How to stop user entering char as int input in C

问题

Heres a part of my code:

    printf("\nEnter amount of adult tickets:");
    scanf("%d", &TktAdult);
    while (TktAdult<0){
        printf("\nPlease enter a positive number!");
        printf("\nEnter amount of adult tickets:");
        scanf("%d", &TktAdult);
    }

Right now it can only stop user from entering a negative value, but how do I add to it so it also stops user from entering a char??

回答1:

The following code rejects user input that are:

non-numeric as handled by // 1;
negative numbers as handled by // 2;

positive numbers followed by non-numeric chars as handled by //3

while (1) {
    printf("\nEnter amount of adult tickets: ");
    if (scanf("%d", &TktAdult) < 0 ||  // 1
            TktAdult < 0 ||  // 2
            ((next = getchar()) != EOF && next != '\n')) {  // 3
        clearerr(stdin);
        do
            next = getchar();
        while (next != EOF && next != '\n');  // 4
        clearerr(stdin);
        printf("\nPlease enter a positive number!");
    } else {
        break;
    }
}

Also, // 4 clears the standard input of buffered non-numeric characters following case // 3 (i.e. 123sda - scanf takes 123 but leaves 'sda' in the buffer).

回答2:

... stop user from entering a negative value ...

is not possible. Users enter all sorts of gibberish. Instead, read a line of user input and parse it for correctness. Use fgets() for input, then sscanf(), strtol(), etc. for parsing.

// return -1 on EOF
int GetPositiveNumber(const char *prompt, const char *reprompt) {
  char buf[100];
  fputs(prompt, stdout);
  fflush(stdout);
  while (fgets(buf, sizeof buf, stdin)) [
    int value;
    if (sscanf(buf, "%d", &value) == 1 && value > 0) {
      return value;
    }
    fputs(reprompt, stdout);
    fflush(stdout);
  }
  return -1;
}

// Usage
int TktAdult = GetPositiveNumber(
    "\nEnter amount of adult tickets:" , 
    "\nPlease enter a positive number!");
if (TktAdult < 0) Handle_End_of_File();
else Success(TktAdult);

回答3:

but how do I add to it so it also stops user from entering a char??

You can't prevent user from entering character values. What you can do is check if tktAdult was successfully scanned. If not, flush out everything from stdin:

 bool valid = false;
 while (true) {
 ...

    if (scanf("%d", &TktAdult) != 1) {
       int c;
       while ((c = getchar()) != EOF && c != '\n');
    } else {
       break;
    }
}

The better alternative is to use fgets() and then parse the line using sscanf().

Also see: Why does everyone say not to use scanf? What should I use instead?.

回答4:

// You can try this,  Filter all except for the digital characters
int TktAdult;
char ch;
char str[10];
int i = 0;
printf("\nEnter amount of adult tickets:");
while ((ch = getchar()) != '\n')
{
    // Filter all except for the digital characters
    if(!isalpha(ch) && isalnum(ch))
        str[i++] = ch;
}
str[i] = '\0';
TktAdult = atoi(str);
printf("Done [%d]\n", TktAdult);

来源：https://stackoverflow.com/questions/41145908/how-to-stop-user-entering-char-as-int-input-in-c

标签

user-input