验证特定字符的用户输入

Question

I'm creating a program that runs a card game, I need to get a trump suit (H, D, S, C) and validate user input. I am able to catch invalid stuff like j or 3, but if they enter HH, or SS (which is invalid) it thinks it's correct.

I've tried using strlen to see if there's more chars, and I thought maybe I could check if the buffer has other garbage in it, but have no idea how to do that. I also tried doing something like while(scanf(" %c", &trump) != 1 , but that didn't work. I've only been coding in c for a week.

my tump function

char getTrump(){
  char trump;
  char ch;

  char correct = 1;
  while(correct == 1){
    printf("Please choose a trump suit (H, D, S, C): ");
    scanf(" %c", &trump);
    while(getchar() != '\n');

    if(isValidSuit(trump) == 1){
      printf("Correct\n");
      return trump;
      correct = 0;
    }else{
      printf("Please enter a valid suit.\n");
    }
  }
}

and if it helps my isValidSuit function (which has to have a char as a parameter)

int isValidSuit(char c){
  char suits[] = {'H', 'D', 'S', 'C'};

  for(int i = 0; i < 4; i++){
    if(suits[i] == c){
      return 1;
    }
  }

  return 0;
}

an example of running it:

Please choose a trump suit (H, D, S, C): j
Please enter a valid suit.
Please choose a trump suit (H, D, S, C): HH
Correct

Answer 1

if they enter HH, or SS (which is invalid) it thinks it's correct.

Of course because when you give input HH , the scanf(" %c", &trump); read first H of input in trump variable and left the second H of input in the input buffer. After this, the getchar() in while loop reads the leftover user input. The isValidSuit() finds H as value of trump variable and returns 1 . Hence, you are getting Correct for HH or SS input.

To fix this problem, you can check if the input if more than 1 character and that character is not newline character than prompt user to enter valid input, like this:

char getTrump(){
    char trump;
    char ch;

    char correct = 1;
    while(correct == 1){
        printf("Please choose a trump suit (H, D, S, C): ");
        scanf(" %c", &trump);
        if (getchar() != '\n') {
            while(getchar() != '\n');
            printf("Please enter a valid suit.\n");
            continue;
        }
    .......
    .......

Answer 2

I think you would be better off (generally speaking anyway) using fgets for the user input. scanf is problematic by comparision. fgets reads input in this case from stdin (the keyboard). Now it's simple to remove any newline character and test the length.

char getTrump(){
  const int suitSize = 12;
  char suit[suitSize];
  bool correct = false;
  memset(suit, 0, suitSize);

  do
  {
    printf("Please choose a trump suit (H, D, S, C): ");
    fgets(suit, suitSize-1, stdin);

    if (suit[strlen(suit)-1] == '\n')
        suit[strlen(suit)-1] = '\0';

    // check input length only one char
    if (strlen(suit) == 1 && isValidSuit(*suit) == 1){
        printf("Correct\n");
        correct = true;
    }
    else{
        printf("Please enter a valid suit.\n");
    }
  } while (!correct);
  return *suit;
}