How to add the numbers inside the scanf in loop? - c

Question

Is there some way to add all the numbers in a scanf loop? The loop would stop if the input number is negative. The problem is that the negative must also be included in the sum.
Here, I managed to get the sum of all the positive scanf values repeated in the loop, but the negative number is still not included in the sum of all the numbers.
#include <stdio.h>
main()
{
int z, x;
printf("Enter a number:\n");
z = 0;
scanf("%d", &x);
do
{
z += x;
scanf(" %d", &x);
} while (x >= 0);
printf("Sum = %d ", z);
return 0;
}

Answer 1

A simple rearrangement of the order of statements in your do ... while loop (and removal of the preceding scanf call) will do the trick:
#include<stdio.h>
int main() // You are returning "0" so declare "int" as return type
{
int x = 0, z = 0; // Easier to initialize at the same time as declaration.
printf("Enter a number:\n");
// scanf ("%d", &x); // Don't read here - do that inside the loop.
do {
int test = scanf(" %d", &x); // Read as the FIRST statement in the loop ...
if (test != 1) { // If the "scanf" call failed, we need to clear the input stream ...
int c;
while ((c = getchar()) != '\n' && c != EOF)
; // Clear any offending input
if (c == EOF) break; // Probably can't recover from an EOF, so exit the while loop
}
else z += x; // ... then we can add X even if it's negative
} while (x >= 0); // But end the loop when it IS negative anyway
printf("Sum = %d ", z);
return 0;
}
Note that I have added a test variable to makes sure that the scanf operation succeeded. If the usser enters foo (as mentioned in the comment by William Pursell), then the input buffer is cleared, the addition is skipped, and the read will be attempted again.

Answer 2

I'm not sure if you want to handle an input stream that does not contain a negative value as an error, but you could simply do:
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
int sum = 0, x, rv;
while( 1 == (rv = scanf ("%d", &x)) ){
sum += x;
if( x < 0 ){
break;
}
}
if( rv == 0 ){
fprintf(stderr, "Invalid input\n");
} else {
printf("Sum = %d\n", sum);
}
return rv == 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}
I think the following behavior is reasonable:
$ echo 4 5 8 5| ./a.out
Sum = 22
$ echo 4 5 -1 5| ./a.out
Sum = 8
$ echo 4 5 not-an-integer 5| ./a.out
Invalid input

Answer 3

Inverting the lines inside the loop should do it:
int z, x;
int ch;
printf("Enter a number:\n");
z = 0;
do
{
if (scanf("%d", &x) == 1) // checking if parsing was successful
{
z += x; // if so, perform the sum
}
else
{
puts("Bad input"); // if user inputs a bad value
while((ch = getchar()) != '\n' && ch != EOF){} // you clear the input buffer
}
} while (x >= 0);
printf("Sum = %d ", z);
Note that I also removed the first scanf ouside the loop, which becomes unneeded.