Program is skipping fgets without allowing input

问题:

Basically as the title says.. When my program is run from the console, it'll ask if you'd like to encrypt or decrypt.. and when I input e or E, it creates a new blank line (until I input some kind of text), then shows the "enter the text" and "enter the key" lines all at once..

So, in the console it would look something like:

Would you like to (E)ncrypt or (D)ecrypt? e

asdf jkl; <---- random user input to get the program to continue..

Enter the text you would like to encrypt : Enter a key to use for encryption : (user input)

and then the program exits..

//message to be encrypted
char text[250]; 
//word to use as the key
char key[50];
//stores the encrypted word
char encrypted[250];

char answer;
printf("Would you like to (E)ncrypt or (D)ecrypt? ");
scanf(" %c", &answer);

if(answer == 'e' || answer == 'E')
{
    printf("Enter the text you want to encrypt : ");
    fgets(text, 250, stdin);

    printf("Enter a key to use for encryption : ");
    fgets(key, 50, stdin);

    printf("Encrypted text : ");

    //code that encrypts the text here      
}

So the problem, then, is that it's skipping the fgets entirely and not waiting/allowing the user to input any answers.. why for?

回答1:

The line scanf(" %c", &answer); is leaving a newline in the input buffer which is taken by fgets. The leading space in " %c" consumes leading whitespace but not trailing whitespace.

You can get rid of the newline with the "%*c" format specifier in scanf which reads the newline but discards it. No var argument needs to be supplied.

#include <stdio.h>

int main(void)
{
    char answer;
    char text[50] = {0};
    scanf(" %c%*c", &answer);
    fgets(text, sizeof text, stdin);
    printf ("%c %s\n", answer, text);
    return 0;
}

回答2:

From http://www.cplusplus.com/reference/cstdio/fgets/

"Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first."

Presumably you press Enter after typing E or D. Your scanf() doesn't consume the newline so it remains in the input stream. fgets() sees the newline and returns.