Sum of Positive Numbers Until 999 in C

Algorithm

1. Set sum to 0.
2. Read a number. If it is 999, stop.
3. If the number is greater than 0, add it to sum; negative numbers and 0 are skipped.
4. Repeat from the read step.
5. Print sum.

C program

#include <stdio.h>

#define SENTINEL 999

int main(void) {
    int number, sum = 0;

    printf("Enter numbers (enter %d to stop):\n", SENTINEL);
    while (scanf("%d", &number) == 1) {
        if (number == SENTINEL) {
            break;
        }
        if (number > 0) {
            sum += number;
        }
    }

    printf("Sum of positive numbers: %d\n", sum);
    return 0;
}

Sample input and output

5 -3 10 0 999

Enter numbers (enter 999 to stop):
Sum of positive numbers: 15

999

Enter numbers (enter 999 to stop):
Sum of positive numbers: 0

How the program works

The sentinel value 999 marks the end of the input, so the loop does not need to know how many numbers will be entered. The check comes before the addition so that 999 is never added.

Negative numbers and zero are excluded by the number > 0 test. The while condition also stops if the input is not an integer, which avoids an endless loop on bad input.

Viva questions

Common mistakes

• Adding the sentinel before testing for it.
• Using >= 0 instead of > 0, which includes zero.