How to handle incorrect input with an infinite loop using scanf?

Question

I'm struggling to come up with a solution for a problem that I set myself to complete, I have tried multiple different ways but I can't seem to get it to behave.

What I'm attempting to do is handle when non-numeric characters are input by a user in scanf. The end goal is to have the program ask the user for a value from a series, and keep asking until a float or int is entered, then finish the series.

Seeing as it's a short program, ill post the whole thing. Any help is much appreciated.

int main(void) {
    int numberOfValues, ctr, inputValidation, avg;
    float sum = 0;

    printf("\nHow many values are you averaging?: ");
    scanf("%d", &numberOfValues);

    float inputValue[numberOfValues];

    printf("\n");

    for (ctr = 0; ctr < numberOfValues; ctr = ctr + 1) {
        printf("\tPlease enter value %d: ", ctr + 1);
        inputValidation = scanf("%f", &inputValue[ctr]);
        if (inputValidation != 1) {
            printf("\tPlease enter value %d again: ", ctr + 1);
            scanf("%f", &inputValue[ctr]);
        } else {
            sum += inputValue[ctr];
        }
    }

    avg = sum / numberOfValues;

    printf("\nYour average is: %g", avg);

    return 0;
}

The best way to solve this problem is [not to use `scanf`](https://stackoverflow.com/questions/58403537/). — Steve Summit, Jul 06 '21 at 01:38

niry · Accepted Answer · 2021-07-06T00:44:14.523

1

You need to consume the "bad" data. Since scanf() could not match the format you requested, it will leave the data in the buffer.

Using getchar() as a way to do so:

            int gc;
            do {
                gc = getchar();
                if (gc == EOF) exit(EXIT_FAILURE);
            } while(gc != '\n');

Here is your program modified to do so. Disclaimer: the following program is merely helping with the specific question. It is incomplete and there are other challenges and scenarios to overcome. E.g. try to begin your input with a digit and add non-digit char(s) such as "4abc" and see what happens. More examples of issues in the comments.

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

int main(void)
{
    int numberOfValues, ctr, avg;
    float sum = 0;

    printf("\nHow many values are you averaging?: ");
    scanf("%d", &numberOfValues);

    float inputValue[numberOfValues];

    printf("\n");

    for (ctr = 0; ctr < numberOfValues; ctr = ctr + 1)
    {
        printf("\tPlease enter value %d: ", ctr + 1);

        while (scanf("%f", &inputValue[ctr]) != 1) {
            int gc;
            do {
                gc = getchar();
                if (gc == EOF) exit(EXIT_FAILURE);
            } while(gc != '\n');
            printf("\tPlease enter value %d again: ", ctr + 1);
        }
        sum += inputValue[ctr];
    }

    avg = sum / numberOfValues;

    printf("\nYour average is: %d", avg);

    return 0;
}

edited Jul 06 '21 at 00:44

answered Jul 05 '21 at 16:48

niry

3,238
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4

`while (getchar() != '\n');` would enter an infinite loop on EOF or IO-error. – HAL9000 Jul 05 '21 at 18:35
1

@HAL9000 indeed. I've amended the program to check for EOF (both actual EOF and error apply). I've also added a disclaimer. – niry Jul 05 '21 at 22:25
1

You should also check the return value of `scanf("%d", &numberOfValues);` to avoid undefined behavior on invalid input. – chqrlie Jul 05 '21 at 23:18
`avg` should probably be a `float` too. – chqrlie Jul 05 '21 at 23:19
1

For improved average computation, accumulating the sum of integers as an integer makes sense -perhaps as a `long long` to avoid overflow. `long long sum = 0;` The average calculation and display deserve to be `double`. `double avg = 1.0 * sum / numberOfValues;`. Save `float` for special cases and use `double` for general FP. – chux - Reinstate Monica Jul 06 '21 at 03:06
@chqrlie good point, I did originally have it as a float but in my endless optimisation of the code I seemed to have put it back in the int declarations – Edward Nunn Jul 06 '21 at 15:43

score 1 · Answer 2 · answered Jul 06 '21 at 17:49

You need to discard the 'bad' data before trying to call scanf again. There are a variety of ways to do that. You could discard everything up to the next newline (scanf("%*[^\n]") would do that), though you should probably also check for EOF somewhere. Or you could discard a single character and try again (but you probably don't want to prompt again, as you probably have multiple characters left in the input buffer.)

How to handle incorrect input with an infinite loop using scanf?

2 Answers2