C isupper()函数

Question

C isupper()函数

pal*_*man 6 c

我目前正在阅读"C编程语言第2版",我不清楚这个练习:

可以实现像isupper这样的函数以节省空间或节省时间.探索两种可能性.

我该如何实现这个功能？
我应该如何编写两个版本,一个用于节省时间,一个用于节省空间(一些伪代码会很好)？

我会很感激对此提出一些建议.

Answer 1

Jon*_*ler 11

原始答案

一个版本使用一个用适当的值初始化的数组,代码集中每个字符一个字节(加1以允许EOF,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);
}

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注意,"比特"可以被所有的各种功能等中使用isupper(),islower(),isalpha(),等具有掩模适当的值.如果在运行时使'bits'数组可以更改,则可以适应不同的(单字节)代码集.

这需要空间 - 数组.

另一个版本假设有关大写字符的连续性,以及有限的有效大写字符集(对于ASCII很好,对ISO 8859-1或其亲属不太好):

int isupper(int ch)
{
    return (ch >= 'A' && ch <= 'Z');  // ASCII only - not a good implementation!
}

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这可以(几乎)在宏中实现; 很难避免两次评估字符,这在标准中实际上是不允许的.使用非标准(GNU)扩展,它可以实现为只评估一次字符参数的宏.要将其扩展到ISO 8859-1,则需要第二个条件,即:

int isupper(int ch)
{
    return ((ch >= 'A' && ch <= 'Z')) || (ch >= 0xC0 && ch <= 0xDD));
}

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经常重复这个宏作为宏,并且'节省空间'迅速成为成本,因为位掩码具有固定的大小.

鉴于现代代码集的要求,映射版本几乎总是在实践中使用; 它可以在运行时适应当前的代码集等,基于范围的版本不能.

扩展答案

我仍然无法弄清楚UPPER_MASK的工作原理.你能更具体地解释一下吗？

忽略标题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()和之间的区别isblank()是:

isspace()- space(' '),换页('\f'),换行('\n'),回车('\r'),水平制表符('\t')和垂直制表符('\v').
isblank()- space(' ')和horizontal tab('\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),如果为true,则为islower()或者为true,但在其他语言环境中不需要为true.isupper()isalpha()

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

从这十个面具中,您可以创建另外两个:

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,字母字符既不是大写也不是小写.

所以,我们可以定义如下掩码:

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[];

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字符集的数据; 显示的数据是ISO 8859-1的前半部分,但是对于所有8859-x代码集的前半部分是相同的.我使用C99指定的初始化程序作为纪录片辅助工具,即使条目全部按顺序排列:

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)

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如前所述,这里的名称实际上位于为用户保留的名称空间中,因此如果您查看<ctype.h>标题,您会发现更多含义模糊的名称,并且它们可能都以一个或两个下划线开头.

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