Validating Integer in C

Question

I am creating a program in C where i have to accept an integer to continue.I read some questions to use the following code but my program just goes into infinite loop soon after entering any character.Please help me

    int tnum,scancheck;
printf("Enter the number of teams(Maximum 4)\n");
scancheck = scanf("%d",&tnum);
while(scancheck != 1)
{
    printf("ERROR: Please enter numbers only\n\n");
    printf("Enter the number of teams(Maximum 4)\n");
    scancheck = scanf("%d",&tnum);  
}

I used Kirilenko's method but my program goes into infinite loop on entering a special character such as l=

EDIT PROBLEM HAS BEEN SOLVED.Thanks guys

score 2 · Accepted Answer · answered Oct 19 '12 at 16:41

2

You have to clean stdin between your calls to scanf. Otherwise the behaviour of scanf with an invalid input is to put back the character on the stream. Then, it go to an infinite loop where you read the same wrong character every iteration.

#include <stdio.h>

void clean_stdin(void) {
    int c;
    while ((c = getchar()) != '\n' && c != EOF)
        ;
}

/* ... */

int ret;
int n;

do {
   ret = scanf("%d", &n);
   clean_stdin();
} while (ret != 1);

answered Oct 19 '12 at 16:41

md5

23,373
3
44
93

can you tell me what stdin is? – Dhruv Jain Oct 19 '12 at 16:47
It is the input stream (read [this](http://en.wikipedia.org/wiki/Standard_streams#Standard_input_.28stdin.29)). – md5 Oct 19 '12 at 16:48
Thanks now i know about it.Thanks a lot – Dhruv Jain Oct 19 '12 at 16:51
I am using fflush(stdin); now Is there anything wrong with it – Dhruv Jain Oct 19 '12 at 16:52
`fflush(stdin)` is a very bad idea: use `fflush` on an input stream (such as `stdin`) is an undefined behavior. For example, the behavior of GNU in that case is to do nothing. – md5 Oct 19 '12 at 16:54
Thanks.. one last question, where do you gather such knowledge.I mean i am learning c for some time i don't think i will never know this kind of stuff – Dhruv Jain Oct 19 '12 at 16:57

Aubin · Answer 2 · 2012-10-19T17:05:37.690

0

If you wish to get an input from the user, get the line and analyze it.

char   line[1024];
char * err = NULL;
int    num;

while( err == NULL || *err )
{
   fgets( stdin, line );
   num = strtol( line, &err, 10 );
}

Until it satisfy the requirement.

edited Oct 19 '12 at 17:05

answered Oct 19 '12 at 16:53

Aubin

14,617
9
61
84

1

Rather than only post a block of code, please *explain* why this code solves the problem posed. Without an explanation, this is not an answer. – Martijn Pieters Oct 19 '12 at 17:03

Vivek · Answer 3 · 2012-10-19T16:50:01.017

-2

Scanf puts the character back on the stdin stream if the input is invalid. Since, we are not clearing the stdin stream, it is going towards an infinite loop.

Try writing this below your scanf statement

fflush(stdin);

edited Oct 19 '12 at 16:50

answered Oct 19 '12 at 16:42

Vivek

1,640
1
17
34

It worked for me!!Thanks a lot bro.One of the fastest helping community here – Dhruv Jain Oct 19 '12 at 16:47
As @Kirilenko already pointed, flushing stdin is undefined behaviour. You should use getchar or scanf("%*c") to manually empty the buffer. – Spidey Oct 19 '12 at 16:58
Yea..I am using Kirilenko's method.Anyway, thanks for your advise – Dhruv Jain Oct 19 '12 at 17:01
Agreed. I was just trying giving a different solution than what was already given. :) – Vivek Oct 19 '12 at 17:02

Validating Integer in C

3 Answers3