Way on c & c++ C岁月 Part 1 —— 【篇二】
<!-- [if !mso]> <mce:style><!-- v/:* {behavior:url(#default#VML);} o/:* {behavior:url(#default#VML);} w/:* {behavior:url(#default#VML);} .shape {behavior:url(#default#VML);} --> <!-- [endif]--><!-- [if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:PunctuationKerning/> <w:DrawingGridVerticalSpacing>7.8 磅</w:DrawingGridVerticalSpacing> <w:DisplayHorizontalDrawingGridEvery>0</w:DisplayHorizontalDrawingGridEvery> <w:DisplayVerticalDrawingGridEvery>2</w:DisplayVerticalDrawingGridEvery> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:SpaceForUL/> <w:BalanceSingleByteDoubleByteWidth/> <w:DoNotLeaveBackslashAlone/> <w:ULTrailSpace/> <w:DoNotExpandShiftReturn/> <w:AdjustLineHeightInTable/> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> <w:UseFELayout/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!-- [if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--><!-- [if !mso]> <object classid="clsid:38481807-CA0E-42D2-BF39-B33AF135CC4D" id=ieooui> </object> <mce:style><!-- st1/:*{behavior:url(#ieooui) } --> <!-- [endif]--> <!-- /* Font Definitions */ @font-face {font-family:宋体; panose-1:2 1 6 0 3 1 1 1 1 1; mso-font-alt:SimSun; mso-font-charset:134; mso-generic-font-family:auto; mso-font-pitch:variable; mso-font-signature:3 135135232 16 0 262145 0;} @font-face {font-family:"Comic Sans MS"; panose-1:3 15 7 2 3 3 2 2 2 4; mso-font-charset:0; mso-generic-font-family:script; mso-font-pitch:variable; mso-font-signature:647 0 0 0 159 0;} @font-face {font-family:"/@宋体"; panose-1:2 1 6 0 3 1 1 1 1 1; mso-font-charset:134; mso-generic-font-family:auto; mso-font-pitch:variable; mso-font-signature:3 135135232 16 0 262145 0;} @font-face {font-family:新宋体; panose-1:2 1 6 9 3 1 1 1 1 1; mso-font-charset:134; mso-generic-font-family:modern; mso-font-pitch:fixed; mso-font-signature:3 135135232 16 0 262145 0;} @font-face {font-family:"/@新宋体"; panose-1:2 1 6 9 3 1 1 1 1 1; mso-font-charset:134; mso-generic-font-family:modern; mso-font-pitch:fixed; mso-font-signature:3 135135232 16 0 262145 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; text-align:justify; text-justify:inter-ideograph; mso-pagination:none; font-size:10.5pt; mso-bidi-font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:宋体; mso-font-kerning:1.0pt;} a:link, span.MsoHyperlink {color:blue; text-decoration:underline; text-underline:single;} a:visited, span.MsoHyperlinkFollowed {color:purple; text-decoration:underline; text-underline:single;} /* Page Definitions */ @page {mso-page-border-surround-header:no; mso-page-border-surround-footer:no;} @page Section1 {size:612.0pt 792.0pt; margin:72.0pt 90.0pt 72.0pt 90.0pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!