为什么我在 C 中得到错误的平均值？

Question

I'm new at this and having some trouble. I'm trying to find the average of the grades that are inputed by the user but I realized that if you use a decimal in any of the grades, it's just being calculated as if they are whole numbers.

#include <stdio.h>

int main(void)
{
    unsigned int counter;
    float grade;
    int total;
    float average;
    int number;
    
    total = 0;
    counter = 1;
    
    printf("Number of scores to enter:\t");
    scanf("%d", &number);

    printf("\n");
    
    while (counter <= number) {
        printf("%s%d%s", "Enter the score for Lab ", counter, ":\t");
        scanf("%f", &grade);
        total = total + grade;
        counter = counter + 1;
    }
    
    printf("\n");
    
    average = (float) total / number;
    
    printf("Average lab score: %.1f\n", average);
    
    if (grade>=90) {
        puts("Letter grade: A");
    }
    else if (grade>=80) {
        puts("Letter grade: B");
    }
    else if (grade>=70) {
        puts("Letter grade: C");
    }
    else if (grade>=60) {
        puts("Letter grade: D");
    }
    else {
        puts("Letter grade: F");
    }

    return 0;
}

Answer 1

You are capturing scanf("%f", &grade); as a float and then calculating total = total + grade; .

You have defined int total; . You would need to define it as float total; .

You are moving a float variable into an integer which is truncating the decimals you had previously entered.

Answer 2

There's no need to ask up front how many data points will be entered. Indeed, that is an anti-pattern. Just do something like:

#include <stdio.h>

int
main(void)
{
        unsigned int count = 0;
        float grade;
        float total = 0.0;
        float average;

        while( scanf("%f", &grade) == 1 ) {
                total += grade;
                count += 1;
        }
        average = total / (float) count;
        printf("Average lab score: %.1f\n", average);
        fputs("Letter grade: ", stdout);
        putchar( average >= 90.0 ? 'A' : average >= 80.0 ? 'B' :
                average >= 70.0 ? 'C' : average >= 60.0 ? 'D' : 'F');
        putchar('\n');

        return average >= 60.0;
}

$ echo 78.2 96.5 80 | ./a.out
Average lab score: 84.9
Letter grade: B

Key points: total needs to be a float type. You must check the value returned by scanf . Always. Probably you want to handle bad input more cleanly that this does. This just throws away all data after an error and computes the average based on whatever data was entered prior to the error. A cleaner solution would abort with an error message. Exercise left for the reader.

A reasonable argument can be made that this is an abuse of the ternary operator; however you want to refactor it, don't repeat yourself by hardcoding the string "Letter grade: " multiple times.

Rather than abusing the ternary operator as above, you may prefer something like:

#include <stdio.h>

int
main(void)
{
        unsigned int count = 0;
        float grade;
        float total = 0.0;
        float average;

        while( scanf("%f", &grade) == 1 ) {
                total += grade;
                count += 1;
        }
        average = total / (float) count;
        int s = 'A' + (99 - (int)average) / 10;
        printf("Average lab score: %.1f\n", average);
        printf("Letter grade: %c\n", s > 'D' ? 'F' : s);

        return average >= 60.0;
}