C语言7 switch语句为什么高效

switch语句的定义

语法

switch(表达式) {     case 常亮表达式1:         语句;         break;     case 常亮表达式:         语句;         break;     case 常亮表达式:         语句;         break;      ......     default:         语句;         break; }

需要注意的点

1. 表达式结束不能使浮点数
2. case后的值不能一样
3. case后的值必须是常量

• break:
• break非常重要，当执行到一个分支后 如果没有break就会继续向下执行，遇到break才跳出switch语句
• default 语句与位置无关，但是当default写在其他条件的前面时。如果没有break;，就会向下继续匹配

Switch语句与 if..else语句的区别：

1. switch语句只能进行等值判断，而if..else可以进行区间判断
2. switch语句的执行效率远远高于if..else 在分支条件比较多的情况下，这种趋势愈发明显

观察switch语句的反汇编，看看switch语句为啥效率高

为啥switch比if..else效率高

### 游戏中的switch语句

switch(x) {    case 1:        printf("A /n");        break;    case 2:        printf("B /n");        break;    case 3:        printf("C /n");        break;    default:        printf("default /n");        break; } 

编译后反汇编代码

11:        switch(x) 12:    { 00401038 8B 45 08             mov         eax,dword ptr [ebp+8] 0040103B 89 45 FC             mov         dword ptr [ebp-4],eax 0040103E 83 7D FC 01          cmp         dword ptr [ebp-4],1 00401042 74 0E                je          MyPrint+32h (00401052) 00401044 83 7D FC 02          cmp         dword ptr [ebp-4],2 00401048 74 17                je          MyPrint+41h (00401061) 0040104A 83 7D FC 03          cmp         dword ptr [ebp-4],3 0040104E 74 20                je          MyPrint+50h (00401070) 00401050 EB 2D                jmp         MyPrint+5Fh (0040107f) 13:       case 1: 14:           printf("A /n"); 00401052 68 30 00 42 00       push        offset string "A /n" (00420030) 00401057 E8 B4 00 00 00       call        printf (00401110) 0040105C 83 C4 04             add         esp,4 15:           break; 0040105F EB 2B                jmp         MyPrint+6Ch (0040108c) 16:       case 2: 17:           printf("B /n"); 00401061 68 2C 00 42 00       push        offset string "B /n" (0042002c) 00401066 E8 A5 00 00 00       call        printf (00401110) 0040106B 83 C4 04             add         esp,4 18:           break; 0040106E EB 1C                jmp         MyPrint+6Ch (0040108c) 19:       case 3: 20:           printf("C /n"); 00401070 68 28 00 42 00       push        offset string "C /n" (00420028) 00401075 E8 96 00 00 00       call        printf (00401110) 0040107A 83 C4 04             add         esp,4 21:           break; 0040107D EB 0D                jmp         MyPrint+6Ch (0040108c) 22:       default: 23:           printf("default /n"); 0040107F 68 1C 00 42 00       push        offset string "default /n" (0042001c) 00401084 E8 87 00 00 00       call        printf (00401110) 00401089 83 C4 04             add         esp,4 24:           break; 25:    } 26:   } 

### 区别

当调教比较少的时候 没啥区别 。但是当条件比较多的时候：

 11:        switch(x) 12:    { 0040B818 8B 45 08             mov         eax,dword ptr [ebp+8] 0040B81B 89 45 FC             mov         dword ptr [ebp-4],eax 0040B81E 8B 4D FC             mov         ecx,dword ptr [ebp-4] 0040B821 83 E9 01             sub         ecx,1 0040B824 89 4D FC             mov         dword ptr [ebp-4],ecx 0040B827 83 7D FC 03          cmp         dword ptr [ebp-4],3 0040B82B 77 46                ja          $L539+0Fh (0040b873) 0040B82D 8B 55 FC             mov         edx,dword ptr [ebp-4] 0040B830 FF 24 95 91 B8 40 00 jmp         dword ptr [edx*4+40B891h] 13:       case 1: 14:           printf("A /n"); 0040B837 68 34 0F 42 00       push        offset string "A /n" (00420f34) 0040B83C E8 CF 58 FF FF       call        printf (00401110) 0040B841 83 C4 04             add         esp,4 15:           break; 0040B844 EB 3A                jmp         $L539+1Ch (0040b880) 16:       case 2: 17:           printf("B /n"); 0040B846 68 30 00 42 00       push        offset string "B /n" (00420030) 0040B84B E8 C0 58 FF FF       call        printf (00401110) 0040B850 83 C4 04             add         esp,4 18:           break; 0040B853 EB 2B                jmp         $L539+1Ch (0040b880) 19:       case 3: 20:           printf("C /n"); 0040B855 68 2C 00 42 00       push        offset string "C /n" (0042002c) 0040B85A E8 B1 58 FF FF       call        printf (00401110) 0040B85F 83 C4 04             add         esp,4 21:           break; 0040B862 EB 1C                jmp         $L539+1Ch (0040b880) 22:       case 4: 23:           printf("D /n"); 0040B864 68 28 00 42 00       push        offset string "D /n" (00420028) 0040B869 E8 A2 58 FF FF       call        printf (00401110) 0040B86E 83 C4 04             add         esp,4 24:           break; 0040B871 EB 0D                jmp         $L539+1Ch (0040b880) 25:       default: 26:           printf("default /n"); 0040B873 68 1C 00 42 00       push        offset string "default /n" (0042001c) 0040B878 E8 93 58 FF FF       call        printf (00401110) 0040B87D 83 C4 04             add         esp,4 27:           break; 28:    } 29:   } 

我们发现 生成一张内存表。
switch通过算法，直接一步算出地址 从表中找到需要跳转的地址

大表和小表

当生成的函数跳转地址表 每个成员有4个字节的时候 我们称之为 大表

总结：

1. 即没有大表也没有小表：

   • case 项小于等于3项
   • case最大值和最小值差值>=255

2. 有大表没小表：

   • case项大于3项并且大表项空隙小于等于6个

3. 有大表有小表

   • case项大于3项并且大表项空隙大于6个并且case最大最小差值<255