C isupper() function

Question

I'm currently reading "The C Programming Language 2nd edition" and I'm not clear about this exercise: Functions like isupper can be implemented to save space or to save time. Explore both possibilities. - How can I implement this function? - How should I write two versions, one to save time and one to save space (some pseudo code would be nice)? I would appreciate some advice about this.

Accepted Answer

Original answer One version uses an array initialized with appropriate values, one byte per character in the code set (plus 1 to allow for EOF, which may also be passed to the classification functions): ``` static const char bits[257] = { ...initialization... }; int isupper(int ch) { assert(ch == EOF || (ch >= 0 && ch <= 255)); return((bits+1)[ch] & UPPER_MASK); } ``` Note that the 'bits' can be used by all the various functions like `isupper()`, `islower()`, `isalpha()`, etc with appropriate values for the mask. And if you make the 'bits' array changeable at runtime, you can adapt to different (single-byte) code sets. This takes space - the array. The other version makes assumptions about the contiguity of upper-case characters, and also about the limited set of valid upper-case characters (fine for ASCII, not so good for ISO 8859-1 or its relatives): ``` int isupper(int ch) { return (ch >= 'A' && ch <= 'Z'); // ASCII only - not a good implementation! } ``` This can (almost) be implemented in a macro; it is hard to avoid evaluating the character twice, which is not actually permitted in the standard. Using non-standard (GNU) extensions, it can be implemented as a macro that evaluates the character argument just once. To extend this to ISO 8859-1 would require a second condition, along the lines of: ``` int isupper(int ch) { return ((ch >= 'A' && ch <= 'Z')) || (ch >= 0xC0 && ch <= 0xDD)); } ``` Repeat that as a macro very often and the 'space saving' rapidly becomes a cost as the bit masking has a fixed size. Given the requirements of modern code sets, the mapping version is almost invariably used in practice; it can adapt at run-time to the current code set, etc, which the range-based versions cannot. Extended answer I still can't figure out how UPPER_MASK works. Can you explain it more specifically? Ignoring issues of namespaces for symbols in headers, you have a series of twelve classification macros: - `isalpha()` - `isupper()` - `islower()` - `isalnum()` - `isgraph()` - `isprint()` - `iscntrl()` - `isdigit()` - `isblank()` - `isspace()` - `ispunct()` - `isxdigit()` The distinction between `isspace()` and `isblank()` is: - `isspace()` — space (`' '`), form feed (`'\f'`), new-line (`' '`), carriage return (`' '`), horizontal tab (`' '`), and vertical tab (`'\v'`). - `isblank()` — space (`' '`), and horizontal tab (`' '`). There are definitions for these sets of characters in the C standard, and guidelines for the C locale. For example (in the C locale), either `islower()` or `isupper()` is true if `isalpha()` is true, but that need not be the true in other locales. I think the necessary bits are: - `DIGIT_MASK` - `XDIGT_MASK` - `ALPHA_MASK` - `LOWER_MASK` - `UPPER_MASK` - `PUNCT_MASK` - `SPACE_MASK` - `PRINT_MASK` - `CNTRL_MASK` - `BLANK_MASK` From these ten masks, you can create the other two: - ALNUM_MASK = ALPHA_MASK | DIGIT_MASK - GRAPH_MASK = ALNUM_MASK | PUNCT_MASK Superficially, you can also use `ALPHA_MASK = UPPER_MASK | LOWER_MASK`, but in some locales, there are alphabetic characters that are neither upper-case nor lower-case. So, we can define masks as follows: ``` enum CTYPE_MASK { DIGIT_MASK = 0x0001, XDIGT_MASK = 0x0002, LOWER_MASK = 0x0004, UPPER_MASK = 0x0008, ALPHA_MASK = 0x0010, PUNCT_MASK = 0x0020, SPACE_MASK = 0x0040, PRINT_MASK = 0x0080, CNTRL_MASK = 0x0100, BLANK_MASK = 0x0200, ALNUM_MASK = ALPHA_MASK | DIGIT_MASK, GRAPH_MASK = ALNUM_MASK | PUNCT_MASK }; extern unsigned short ctype_bits[]; ``` The data for the character set; the data shown is for the first half of ISO 8859-1, but is the same for the first half of all the 8859-x code sets. I'm using C99 designated initializers as a documentary aid, even though the entries are all in order: ``` unsigned short ctype_bits[] = { [EOF +1] = 0, ['\0' +1] = CNTRL_MASK, ['\1' +1] = CNTRL_MASK, ['\2' +1] = CNTRL_MASK, ['\3' +1] = CNTRL_MASK, ['\4' +1] = CNTRL_MASK, ['\5' +1] = CNTRL_MASK, ['\6' +1] = CNTRL_MASK, ['\a' +1] = CNTRL_MASK, ['\b' +1] = CNTRL_MASK, [' ' +1] = CNTRL_MASK|SPACE_MASK|BLANK_MASK, [' ' +1] = CNTRL_MASK|SPACE_MASK, ['\v' +1] = CNTRL_MASK|SPACE_MASK, ['\f' +1] = CNTRL_MASK|SPACE_MASK, [' ' +1] = CNTRL_MASK|SPACE_MASK, ['\x0E'+1] = CNTRL_MASK, ['\x0F'+1] = CNTRL_MASK, ['\x10'+1] = CNTRL_MASK, ['\x11'+1] = CNTRL_MASK, ['\x12'+1] = CNTRL_MASK, ['\x13'+1] = CNTRL_MASK, ['\x14'+1] = CNTRL_MASK, ['\x15'+1] = CNTRL_MASK, ['\x16'+1] = CNTRL_MASK, ['\x17'+1] = CNTRL_MASK, ['\x18'+1] = CNTRL_MASK, ['\x19'+1] = CNTRL_MASK, ['\x1A'+1] = CNTRL_MASK, ['\x1B'+1] = CNTRL_MASK, ['\x1C'+1] = CNTRL_MASK, ['\x1D'+1] = CNTRL_MASK, ['\x1E'+1] = CNTRL_MASK, ['\x1F'+1] = CNTRL_MASK, [' ' +1] = SPACE_MASK|PRINT_MASK|BLANK_MASK, ['!' +1] = PUNCT_MASK|PRINT_MASK,…

C isupper() function

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