I read the FAQ on isdigit() but...

...I still do not understand how to check a user's input to ensure that it is a numeric character. I want to use the isdigit() function and I read the FAQ regarding the syntax of this function, but either the FAQ is too ambiguous or I'm not grasping the concept. (Probably the latter). I know what the isdigit() function is supposed to do -- and it's exactly what I need for my program -- but writing the blasted thing is my problem. I need more than just:

#include <ctype.h>

int isdigit(int c);

to help me write the function.

Furthermore, I want this thing to work inside a loop that already checks the input to make sure it falls within the range of acceptable numbers. Now I need the isdigit() function to first make sure the input is a number and not a letter, then it can check to see if it is within the range of acceptable numbers.

Any help would be appreciated.

CaptainJack

you should check for a digit by it's ascii value.

if (c>'48'||c<'57')

You should NOT check for a digit by its ascii value (nothing says the computer has to use ascii) and even then the code shown won't work...

I'm not totally sure I understand the original question. Maybe posting some of the code you are working on would help explain.

In the meantime take a look at this:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>

void checkChar(char ch)
{
  if (isdigit(ch)) {
    printf("%c is a number\n", ch);
  } 
  else {
    printf("%c is not a number\n", ch);
  }
}

int main ( void )
{
  char c1 = 'a';
  char c2 = '3';

  checkChar(c1);
  checkChar(c2);

  return 0;
}

HTH

so what is wrong with checking it's ascii value?
would it run slower?

ASCII is not the mandatory representation for characters in C
(although I will admit it is widely used). What is mandated in
the C specification is that numbers are coded sequentially, so
you could do this:

Code:

if ((ch >= '0') && (ch <= '9')) {
  /* ch is a digit */
}

but there is a perfectly good function, isdigit(), that does all this
for us so why bother? Speed is actually the only reason I can
think of not to use isdigit() - but there are probably better ways
to speed up your program!

A lot of people have been asking about isdigit recently. Just search the boards, I'm quite sure you question has already been answered in the past two days or so.