我目前正在阅读“The C Programming Language 2nd edition”，我对这个练习不太清楚:
Functions like isupper can be implemented to save space or to save time. Explore both possibilities.
- 如何实现这个功能？
- 我应该如何写两个版本，一个是为了节省时间，一个是为了 节省空间(一些伪代码会很好)？
如果能给我一些建议，我将不胜感激。
最佳答案
原始答案
一个版本使用一个用适当值初始化的数组，代码集中每个字符一个字节(加 1 以允许 EOF，它也可以传递给分类函数):
static const char bits[257] = { ...initialization... };
int isupper(int ch)
{
assert(ch == EOF || (ch >= 0 && ch <= 255));
return((bits+1)[ch] & UPPER_MASK);
}
请注意，“位”可由所有各种函数使用，如 isupper() 、 islower() 、 isalpha() 等，并为掩码设置适当的值。如果您使“位”数组在运行时可变，则可以适应不同的(单字节)代码集。
这需要空间——数组。
另一个版本假设大写字符的连续性，以及有限的有效大写字符集(对 ASCII 没问题，对 ISO 8859-1 或其相关标准不太好):
int isupper(int ch)
{
return (ch >= 'A' && ch <= 'Z'); // ASCII only - not a good implementation!
}
这可以(几乎)用宏来实现；很难避免对字符进行两次评估，这在标准中实际上是不允许的。使用非标准 (GNU) 扩展，它可以实现为仅对字符参数求值一次的宏。将其扩展到 ISO 8859-1 需要第二个条件，即:
int isupper(int ch)
{
return ((ch >= 'A' && ch <= 'Z')) || (ch >= 0xC0 && ch <= 0xDD));
}
经常将其作为宏重复，由于位掩码具有固定大小，“空间节省”很快成为一种成本。
鉴于现代代码集的要求，映射版本在实践中几乎无一异常(exception)地被使用；它可以在运行时适应当前代码集等，而基于范围的版本则不能。
扩展答案
I still can't figure out how UPPER_MASK works. Can you explain it more specifically?
忽略 header 中符号的命名空间问题，您有一系列十二个分类宏:
-
isalpha() -
isupper() -
islower() -
isalnum() -
isgraph() -
isprint() -
iscntrl() -
isdigit() -
isblank() -
isspace() -
ispunct() -
isxdigit()
isspace()和isblank()的区别是:
-
isspace()—空格 (' ')、换页符 ('\f')、换行符 ('\n')、回车符 ('\r')、水平制表符 ('\t') 和垂直制表符 ('\v'< )/li> -
isblank()— 空格 (' ') 和水平制表符 ('\t')。
C 标准中有这些字符集的定义，以及 C 语言环境的指南。
例如(在 C 语言环境中)，如果 islower() 为真，则 isupper() 或 isalpha() 为真，但在其他语言环境中则不必为真。
我认为必要的部分是:
-
DIGIT_MASK -
XDIGT_MASK -
ALPHA_MASK -
LOWER_MASK -
UPPER_MASK -
PUNCT_MASK -
SPACE_MASK -
PRINT_MASK -
CNTRL_MASK -
BLANK_MASK
从这十个面具，你可以创建另外两个:
- ALNUM_MASK = ALPHA_MASK | DIGIT_MASK
- GRAPH_MASK = ALNUM_MASK | PUNCT_MASK
从表面上看，您也可以使用 ALPHA_MASK = UPPER_MASK | LOWER_MASK ，但在某些语言环境中，存在既不是大写也不是小写的字母字符。
因此，我们可以如下定义掩码:
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[];
字符集的数据；显示的数据适用于 ISO 8859-1 的前半部分，但与所有 8859-x 代码集的前半部分相同。我使用 C99 指定的初始值设定项作为文档辅助，即使条目都是按顺序排列的:
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,
['\t' +1] = CNTRL_MASK|SPACE_MASK|BLANK_MASK,
['\n' +1] = CNTRL_MASK|SPACE_MASK,
['\v' +1] = CNTRL_MASK|SPACE_MASK,
['\f' +1] = CNTRL_MASK|SPACE_MASK,
['\r' +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,
['"' +1] = PUNCT_MASK|PRINT_MASK,
['#' +1] = PUNCT_MASK|PRINT_MASK,
['$' +1] = PUNCT_MASK|PRINT_MASK,
['%' +1] = PUNCT_MASK|PRINT_MASK,
['&' +1] = PUNCT_MASK|PRINT_MASK,
['\'' +1] = PUNCT_MASK|PRINT_MASK,
['(' +1] = PUNCT_MASK|PRINT_MASK,
[')' +1] = PUNCT_MASK|PRINT_MASK,
['*' +1] = PUNCT_MASK|PRINT_MASK,
['+' +1] = PUNCT_MASK|PRINT_MASK,
[',' +1] = PUNCT_MASK|PRINT_MASK,
['-' +1] = PUNCT_MASK|PRINT_MASK,
['.' +1] = PUNCT_MASK|PRINT_MASK,
['/' +1] = PUNCT_MASK|PRINT_MASK,
['0' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['1' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['2' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['3' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['4' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['5' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['6' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['7' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['8' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
['9' +1] = DIGIT_MASK|PRINT_MASK|XDIGT_MASK,
[':' +1] = PUNCT_MASK|PRINT_MASK,
[';' +1] = PUNCT_MASK|PRINT_MASK,
['<' +1] = PUNCT_MASK|PRINT_MASK,
['=' +1] = PUNCT_MASK|PRINT_MASK,
['>' +1] = PUNCT_MASK|PRINT_MASK,
['?' +1] = PUNCT_MASK|PRINT_MASK,
['@' +1] = PUNCT_MASK|PRINT_MASK,
['A' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['B' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['C' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['D' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['E' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['F' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK|XDIGT_MASK,
