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Xcode – how not to get an extra count in a while loop?

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;
}
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2

Answers


  1. 0

    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("%.2lfn", average, sum, count);

    return;
}

int main()
{
    while (1)
        calculateAverage();

    return 0;
}
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  2. 0

    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
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