为什么char比较“if（c> ='a'&& c <='z'）”不可移植？

Question

Some one told me that the char comparison if(c>='a' && c<='z') is not portable.

example

int main() {
    char c;
    scanf("%c", &c);
    if(c>='a' && c<='z')
        printf("lower case\n");
}

Any proof that the char comparison if(c>='a' && c<='z') is not portable?

Answer 1

The C standard does not guarantee that only lower case letters appear between 'a' and 'z' in the execution character set.

In the EBCDIC encoding, there are other characters between 'a' and 'z' , and some C implementations use EBCDIC.

The C standard does guarantee that the digits are consecutive, so '0' <= d && d <= '9' does test whether d is a decimal digit character.

Answer 2

Any proof that the char comparison if(c>='a' && c<='z')) is not portable?

c = '~';

if  (c>='a' && c<='z'))
{
   printf("Yes\n");
}

will print Yes in a AS/400 from IBM because in EBCDIC encoding ~ is between a and z .