['G' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['H' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['I' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['J' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['K' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['L' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['M' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['N' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['O' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['P' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['Q' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['R' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['S' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['T' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['U' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['V' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['W' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['X' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['Y' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['Z' +1] = ALPHA_MASK|UPPER_MASK|PRINT_MASK,
['[' +1] = PUNCT_MASK|PRINT_MASK,
['\\' +1] = PUNCT_MASK|PRINT_MASK,
[']' +1] = PUNCT_MASK|PRINT_MASK,
['^' +1] = PUNCT_MASK|PRINT_MASK,
['_' +1] = PUNCT_MASK|PRINT_MASK,
['`' +1] = PUNCT_MASK|PRINT_MASK,
['a' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['b' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['c' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['d' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['e' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['f' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK|XDIGT_MASK,
['g' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['h' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['i' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['j' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['k' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['l' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['m' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['n' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['o' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['p' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['q' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['r' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['s' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['t' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['u' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['v' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['w' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['x' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['y' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['z' +1] = ALPHA_MASK|LOWER_MASK|PRINT_MASK,
['{' +1] = PUNCT_MASK|PRINT_MASK,
['|' +1] = PUNCT_MASK|PRINT_MASK,
['}' +1] = PUNCT_MASK|PRINT_MASK,
['~' +1] = PUNCT_MASK|PRINT_MASK,
['\x7F'+1] = CNTRL_MASK,
...continue for second half of 8859-x character set...
};
#define isalpha(c) ((ctype_bits+1)[c] & ALPHA_MASK)
#define isupper(c) ((ctype_bits+1)[c] & UPPER_MASK)
#define islower(c) ((ctype_bits+1)[c] & LOWER_MASK)
#define isalnum(c) ((ctype_bits+1)[c] & ALNUM_MASK)
#define isgraph(c) ((ctype_bits+1)[c] & GRAPH_MASK)
#define isprint(c) ((ctype_bits+1)[c] & PRINT_MASK)
#define iscntrl(c) ((ctype_bits+1)[c] & CNTRL_MASK)
#define isdigit(c) ((ctype_bits+1)[c] & DIGIT_MASK)
#define isblank(c) ((ctype_bits+1)[c] & BLANK_MASK)
#define isspace(c) ((ctype_bits+1)[c] & SPACE_MASK)
#define ispunct(c) ((ctype_bits+1)[c] & PUNCT_MASK)
#define isxdigit(c) ((ctype_bits+1)[c] & XDIGT_MASK)
如前所述，此处的名称实际上位于为用户保留的命名空间中，因此如果您查看 <ctype.h> header ，您会发现更多神秘名称，它们可能都以一两个下划线开头。
关于C isupper() 函数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/2169261/