如何防止用户输入的错误值存储在变量中？

Question

I'm trying to prevent the user from entering a wrong value in this simple C program by using an if statement within while loop. But the problem is that whenever the user enters a wrong value, it gets stored in the variable and the same value is then use for further calculations. Here's the actual program:

#include <stdio.h>
#include <stdlib.h>
/*Program to calculate the marks obtained scored by the class in a quiz*/

    int main()
    {
    float marks, average, total = 0.0;
    int noStudents;
    printf("Enter the total number of students: ");
    scanf("%d", &noStudents);
    int a=1;
    while(a<=noStudents)
    {
        printf("Enter marks obtained out of 20: ");
        scanf("%f", &marks);
        if(marks<0 || marks >20)
        {
            printf("You have entered wrong marks\nEnter again: ");
            scanf("%d", &marks);
        }
        total = total+marks;
        a++;
    }
    average = total/(float)noStudents;
    printf("Average marks obtained by the class are: %f", average);


    return 0;
}

Answer 1

The first problem is the inconsistency in your code. Inside the condition statement body, you wrote

 scanf("%d", &marks);

which uses mismatched argument type for %d . This invokes undefined behavior . You should be using %f , as before.

That said,

• you're relying on user to correct themselves in the second attempt, don't do that. Use a loop and only after you get a valid value, break out of that.
• In the statement average = total/(float)noStudents; , you don't need the cast. One of the operand, total is already of float type, so the other operand will be automatically promoted and floating point division will take place, even in absence of the explicit cast.

Answer 2

Have slightly tweaked your code. Hope it helps. As already mentioned in one of the comment, don't expect user to give the correct value in out of range scenario. You should keep on asking the user to enter within the range unless he feeds in the correct value.

#include<stdio.h>
#include<stdlib.h>

int main()
{
    float marks, average, total = 0.0;
    int noStudents;
    printf("Enter the total number of students: ");
    scanf("%d", &noStudents);
    int a=1;

     while(a<=noStudents)
     {
        printf("Enter marks obtained out of 20: ");
        scanf("%f", &marks);
        if(marks<0 || marks >20)
        {
            printf("You have entered wrong marks.Enter again:\n ");
            continue;
        }
        total = total+marks;
        a++;
     }
    average = total/noStudents;
    printf("Average marks obtained by the class are: %f", average);

    return 0;
}