About scanf()

**Antigloss** · 05-29-2005

It was once a bad headache for me. The C code like this:

Code:

      while (gross != -1) {
           printf("input: ");
           scanf("%f", &gross);
      }

When the code is compiled and executed, if input characters such as a, b, c, d...

It'll become an infinite loop printing "input:". But yesterday, I found a

solution to this all by chance. That is, change the code as follows:

Code:

     while (gross != -1) {
          printf("input: ");
          scanf("%f", &gross);
          getchar(); /* Add getchar() here */
     }

After adding 'getchar()' into the program, the infinite loop problem is solved.

But I don't know why. Could you please tell me? And please tell me if there are

other solutions to it. Thanks a lot.

I also noticed that if any character is inputed, getchar() gets the

character. If number is inputed, getchar() gets '\n'.
I took advantage of this and wrote a small program, here is the code. Could

you please give me some advice about my code? Thanks.

Code:

#include<stdio.h>

#define BASE_PAY         200
#define salary_cal(x)    9*(x)/100 + BASE_PAY

int main()
{
    float gross = 0.0;
    char  flag;
    
    while ( 1 ) {
        printf("Enter sales in dollars ( q to end ): ");
        scanf("%f", &gross);
        if ( ( flag = getchar() ) != '\n' ) {  /* If character is inputed */
            if ( flag == 'q' || flag == 'Q') { /* If 'q' or 'Q' is inputed, exit. */
                printf("Thanks for using our product!\n");
                return 0;
            }
            /* If the inputed character is not 'q' or 'Q' */
            printf("Invalid input!\n");
            continue;
        }
        printf("Salary: %.2f\n", salary_cal(gross) );
    }

    return 0;
}

**sand_man** · 05-29-2005

'\n' is left in the input buffer after calling scanf
calling getchar reads the '\n' and removing from the buffer.
flush the buffer

**Dave_Sinkula** · 05-29-2005

>please give me some advice about my code?
Read input as a string, then check for the 'q'; if not quitting, then attempt to convert the text to a float.

Code:

#include <stdio.h>
#include <ctype.h>

#define BASE_PAY         200
#define salary_cal(x)    (9*(x)/100 + BASE_PAY)

int main()
{
   while ( 1 )
   {
      char line[80];
      fputs("Enter sales in dollars ( q to end ): ", stdout);
      fflush(stdout);
      if ( fgets(line, sizeof line, stdin) )
      {
         float gross;
         if ( toupper((unsigned char)*line) == 'Q' )
         {
            break;
         }
         if ( sscanf(line, "%f", &gross) == 1 )
         {
            printf("Salary: %.2f\n", salary_cal(gross) );
         }
         else
         {
            puts("Invalid input!");
         }
      }
   }
   printf("Thanks for using our product!\n");
   return 0;
}

/* my output
Enter sales in dollars ( q to end ): one thousand
Invalid input!
Enter sales in dollars ( q to end ): 1000
Salary: 290.00
Enter sales in dollars ( q to end ): q
Thanks for using our product!
*/

**Antigloss** · 05-30-2005

Thanks all for your kindly help.

Dear Dave_Sinkula, thanks a lot, I learnt so much from your code.
I found that without calling "fflush(stdout)", your program still run correctly, and exactly the same as fflush(stdout) is called, so I don't know why you call fflush(stdout) here. Is it neccessary to call fflush(stdout)? Could you please tell me?

**dwks** · 05-30-2005

Without fflush(stdout): If the user inputs more than 80 characters, fgets() will read in the first 80 chars and leave the rest in the buffer. The loop will iterate again, and the remainder (ie, the chars beyond 80 chars) will be used the second time round.

With fflush(stdout): If the user enters more than 80 chars, the excess chars are discarded.

[EDIT]
Ugh, the above only applies to fflush(stdin).

For fflush(stdout), it's flushing the output stream.

Was fflush(stdout) meant to be flush(stdin)?
[/EDIT]

(dave_sinkula, I think the code

Code:

fgets(line, sizeof line, stdin)

should be

Code:

fgets(line, sizeof(line), stdin)

)

**quzah** · 05-30-2005

You do know that fflush only works on output streams, right? Any other use of it is undefined, which means nothing at all has to happen. If it does, it's purely implemenation specific. It could, and still be 100% ANSI conformant, crash whatever it's running on, and start a World War.

Quzah.

**Dave_Sinkula** · 05-30-2005

Originally Posted by Antigloss

I found that without calling "fflush(stdout)", your program still run correctly, and exactly the same as fflush(stdout) is called, so I don't know why you call fflush(stdout) here. Is it neccessary to call fflush(stdout)? Could you please tell me?

http://www.eskimo.com/~scs/C-faq/q12.4.html

In my preferred command shell, the prompts do not appear.

Originally Posted by dwks

(dave_sinkula, I think the code

Code:

fgets(line, sizeof line, stdin)

should be

Code:

fgets(line, sizeof(line), stdin)

)

The parentheses are only necessary for types; they are not necessary for objects and I prefer to write it that way.

**Antigloss** · 05-30-2005

Originally Posted by quzah

You do know that fflush only works on output streams, right? Any other use of it is undefined, which means nothing at all has to happen. If it does, it's purely implemenation specific. It could, and still be 100% ANSI conformant, crash whatever it's running on, and start a World War.

Quzah.

Do you mean that fflush(stdin) isn't standard C, and should never be used, or it'll lead to many unexpected problems?

**sand_man** · 05-30-2005

Originally Posted by Antigloss

Do you mean that fflush(stdin) isn't standard C, and should never be used, or it'll lead to many unexpected problems?

Yes, it is in the FAQ.

**Antigloss** · 05-30-2005

Then, how to solve this problem?

Originally Posted by dwks

If the user inputs more than 80 characters, fgets() will read in the first 80 chars and leave the rest in the buffer. The loop will iterate again, and the remainder (ie, the chars beyond 80 chars) will be used the second time round.

**Antigloss** · 05-30-2005

I found the answer in the FAQ.
Thanks all~ ^_^

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