Validate integer against chars

Question

I'm trying to make a program where the user inputs value to an array. What is actually required is that the program should validate against a char character. So if the user inputs a random char such as 'n' the program should tell him "You introduced a char, please input an integer: ".

How is that possible to make that without using a char variable?

for (i = 1; i <= size; i++) {
      printf("Introduce the value #%d of the list: ", i);
      scanf("%d", &list[i]);
      if () { // I'm blocked right in this line of code.
        printf("What you tried to introduce is a char, please input an integer: ");
        scanf("%d", &list[i]);
      }

Thanks in advance.

Answer 1

As @MFisherKDX says, check the return value of scanf. From the scanf man page:

These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errno is set indicate the error.

So capturing the return value of scanf in an int variable and then comparing that variable to 1 (in your case, because you are only attempting to read 1 item) should tell you if scanf successfully read an integer value.

However, there is a nasty pitfall when using scanf that you should be aware of. If you do type n at the prompt, scanf will fail and return 0, but it will also not consume the input you typed. Which means that the next time you call scanf, it will read the same input (the n character you typed), and fail again. And it will keep doing so no matter how many times you call scanf. It always amazes me that computer science educators continue to teach scanf to students, given not only this potential pitfall, but several other pitfalls as well. I wish I had a nickel for every hour that some CS student somewhere has spent struggling to get scanf to behave the way their intuition tells them it should. I'd be retired on my own private island by now. But I digress.

One way around this particular pitfall is to check if scanf failed, and if so, to purposely consume and discard all input from stdin up to and including the next newline character or EOF, whichever comes first.

First let's look at some unfixed code that causes an infinite loop if you enter a non-integer as input:

// Typing the letter 'n' and hitting <Enter> here causes an infinite loop:
int num, status;
while (1) {
  printf("Enter a number: ");
  status = scanf("%d", &num);
  if (status == 1)
    printf("OK\n");
  else
    printf("Invalid number\n");
}

The above code will (after you type n and hit <Enter>), will enter an infinite loop, and just start spewing "Invalid number" over and over. Again, this is because the n you entered never gets cleared out of the input buffer.

There are a few possible ways to get around this problem, but the consensus seems to be that the most portable and reliable way to do so is as follows:

// Fixed. No more infinite loop.
int num, status;
while (1) {
  printf("Enter a number: ");
  status = scanf("%d", &num);
  if (status == 1)
    printf("OK\n");
  else {
    printf("Invalid number\n");
    // Consume the bad input, so it doesn't keep getting re-read by scanf
    int ch;
    while ((ch = getchar()) != '\n' && ch != EOF) ;
    if (ch == EOF) break;
  }
}

Answer 2

The function scanf() will returns the number of elements read, so in this case it will return 1 every time it reads an int and 0 when it reads a char, so you just need to verify that return value. Keep in mind that after reading a character it will remain in the buffer so if you use the scanf() command again it will read the character again and repeat the error. To avoid that you need to consume the character with while(getchar() != '\n');

With that in mind I modified your code so that it works properly printing an error message if a character is introduced and asking for a new int.

for (int i = 1; i <= size; i++) {
      printf("Introduce the value #%d of the list: ", i);
      while (!scanf("%d", &list[i])) { //verifies the return of scanf
        while(getchar() != '\n'); //consumes the character in case of error
        printf("What you tried to introduce is a char\n");
        printf("please introduce the value #%d of the list: ", i);
      }
    }