Input in C. Scanf before gets. Problem

问题:

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!

回答1:

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);
...

回答2:

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

回答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;
}

回答4:

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

int main(void) {
        int a;
        char b[20];
        printf("Input your ID: ");
        scanf("%d", &a);
        getchar();
        printf("Input your name: ");
        gets(b);
        printf("---------");
        printf("Name: %s", b);
        return 0;
}



Note: 
  If you use the scanf first and the fgets second, it will give problem only. It will not read the second character for the gets function. 

  If you press enter, after give the input for scanf, that enter character will be consider as a input f or fgets.

回答5:

you should do this way.

    fgetc(stdin);
    scanf("%c",&c);
    if(c!='y')
    {
        break;
    }
    fgetc(stdin);

to read input from scanf after reading through gets.

回答6:

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]);

回答7:

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;
}

回答8:

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