Infinite loop code in C

Question

In the below program I try to input a number between 1 to 100 but if I enter a 'character' or "string" ( like s or sova ) during the execution time of scanf() statement it creates a infinite loop. so I try to do .... when I input a string or a character it shown me a message like "wrong value entered. enter again" and it will enter again...
Thanx;

#include<stdio.h>
int main()
{
    int a;
    scanf("%d",&a);
    while(!(a>=1&&a<=100))
    {
        printf("wrong value entered. enter again\n");
        scanf("%d",&a);
    }
    printf("you enter %d. Thanxz",a);
    return 0;
}

First not able to get why u are using while loop.You can simply do in if and else as. — kumar123, Aug 30 '13 at 05:16
I have to say both comments do nothing to answer or provide insight to the problem. ^^^^ — ChiefTwoPencils, Aug 30 '13 at 05:18
If you don't enter `int` value scanf doesn't consume input and that input remain stdin buffer you need to fflush that buffer by reading unscanned chars-- Read: [Scanf won't execute for second time-infinite-loop](http://stackoverflow.com/questions/17827603/scanf-wont-execute-for-second-time/17827635#17827635) I explain same problem elaborately — Grijesh Chauhan, Aug 30 '13 at 05:29

score 5 · Accepted Answer · answered Aug 30 '13 at 05:17

5

You need to check the return value of scanf
If the user has not entered a integer, you need to eat the input. The scanf function will continually say not a integer, try again. So if scanf returns 0 you need to deal with it

answered Aug 30 '13 at 05:17

Ed Heal

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AndersK · Answer 2 · 2013-08-30T05:31:01.280

When you use scanf you are working with buffered input, this means that when you enter a value say "123" and press ENTER then "123" plus the ending character (ENTER) will all be added to the buffer. scanf then removes 123 since %d specifies that an integer should be read but if a user enters something invalid like a string instead then the buffer will not be emptied.

A better way to read input from the keyboard is to use fgets() where you specify a max length to read. Once you have the input you can use sscanf() to retrieve the numeric value from it. The ENTER till then not irritate your input.

char buffer[128];
fgets( buffer, 128, stdin );
sscanf( buffer, "%d", &a );

Also always check return values from functions as a rule of thumb so that you can do appropriate action if the function fails.

But the `%d` conversion specification does skip leading whitespace; in this case it won't matter that the newline character is still in the buffer. — verbose, Aug 30 '13 at 05:26

verbose · Answer 3 · 2013-08-30T06:43:55.843

Try this.

#include <stdio.h>
#define FLUSH while (getchar() != '\n')  // macro to eat invalid input

int main (void) {

    int a = 0;
    printf ("Enter an integer: ");
    scanf("%d", &a);

    while (a < 1 || a > 100) {

        FLUSH;
        printf("Invalid input. Please try again: ");
        scanf("%d",&a);
    }

    printf("You entered %d.\nThanks!\n", a);
    return 0;
}

Your code shows several coding habits that need to be changed:

Include (void) in the parameter list of main().
Leave spaces on either side of binary operators: while(!(a>=1&&a<=100)) is needlessly ugly and hard to read.
Simplify your logical expressions. Why use (! (a>=1 && a<=100)) when it means the same thing as (a < 1 || a > 100), and the latter is so much easier to read?
Prompt for user input when needed. Don't have the cursor just sit at a blank line with no indication to the user about what to do.
Use proper grammar and capitalization in your prompts. There's no reason to be lazy in your programming.

score 0 · Answer 4 · answered Aug 30 '13 at 06:19

If the return value from scanf is not equal to the number of item you like the user to input, read all characters of the input buffer until there is a '\n'. But instead of copying a whole loop over and over again to the places in your code where the user should input something, you could wrap the loop in a function like this:

#include <stdio.h>
#include <stdarg.h>
#include <string.h>

void input(const char *format,...)
{
    va_list ap;
    int r;
    /* number of items [to read] */
    int noi=0;

    for(r=0;r<strlen(format)-1;r++)
    {
        if(format[r]=='%')
        {
            if(format[r+1]!='%')
                noi++;
            else
                r++;
        }
    }

    do
    {
        va_start(ap,format);
        r=vscanf(format,ap);
        va_end(ap);

        if(r!=noi)
        {
            switch(r)
            {
                case EOF:
                case 0:
                    printf("All wrong, try again!\n");
                break;

                default:
                    printf("Unexpected value after item no %d!\n",r);
            }

             while(getc(stdin)!='\n');
        }
        else
            break;

    } while(1);
}

Hope that helps,

Jan

Infinite loop code in C

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