c语⾔的If与Switch效率⽐较最近看到⽂章表⽰Switch⽐If慢，这⾥就来测试⼀下是否属实。
⽴刻打开VS，编写以下代码：
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define MAX_LOOP 100000000
int nums[5];
void switchFunc()
{
int i, r;
srand((unsigned)time(NULL));
for(i =0; i < MAX_LOOP; i++)
{
r =rand()%5;
switch(r)
{
case3:
nums[3]++;
break;
case1:
nums[1]++;
break;
case2:
nums[2]++;
break;
case0:
nums[0]++;
break;
default:
nums[4]++;
break;
}
}
}
void ifFunc()
{
int i, r;
srand((unsigned)time(NULL));
for(i =0; i < MAX_LOOP; i++)
{
r =rand()%5;
if(r ==3)
nums[3]++;
else if(r ==1)
nums[1]++;
else if(r ==2)
nums[2]++;
else if(r ==0)
nums[0]++;
switch的用法c语言else
nums[4]++;
}
}
int main()
{
clock_t start, finish, i;
double if_time, switch_time, if_total_time =0, switch_total_time =0;
int test_time =10;
printf("Start test.\n");
printf("Start test.\n");
printf(" \tswitch time\tif time\n");
for(i =0; i < test_time; i++)
{
memset(nums,0,sizeof(nums));
start =clock();
switchFunc();
finish =clock();
switch_time =(double)1000.0*(finish - start)/ CLOCKS_PER_SEC;
switch_total_time += switch_time;
start =clock();
ifFunc();
finish =clock();
if_time =(double)1000.0*(finish - start)/ CLOCKS_PER_SEC;
if_total_time += if_time;
printf("#%d\t%0.0fms\t\t%0.0fms\n", i +1, switch_time, if_time);
}
printf("\n");
printf("total\t%0.0fms\t\t%0.0fms\n", switch_total_time / test_time, if_total_time / test_time);
return0;
}
使⽤VS2022编译，在ReleaseX84环境下运⾏结果为：
Start test.
switch time if time
#1 3929ms 3673ms
#2 3590ms 3584ms
#3 3606ms 3587ms
#4 3604ms 3585ms
#5 3601ms 3589ms
#6 3589ms 3586ms
#7 3605ms 3584ms
#8 3596ms 3581ms
#9 3591ms 3592ms
#10 3605ms 3593ms
total 3632ms 3595ms
测试结果表明，switch确实⽐if慢。这个原因来⾃CPU 分⽀预测。
编译器根据case的数量和case值的稀疏程度来翻译switch语句，当case情况⽐较多(例如4个以上)，并且值的范围跨度⽐较⼩时，就会使⽤跳转表。
构建的跳转表为
.text:00415A6C jpt_4159A7 dd offset $LN10_0 ; DATA XREF: _switchFunc+97↑r
.text:00415A6C dd offset $LN8 ; jump table for switch statement
.text:00415A6C dd offset $LN9_0
.text:00415A6C dd offset $LN7
增加了条件到0-9以后，swtich⽐if快。
修改后的代码如下：
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define MAX_LOOP 100000000
int nums[10];
void switchFunc()
{
int i, r;
srand((unsigned)time(NULL));
for(i =0; i < MAX_LOOP; i++)
for(i =0; i < MAX_LOOP; i++) {
r =rand()%10;
switch(r)
{
case8:
nums[8]++;
break;
case3:
nums[3]++;
break;
case7:
nums[7]++;
break;
case1:
nums[1]++;
break;
case4:
nums[4]++;
break;
case2:
nums[2]++;
break;
case0:
nums[0]++;
break;
case6:
nums[6]++;
break;
case5:
nums[5]++;
break;
default:
nums[9]++;
break;
}
}
}
void ifFunc()
{
int i, r;
srand((unsigned)time(NULL)); for(i =0; i < MAX_LOOP; i++) {
r =rand()%10;
if(r ==8)
nums[8]++;
else if(r ==3)
nums[3]++;
else if(r ==7)
nums[7]++;
else if(r ==1)
nums[1]++;
else if(r ==4)
nums[4]++;
else if(r ==2)
nums[2]++;
else if(r ==0)
nums[0]++;
else if(r ==6)
nums[6]++;
else if(r ==5)
nums[5]++;
else
nums[9]++;
nums[9]++;
}
}
int main()
{
clock_t start, finish, i;
double if_time, switch_time, if_total_time =0, switch_total_time =0;
int test_time =10;
printf("Start test.\n");
printf(" \tswitch time\tif time\n");
for(i =0; i < test_time; i++)
{
memset(nums,0,sizeof(nums));
start =clock();
switchFunc();
finish =clock();
switch_time =(double)1000.0*(finish - start)/ CLOCKS_PER_SEC;
switch_total_time += switch_time;
start =clock();
ifFunc();
finish =clock();
if_time =(double)1000.0*(finish - start)/ CLOCKS_PER_SEC;
if_total_time += if_time;
printf("#%d\t%0.0fms\t\t%0.0fms\n", i +1, switch_time, if_time);
}
printf("\n");
printf("total\t%0.0fms\t\t%0.0fms\n", switch_total_time / test_time, if_total_time / test_time); return0;
}
结果如下：
Start test.
switch time if time
#1 3962ms 3900ms
#2 3732ms 3787ms
#3 3750ms 3794ms
#4 3738ms 3794ms
#5 3727ms 3796ms
#6 3735ms 3785ms
#7 3729ms 3797ms
#8 3725ms 3799ms
#9 3744ms 3800ms
#10 3739ms 3784ms
total 3758ms 3804ms
结论
当条件分⽀⽐较少的情况下，if⽐较快，当条件分⽀⽐较多的情况下，swtich⽐较快。
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