Handling Extra Characters

I've been studying C Programming for the past 5 months...I've noticed many people use the fflush(stdin) command to clear any extra characters. This to me is great because people are human and make mistakes.

My question is how often should you use flush? Would you use it any time you are using a scanf?

You shouldn't use fflush(stdin). Doing so greatly limits where your program can run.
As for when you should clear out the input buffer: Whenever you use a function that has the possiblity of leaving stuff in the buffer.

One should refrain from discarding data left in a stream. It severely limits the flexibility that users have in working productively. As an example, say you have a program that reads five numbers and prints the result of an addition on all of them.

Code:

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

int main(void)
{
    int n = 0;
    int number;
    int sum = 0;

    printf("Enter five numbers: ");
    fflush(stdout);
    while (scanf("%d", &number) == 1) {
        sum += number;
        if (++n == 5)
            break;
    }
    if (n != 5) {
        fprintf(stderr, "Input error\n");
        return EXIT_FAILURE;
    }
    printf("The sum is %d\n", sum);

    return 0;
}

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By restricting input and discarding anything in the stream, the following input would not work properly.

Enter five numbers: 1 2 3 4 5

Instead, it would read one number and discard everything else, thus requiring the user to enter numbers one by one. This would slow down all but novice users of the program. If you must discard left over characters, do so only when there is no possibility that the data would be required for further successful operation.

Quote:

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Originally posted by Edward
One should refrain from discarding data left in a stream. It severely limits the flexibility that users have in working productively. As an example, say you have a program that reads five numbers and prints the result of an addition on all of them.

...

By restricting input and discarding anything in the stream, the following input would not work properly.

Enter five numbers: 1 2 3 4 5

Instead, it would read one number and discard everything else, thus requiring the user to enter numbers one by one.

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Only if you flush within the while loop. I for one would flush after all the inputs or when there is a potential for misreads because of scanf's penchant for leaving crap in the buffer.

Planning is what makes the flushing work without slowing the user.