is a good program.

Answer 2

I used a simple formula from Arithmetic  Progression

#include<stdio.h>

void main()

 {

  int n,sum;
  printf("ENTER VALUE OF n:");
  scanf("%d",&n);
  sum=0.5*n*(1+n);
  printf("\n sum=%d",sum);
  
}

Answer 3

// for calculate sum of all positive integers upto n

#include<stdio.h>

void main()

 {

  int n,sum,i;

sum=o;

if(n<0)

 {

 printf("enteres number is  a negative number");

}

else if(n=0)

{

printf("sum is 0");

}

else

{

for(i=0;i<=n;i++)

 {

  sum=sum+i;

}

}

printf("sum of all integer upto %d is %d",n,sum);

}

Answer 4

https://onlinegdb.com/BJBXXhDwL

Answer 5

Here is a simple program to calculate the sum using recursion. And I have used a do while loop to check whether the user inputed number is positive or not.

#include <iostream>

int sum(int number)
{
    if(number > 0)
    {
        return number + sum(number - 1);
    }
}

int main()
{
    int n{0};
    do{
        std::cout << "Enter the number: ";
        std::cin >> n;
    }while(n <= 0);
    std::cout << sum(n);
    return 0;
}