-- [if gte mso 10]> <mce:style><!-- /* Style Definitions */ table.MsoNormalTable {mso-style-name:普通表格; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} --> <!-- [endif]--><!-- [if gte mso 9]><xml> <o:shapedefaults v:ext="edit" spidmax="1047"/> </xml><![endif]--><!-- [if gte mso 9]><xml> <o:shapelayout v:ext="edit"> <o:idmap v:ext="edit" data="1"/> <o:rules v:ext="edit"> <o:r id="V:Rule1" type="connector" idref="#_x0000_s1044"> <o:proxy start="" idref="#_x0000_s1042" connectloc="7"/> <o:proxy end="" idref="#_x0000_s1043" connectloc="0"/> </o:r> </o:rules> </o:shapelayout></xml><![endif]-->
<!-- [if supportFields]><span lang=EN-US><span style="mso-element:field-begin" mce_style="mso-element:field-begin"></span><span style='mso-spacerun:yes'> </span>= 2 /* GB2 <span style="mso-element:field-separator" mce_style="mso-element:field-separator"></span></span><![endif]-->⑵ <!-- [if supportFields]><span lang=EN-US><span style="mso-element:field-end" mce_style="mso-element:field-end"></span></span><![endif]-->简单的数据类型？感觉不那么简单。
C 语言的数据类型我认为可以划分为 3 类，分别是空类型 void 、基本类型（字符型、整型、浮点型和枚举类型）和派生类型（数组、函数、指针、结构体和共用体）。
标准 C 上规定 int 类型范围为 -32768 ~ 32767 ，然而实际上往往并非如此，这取决于编译器的设计，比如下述代码：
#include <stdio.h>
int main(int argc, char *argv[]){
printf("%d/n" , sizeof (int ));
printf("%d/n" , sizeof (float ));
printf("%d/n" , sizeof (double ));
printf("%d/n" , sizeof (long long ));
return 0;
}
在 vc2008 中的输出为： 4 4 8 8 。
这说明了在 vc2008 编译环境下， int 类型占据了 4 个字节的内存空间。而看到 int 类型，很容易让人联想到自加运算（ ++ ），关于自加运算不妨看看 a+++b 、 a+++++b 和 a+++(++b) 有何区别。下面是测试的结果：
#include <stdio.h>
int main(int argc, char *argv[]){
int a,b,c;
a = b = c = 0;
//c = a+++b; // 输出 1,0,0
//c = a+++++b; // 无法通过编译，提示 ++ 需要左值
//c = a+++(++b);// 输出 1,1,1
printf("%d,%d,%d/n" , a,b,c);
return 0;
}
插入旁白：关于左值、右值，网上有人说是翻译错误，应理解为可寻址的和可读的，我觉得也颇有几分道理，但这里就引用编译器报错原文。
第一种结果很容易理解，先执行 c=a+b ，之后 a 自加 1 ；第三种结果也好理解，在赋值运算前 ++b 先得到执行，所以 c 为 1 ，赋值之后 a++ 得到执行。关键在第二种，为什么会编译出错？“ ++ 需要左值”是指哪个“ ++ ”？为什么是那个“ ++ ”？在此，不妨反问一句，如果让自己设计编译器，那么遇到 a+++++b 这样的语句应该怎样处理？先带着这个疑问继续探寻。
编译器会对 a+++++b 这样的表达式进行出错处理，但却接受 a+++b 这样的表达式，从一定程度上说明了编译器“聪明”到可以处理 a++ ，但却还不够“聪明”去处理 ++b 。当然，这里指的 ++b 是特定地、位于 a+++++b 这样表达式当中的限定场合的。为什么呢？这就属于词法分析的问题了。仍然回到上面那个问题，如果让自己设计编译器，遇到 a+++b 要怎么办呢？
为了找出是哪个“ ++ ”需要左值，我们不妨将代码稍加改动为：
#include <stdio.h>
int main(int argc, char *argv[]){
int a,b,c;
a = b = c = 0;
c = ((a++)++)+b;
