how not to get an extra count in a while loop? - c

Question

I have the following code and every time i run it, I get an extra count. Say for example that I input 8 first round and then a 2 next round and exit with the sentinel -1, the sum will be 10 as expected, but the count will be 3. I've debugged the program and no matter if count comes before the scanf() or after, I still get a 3 value. One possible solution is to initialized count to -1. However, I feel I shouldn't have to and setting count equal to zero should work. Do I have to set count to -1?
#include <stdio.h>
void calculateAverage()
{
int grade = 0, count, sum = 0;
double average;
count = 0;
while(grade != -1)
{
sum += grade;
count++;
printf("input a grade: \n");
scanf("%d", &grade);
}
average = (double)(sum)/(double)(count);
printf("%.2lf", average);
return;
}
int main( )
{
while (1)
calculateAverage();
return 0;
}

Answer 1

For starters there is no great sense to declare the variable sum as having the type int because in any case you are casting it to the type double
average = (double)(sum)/(double)(count);
You are increasing the variable count before the user will enter something.
The function can be defined the following way as it is shown in the demonstrative program below.
#include <stdio.h>
void calculateAverage( void )
{
const int Sentinel = -1;
size_t count = 0;
double sum = 0.0;
printf( "input a grade (%d - stop): ", Sentinel );
for ( int grade; scanf( "%d", &grade ) == 1 && grade != Sentinel; )
{
sum += grade;
++count;
printf( "input a grade (%d - stop): ", Sentinel );
}
double average = count == 0 ? sum : sum / count;
printf( "%.2lf", average );
}
int main(void)
{
calculateAverage();
return 0;
}
The program output might look like
input a grade (-1 - stop): 1
input a grade (-1 - stop): 2
input a grade (-1 - stop): 3
input a grade (-1 - stop): 4
input a grade (-1 - stop): 5
input a grade (-1 - stop): 6
input a grade (-1 - stop): 7
input a grade (-1 - stop): 8
input a grade (-1 - stop): 9
input a grade (-1 - stop): 10
input a grade (-1 - stop): -1
5.50

Answer 2

You are incrementing count just before you enter -1. So, it counts -1 as well. Just in increase is if it's not -1.
#include <stdio.h>
void calculateAverage()
{
int grade = 0, count = 0, sum = 0;
double average;
while (grade != -1)
{
sum += grade;
printf("input a grade (-1 - stop): ");
scanf("%d", &grade);
if (grade != -1) // Check this line
count++;
}
average = (double)sum / count;
printf("%.2lf\n", average, sum, count);
return;
}
int main()
{
while (1)
calculateAverage();
return 0;
}