Question

I have a C code in which I am using standard library function isalpha() in ctype.h, This is on Visual Studio 2010-Windows.
In below code, if char c is ‘£’, the isalpha call returns an assertion as shown in the snapshot below:

char c='£';

if(isalpha(c))
{
    printf ("character %c is alphabetic\n",c);

}
else
{
    printf ("character %c is NOT alphabetic\n",c);
}

I can see that this might be because 8 bit ASCII does not have this character.

So how do I handle such Non-ASCII characters outside of ASCII table?

What I want to do is if any non-alphabetic character is found(even if it includes such character not in 8-bit ASCII table) i want to be able to neglect it.

Answer 1

You may want to cast the value sent to isalpha (and the other functions declared in <ctype.h>) to unsigned char

isalpha((unsigned char)value)

It’s one of the (not so) few occasions where a cast is appropriate in C.

---

Edited to add an explanation.

According to the standard, emphasis is mine

7.4

1 The header <ctype.h> declares several functions useful for classifying and mapping
characters. In all cases the argument is an int, the value of which shall be
representable as an unsigned char or shall equal the value of the macro EOF. If the
argument has any other value, the behavior is undefined.

The cast to unsigned char ensures calling isalpha() does not invoke Undefined Behaviour.