How to stop user entering char as int input in C

Question

Heres a part of my code:

    printf(\"\\nEnter amount of adult tickets:\");
    scanf(\"%d\", &TktAdult);
    while (TktAdult<0){
        printf(\"\\n         


        

Answer 1

... 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);

Answer 2

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?.

Answer 3

#include <stdio.h>

int main() {
    int x = 0.0;
    int readReasult;
    while(1)
    {
        readReasult = scanf("%d",&x);
        if (readReasult < 0) break;
        if (readReasult == 1) {
            if (x >= 0)
                printf("next number: %d\n", x);
            else
                printf("negative value not expected\n");
        } else {
            clearerr(stdin);
            scanf("%*s");
            printf("wrong input\n");
        }
    }

    return 0;
}

https://godbolt.org/z/xn86qz

Answer 4

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).

Answer 5

// 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);