Unexpected result when trying to continuously prompt until they give an integer-only input

Question

Issue: when the user inputs 'q', the prompt (Number: ) will show up again (which is expected). However, when the user inputs, "q q", the prompt will appear twice ie (Number: Number: ). And when the user inputs "q q q", the prompt will appear 3 times ie (Number: Number: Number: ). is there anyway to fix this code? i am new to programming so i would really appreciate it if you try to explain the problem in layman terms.

int main(void)
{

    char number[17];
    while (true)
    {
        printf("Number: ");
        scanf("%s", number);
        int i;
        char c;
        if (sscanf(number, "%i %c", &i, &c) == 1)
        {
            printf("%i\n", i);
            return 0;
        }
    }
}

the prompt (Number: ) should only appear once regardless of how many spaces. — helpmepleaseibad, Feb 24 '20 at 12:23

score 2 · Answer 1 · edited Feb 24 '20 at 12:59

2

i have edited the code according to what u guys have said and it works. Can someone explain to me why this works?

int main(void)
{

    char number[17];
    while (true)
    {
        printf("Number: ");
        fgets(number, 17, stdin);
        int i;
        char c;
        if (sscanf(number, "%i %c", &i, &c) == 1)
        {
            printf("%i\n", i);
            return 0;
        }
    }
}

edited Feb 24 '20 at 12:59

Jabberwocky

48,281
17
65
115

answered Feb 24 '20 at 12:35

helpmepleaseibad

29
2

1

BTW: `fgets(number, 17, stdin);` -> `fgets(number, sizeof(number), stdin);`. That way you specify `17` only at _one_ place. – Jabberwocky Feb 24 '20 at 13:01
i would like to understand y scanf would cause the prompt to appear more than once. – helpmepleaseibad Feb 24 '20 at 13:11
Number: a bc def g h scanned a Number: scanned bc Number: scanned def Number: scanned g Number: scanned h Number: – helpmepleaseibad Feb 24 '20 at 13:27
1

but scanf continues to scan the chars after the space in the next loop. is that due to the buffer still storing the remaining user's input? – helpmepleaseibad Feb 24 '20 at 13:30

Sergey Leonovich · Answer 2 · 2020-02-24T12:58:02.153

-1

Another solution is to call fflush(stdin) to clear input buffer.

 if (sscanf(number, "%i %c", &i, &c) == 1)
    {
        printf("%i\n", i);
        return 0;
    }
 else fflush(stdin);

edited Feb 24 '20 at 12:58

answered Feb 24 '20 at 12:42

Sergey Leonovich

47
5

at which line should i implement it tho. – helpmepleaseibad Feb 24 '20 at 12:44
as `else` statement – Sergey Leonovich Feb 24 '20 at 12:47
1

_fflush(stdin)_: is undefined behaviour. https://stackoverflow.com/questions/2979209/using-fflushstdin – Jabberwocky Feb 24 '20 at 12:58

Unexpected result when trying to continuously prompt until they give an integer-only input

2 Answers2