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Problem With Using fgets()/gets()/scanf() After scanf() in C

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  • Difficulty Level : Easy
  • Last Updated : 04 Jan, 2022
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scanf() is a library function in C. It reads standard input from stdin. fgets() is a library function in C. It reads a line from the specified stream and stores it into the string pointed to by the string variable. It only terminates when either:

  • end-of-file is reached
  • n-1 characters are read
  • the newline character is read

1) Consider a below simple program in C. The program reads an integer using scanf(), then reads a string using fgets(),

Input  

10
test

C




// C program to demonstrate the problem when
// fgets()/gets() is used after scanf()
#include <stdio.h>
  
int main()
{
    int x;
    char str[100];
    scanf("%d", &x);
    fgets(str, 100, stdin);
    printf("x = %d, str = %s", x, str);
    return 0;
}


Output

x = 10, str = 

Explanation: The problem with the above code is scanf() reads an integer and leaves a newline character in the buffer. So fgets() only reads newline and the string “test” is ignored by the program.

2) The similar problem occurs when scanf() is used in a loop. 

Input:

a
b
q 

C




// C program to demonstrate the problem when
// scanf() is used in a loop
#include <stdio.h>
  
int main()
{
    char c;
    printf("Press q to quit\n");
    do {
        printf("Enter a character\n");
        scanf("%c", &c);
        printf("%c\n", c);
    } while (c != 'q');
    return 0;
}


Output 

Press q to quit
Enter a character
a
Enter a character


Enter a character
b
Enter a character


Enter a character
q

Explanation: We can notice that the above program prints an extra “Enter a character” followed by an extra newline. This happens because every scanf() leaves a newline character in a buffer that is read by the next scanf.

How to Solve the Above Problem?  

  • We can make scanf() to read a new line by using an extra \n, i.e., scanf(“%d\n”, &x) . In fact scanf(“%d “, &x) also works (Note the extra space).
  • We can add a getchar() after scanf() to read an extra newline.

The corrected programs for the above points will be,

1) scanf() when there is fgets() after it:

Input:

10
test

C




// C program to demonstrate the problem when
// fgets()/gets() is used after scanf()
#include <stdio.h>
  
int main()
{
    int x;
    char str[100];
    scanf("%d\n", &x);
    fgets(str, 100, stdin);
    printf("x = %d, str = %s", x, str);
    return 0;
}


Output

x = 10, str = test

2) When scanf() is used in a loop: 

Input:

a
b
q 

C




// C program to demonstrate the problem when
// scanf() is used in a loop
#include <stdio.h>
  
// Driver Code
int main()
{
    char c;
    printf("Press q to quit\n");
    do {
        printf("Enter a character\n");
        scanf("%c\n", &c);
        printf("%c\n", c);
    } while (c != 'q');
    return 0;
}


Output: Press q to quit

Enter a character
a
Enter a character
b
Enter a character
q

Must Read: Problem occurs with Scanner in Java when nextLine() is used after nextXXX()

This article is contributed by Dheeraj Gupta. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 


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