为什么 printf 在循环中只打印一次，仅用于第一个输入？

Question

void hexToDec(){
    char numHex, numDec;
    int isNum, isLowAf, isHighAf;
    printf("Enter a reversed number in base 16:\n");
    scanf("%c", &numHex);
    while(numHex != '\n'){
        isNum = isHighAf = isLowAf = 0;
        if(numHex >= 48 && numHex <= 57)
            isNum = 1;
        if(numHex >= 65 && numHex <= 70)
            isHighAf = 1;
        if(numHex >= 97 && numHex <= 102)
            isLowAf = 1;
        if(!isNum && !isLowAf && !isHighAf)
             printf("Error! %c is not a valid digit in base 16\n", numHex);
        //else - Hexadecimal to Decimal converter
        fflush(stdin);
        scanf("%c", &numHex);
    }
}

I can't use string.h or arrays[] this task and I need to check every input I get and print every char that isn't digit in base 16. The problem is that it only check the first letter I enter and print not valid for it.

for example:

input:

lds

output:

Error! l is not a valid digit in base 16

expected:

Error! l is not a valid digit in base 16
Error! s is not a valid digit in base 16

Also I can't figure out why the while loop doesn't stop after I click Enter.

Answer 1

fflush(stdin) is not standard C. But on systems where it works (Windows), it will discard all the buffered input that hasn't yet been. So after scanning the first character l , this will cause ds to be discarded, and it will wait for you to type a new line of input.

Get rid of that call if you want it to process the remaining characters of the line.

Answer 2

The while loop is not exiting because of the space in the second scanf .

Would be a tad nicer if character constants are used instead of hard coded numbers.

 while(numHex != '\n'){
    isNum = isHighAf = isLowAf = 0;
    if(numHex >= '0' && numHex <= '9')
        isNum = 1;
    else if(numHex >= 'A' && numHex <= 'F')
        isHighAf = 1;
    else if(numHex >= 'a' && numHex <= 'f')
        isLowAf = 1;
    if(!isNum && !isLowAf && !isHighAf)
         printf("Error! %c is not a valid digit in base 16\n", numHex);
    //else - Hexadecimal to Decimal converter
    //fflush(stdin);//undefined behavior
    scanf("%c", &numHex);
}