printf("%d,%d,%d/n" , a,b,c);
return 0;
}
编译器仍然提示同样的错误，于是我们可以推出是第三个加号和第四个加号组成的“ ++ ”运算符需要左值，即 a+++++b 等效于 ((a++)++)+b 。
想来现在的最新问题是，(a++) 后面为什么不能接自加运算“++ ”？
回到l-value 和r-value 上来，不管采用左值、右值一说，或者采用可寻址值和可读值一说，其本质都是对内存的读写操作（我无法确定这里是否有寄存器操作），也就是说源操作数和目的操作数的地址要能确定。这问题个人感觉不是很好回答，因为依旧涉及到了编译器的设计问题，在此仅谈谈个人的理解。
++a 是先对a 执行加1 操作，然后返回对a 的引用，自然就可以确定此时a 的地址，所以 (++a)++ 是可以编译通过的（于visual c++ 2008编译环境，经博友Promi测试，VC6和GCC并不能编译通过 ）。而a++ 是新建了一个临时变量并赋予其当前a 的值，再对a 进行增1 运算，最后返回临时变量值，此时就无法再编译的时候获得该临时变量的地址，于是造成了((a++)++)+b 无法编译通过。
由此可知，在++a 和a++ 不影响逻辑结构的情况下，我们应该倾向于使用++a 。需要强调的是，我使用的是vc2008 编译环境。
接着回到编译器设计考虑的问题。在C 语言编译器的词法分析中有“贪心法”一说，因为存在“++ ”运算，所以遇到“+ ”自然要再往前看是否还有“+ ”。或许应该换一种说法，在我的知识范围内，词法分析都有“贪心法”一说，即识别单词过程应该包含尽可能多的字符。这很好理解，比如 C 语言中要求变量名必须以下划线或者字母开头，之后可以下划线、字母和数字混合，那么当我们遇到 _abc123def 时，我们难道分析道 _abc 就停止了吗？自然不是的，我们要继续往前以识别出一个完整的单词。这是词法分析的知识范畴，那么编译器是如何识别出标识符的呢？关于这个问题，杭电 ACM 网站上有这么一道简单题，可以去测试一下自己的思路对不， http://acm.hdu.edu.cn/showproblem.php?pid=2024 。再深入想想，如果是判断合法的数值类型应该怎么操作呢？这里的数值类型包括整形、浮点型和指数型。
对于上述杭电 2024 题目，我第一次的思路是分为两种情况：第一个字母只能有下划线和字母，其它字母只能有下划线、字母或数字，于是产生了以下代码：
#include <stdio.h>
#include <ctype.h>
int main(int argc, char *argv[]){//hdu2024_1
int n,flag;
char id[100];
scanf("%d" , &n);
getchar();// 由于 scanf 会将 '/n' 放在缓冲区中，所以先处理掉它
while (n--){
flag = 0;
gets(id);
if (!(id[0] == '_' || isalpha(id[0]))) flag = 1;
for (int i=1; id[i] != '/0' ; ++i){
if (!(isalpha(id[i]) || isdigit(id[i]) || id[i] == '_' )){
flag = 1;
break ;
}
}
printf("%s/n" , (flag == 1) ? "no" : "yes" );
}
return 0;
}
这种思路比较简单，适用于识别标识符，但如果是要识别数值呢（含整形、浮点和指数型）？这里给出另外一种识别标识符的思路，可以用来识别数值类型，即编译器设计时使用的状态转换图（或其改进后的自动机）：
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上述识别标识符转换图的含义为：开始状态为 0 ，如果遇到字母或者下划线则转向状态 1 ；在状态 1 下，如果遇到字母、数组或者下划线则仍转向状态 1 ，遇到其它则表示识别出一个标识符了。由其可产生如下代码：
#include <stdio.h>
#include <ctype.h>
int main(int argc, char *argv[]){//hdu2024_2
int n, flag, state;
char id[100];
scanf("%d" , &n);
getchar();// 由于 scanf 会将 '/n' 放在缓冲区中，所以先处理掉它
while (n--){
flag = state = 0;
gets(id);
for (int i=0; id[i] != '/0' ; ++i){
switch (state){
case 0:
if (isalpha(id[i]) || id[i] == '_' ) state = 1;
else { flag = 1; break ; }
break ;
case 1:
if (isalpha(id[i]) || isdigit(id[i]) || id[i] == '_' ) state = 1;
else { flag = 1; break ; }
break ;
case 2: // 这里的 case 2 对于 hdu2024 这道题目不是必须的，但在词法分析中却是需要的
flag = 1; break ;
}
}
printf("%s/n" , (flag == 1) ? "no" : "yes" );
}
return 0;
}
数据类型果真不那么简单，上面那么多的“ + ”已经有点眼花缭乱了，但对于数据类型的知识范围而言不过是刚刚开了个头。现在给自己一道饭后甜点， char 类型的单个字符为什么不能表示一个汉字字符呢？
2010-1-26 晚
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