Question

I need a brief explanation on how the two commands isdigit() and isalpha() work. Of course, I read online sources before asking the question, but I tried them and couldn’t get them to work. What is the simplest way of using them?

I know it gives back a value, so I’m assuming I can use it like this:

if(isdigit(someinput)==1)
 return -1;

Is that correct? Can I use this for any type of character? And can I compare it with a float number or array?

Suppose, I want to scanf a text file that has numbers and letter and determine what I’m scanning. Can these two commands be used in this context?

Answer 1

They are not “commands”, they are functions. Functions accept arguments and return values.

#include <ctype.h>
int isdigit( int ch );

This is the signature for the isdigit function: it indicates that it will accept an int value (or something that can be cast to int, like a char), and will return an int. You cannot, therefore, pass it an array (though you can call it on every member of an int[]).

The signature for isalpha is identical (except for the name, obviously).

The documentation says the following:

Description: The function isalpha()
returns non-zero if its argument is a
letter of the alphabet. Otherwise,
zero is returned.

This means your comparison will not be correct for all implementations. Better to do something like:

if (isdigit(someinput)) {
 return -1;
}

In C, 0 will evaluate to false in a boolean expression, and all non-zero values evaluate to true. So this check will cover implementations of isdigit that return -1, 5, whatever.

If you want to apply these to values in a text file, you must read the text one character at a time and pass the characters you receive to those methods.