Thread: Question about how scanf works

Hi I am trying to make it so my program will accept only 2 input at a time if more then 2 are written. For example I want it so when i run main and it asks me to put a number and a string and another string, and I put something like foo bar cake it will ignore the cake completely and throw it away instead of what it currently does which would be take cake as the first input for the next run through.
sample input and output is
Please enter a string and another string
foo bar
String 1 = foo , String 2 = bar
Please enter a string and another string
foo bar cake
String 1 = foo , String 2 = bar
Please enter a string and another string
foo bar
String 1 = cake , String 2 = foo
I thought what would work would be adding the fflush(stdin); after every run through but that didn't change anything. Any help would be appreciated.

Code:

import <stdio.h>
int main ( int num_args, char *args[] ){

    while(!feof(stdin)){
        char str[20];
        char str2[20];
        printf("Please enter a string and another string\n");
        scanf("%s",str);
        scanf("%s",str2);
        printf("String 1 = %s , String 2 = %s\n",str,str2);
     }
}

Not doable?

Code:

char buff[BUFSIZ];
while ( fgets( buff, sizeof buff, stdin ) ) {
        char str[20];
        char str2[20];
        if ( sscanf( buff, "%s %s", str, str2) == 2 ) {
                printf("String 1 = %s , String 2 = %s\n",str,str2);
        }
}

You can't do what you want with scanf() alone.

What you'll need to do is read the string using fgets(), and then interpret it to extract strings you want. Logically, if you extract strings you want, and do nothing with any others the user might type in, they are effectively discarded.

It is generally not a good idea to loop on feof() either. There are various FAQ's around that explain why.

Rough code follows (note: it is written to illustrate the idea, not intended as perfection in coding);

Code:

#include the headers needed

int main()
{
      while (1)
      {
             char str[20];
             char str2[20];
             char buffer[100];
             fprintf(stdout, "Please enter a string and another string\n");
             if (fgets(buffer, sizeof(buffer),  stdin) == NULL) break;  /*  break on error or EOF */
             int strings_read = sscanf(buffer, "%19s %19s", str, str2))
             if (strings_read == 1)
             {
                   /*  Only 1 string input, nag user until we get the second  */
                   do
                   {
                       strings_read = 0;
                       fprintf(stdout, "You only entered one string.  Please input the second");
                   } while (fgets(buffer, sizeof(buffer), stdin) != NULL && (strings_read = sscanf(buffer, "%19s", str2)) != 1);
                    if (strings_read == 1)  ++strings_read;   /*  since this is the second string */
             }
             if (strings_read == 2)
                  fprintf(stdout, "String 1 = %s , String 2 = %s\n",str,str2);
             else  /*  an error occurred */
                  break;
        }
   }
}