为什么 scanf 在第一次使用时有效，但在第二次使用时却无效？

Question

So I'm simply trying to accept input from the user in the form 1 2,or 0 0, 2 2, 0 1, etc. If the user enters this the first time around then everything works as expected. However, if they get the input wrong from the beginning, the program tells them that they have got it wrong and asks for the input again, but then this input is seemingly transposed and the first variable is blank whilst the second assumes the value intended for the first. What's going on?

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

int validation();

int main ( void ) {
    

    int valid = 0;
    
    while ( valid == 0 ){
        valid = validation();
    }

    return 0;
}

int validation(){
    
    char cfirst, csecond;
    int first, second, c;

    printf("Please enter your number:\n");
    scanf("%c %c", &cfirst, &csecond);

    printf("cfirst = %c, csecond = %c\n", cfirst, csecond);

    
    if( cfirst == '0' ){
        first = 0;
    }
    else if( ( cfirst == '1' || cfirst == '2' ) ){
        first = cfirst - '0';      // alternative to atoi for a singular character              
    }    
    else{
        printf("Move rejected. Please try again\n");
        while( ( c = getchar() ) != EOF && c != '\n');
        return 0;
    }
    
    if( csecond == '0'){
        second = 0;
    }
    else if( csecond == '1' || csecond == '2' ){
        second = csecond - '0';    // alternative to atoi for a singular character
    }
    else{
        printf("Move rejected. Please try again\n");
        while( ( c = getchar() ) != EOF && c != '\n');
        return 0;
    }

    //first = cfirst - '0';           
    //second = csecond - '0';

    printf("Your first number = %d\n", first);
    printf("Your second number = %d\n", second);

    return 0;
}

I tried using the line:

while( ( c = getchar() );= EOF && c != '\n');

to clear the input buffer (I think) so that scanf would work as it did the first time. This didn't work. Now I'm at a loss.

Answer 1

When reading a char with scanf you are reading all the characters in the buffer, including the enter '\n'. scanf reads the first data avalibale that matches the type declared and then stops. If your program reads the data succesfully the first time it stores the values, but in the buffer remains the enter '\n'. When trying to read a second time it gets the first two elements in the buffer, in this case the enter '\n' and the first value. When reading data with scanf you should always clean the buffer. You can use while(getchar!='\n') for cleaning the buffer after reading with scanf

Hope it helps!