Consider below simple program in C. The program reads an integer using scanf(), then reads a string using fgets().
// 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; } |
Input
10 test
Output:
x = 10, str =
The problem with above code is scanf() reads an integer and leaves a newline character in buffer. So fgets() only reads newline and the string “test” is ignored by the program.
The similar problem occurs when scanf() is used in a loop.
// C program to demonstrate the problem when // scanf() is used in a loop #include<stdio.h> int main() { char c; printf("......Enter q to quit......
"); do { printf("Enter a character
"); scanf("%c", &c); printf("%c
", c); } while (c != 'q'); return 0; } |
Input
a b q
Output:
......Enter q to quit...... Enter a character a Enter a character Enter a character b Enter a character Enter a character q
We can notice that above program prints an extra “Enter a character” followed by an extra new line. This happens because every scanf() leaves a newline character in buffer that is read by next scanf.
How to solve above problem?
- We can make scanf() to read a new line by using an extra “ ”, i.e., scanf(“%d ”, &x) . In fact scanf(“%d “, &x) also works (Note extra space).
- We can add a getchar() after scanf() to read an extra newline.
See this and this for corrected programs.
Same problem occurs with Scanner in Java when nextLine() is used after nextXXX().
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