"The C Programming Language", 2nd edition, Kernighan and Ritchie
Answer to Exercise 7-9, page 168
Functions like isupper can be implemented to save space or to save time. Explore both possibilities.
This question is best left to an essay rather than code, so here's my take:
The easiest way to implement the eleven is() functions in C90's version
of <ctype.h> is via a table lookup. If UCHAR_MAX is 255, then a table
would take up around 514 8-bit bytes and still have room for five more
is() functions. In modern programs, this is a miniscule expense of both
space and time since a mere table lookup doesn't cost a whole lot
(although space may be a priority for embedded systems).
Additionally, since the is() functions of <ctype.h> are locale-dependent
and therefore subject to locale-specific whims, a table could more
easily be modified than modifying hard calculations. Consider the
following three implementations of isupper() :
Implementation #1:
int isupper(int c)
{
return (c >= 'A' && c <= 'Z');
}
Implementation #2:
int isupper(int c)
{
return (strchr("ABCDEFGHIJKLMNOPQRSTUVWXYZ", c) != NULL);
}
Implementation #3:
/* Presumably, _UP is a power of 2 and
* _Ctype is a table
*/
int isupper(int c)
{
return ((_Ctype[(unsigned char)c] & _UP) != 0);
}
Implementation #1 fails in EBCDIC and implementation #2 fails in a
locale that adds more upperspace characters than the ones mentioned.
Implementation #3, however, suggests that _Ctype[] can be modified to
accommodate new uppercase characters.
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