Input in C. Scanf before gets. Problem

Question

I'm pretty new to C, and I have a problem with inputing data to the program.

My code:

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

int main(void) {
   int a;
   char b[20];

   printf("Input your ID: ");
   scanf("%d", &a);

   printf("Input your name: ");
   gets(b);   

   printf("---------");

   printf("Name: %s", b);   

   system("pause");
   return 0;
}

It allows to input ID, but it just skips the rest of the input. If I change the order like this:

printf("Input your name: ");
   gets(b);   

   printf("Input your ID: ");
   scanf("%d", &a);

It will work. Although, I CANNOT change order and I need it just as-is. Can someone help me ? Maybe I need to use some other functions. Thanks!

Answer 1

scanf doesn't consume the newline and is thus a natural enemy of fgets. Don't put them together without a good hack. Both of these options will work:

// Option 1 - eat the newline
scanf("%d", &a);
getchar(); // reads the newline character

// Option 2 - use fgets, then scan what was read
char tmp[50];
fgets(tmp, 50, stdin);
sscanf(tmp, "%d", &a);
// note that you might have read too many characters at this point and
// must interprete them, too

Answer 2

scanf("%d", &a); can't read the return, because %d accepts only decimal integer. So you add a \n at the beginning of the next scanf to ignore the last \n inside the buffer.

Then, scanf("\n%s", b); now can reads the string without problem, but scanf stops to read when find a white space. So, change the %s to %[^\n]. It means: "read everthing but \n"

scanf("\n%[^\n]", b);

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

int main(void) {
    int a;
    char b[20];

    printf("Input your ID: ");
    scanf("%d", &a);

    printf("Input your name: ");
    scanf("\n%[^\n]", b);
    //first \n says to ignore last 'return'
    //%[^\n] read until find a 'return'  
    printf("---------\n");
    printf("Name: %s\n\n", b);   

    system("pause");
    return 0;
}

Answer 3

scanf will not consume \n so it will be taken by the gets which follows the scanf. flush the input stream after scanf like this.

#include <stdlib.h>
#include <string.h>

int main(void) {
   int a;
   char b[20];

   printf("Input your ID: ");
   scanf("%d", &a);
   fflush(stdin);
   printf("Input your name: ");
   gets(b);   

   printf("---------");

   printf("Name: %s", b);   

   system("pause");
   return 0;
}

Answer 4

Try:

scanf("%d\n", &a);

gets only reads the '\n' that scanf leaves in. Also, you should use fgets not gets: http://www.cplusplus.com/reference/clibrary/cstdio/fgets/ to avoid possible buffer overflows.

Edit:

if the above doesn't work, try:

...
scanf("%d", &a);
getc(stdin);
...

Answer 5

Just use 2 gets() functions

When you want to use gets() after a scanf(), you make sure that you use 2 of the gets() functions and for the above case write your code like:

int main(void) {
   int a;
   char b[20];

   printf("Input your ID: ");
   scanf("%d", &a);

//the change is here*********************
   printf("Input your name: ");
   gets(b);
   gets(b);   
//the change is here*********************

   printf("---------");

   printf("Name: %s", b);   

   system("pause");
   return 0;
}

For explanation ([e-mail]);

Answer 6

The scanf function removes whitespace automatically before trying to parse things other than characters. %c, %n, %[] are exceptions that do not remove leading whitespace.

gets is reading the newline left by previous scanf. Catch the newline usinggetchar();

scanf("%d", &a);
getchar(); // catches the newline character omitted by scanf("%d")
gets(b);

https://wpollock.com/CPlus/PrintfRef.htm