批处理文件中是否存在ELSE语法错误？

Question

我是批处理文件编写的新手，并且正在编写一个脚本，该脚本会随机打开三个网页之一并在延迟后循环。我在运行它时经常遇到语法错误，但是我无法查明它在哪里。

:main
@echo on
set location=""
set /A num=%random% %% 10 
if /A"%num%"=="0"
(
    set location="yahoo.com"
) 
else if /A"%num%"=="1"
(
    set location="msn.com"
)
else
(
    set location="google.com"
)

start "Chrome" chrome --new-window %location%
timeout /t 30 /nobreak >NUL
goto main

帮助将不胜感激，并在此先感谢！

Answer 1

这是根据MC ND和Magoo的良好建议编写的代码：

@echo off
:main
set location=
set /A num=%random% %% 10
if "%num%"=="0" (
   set "location=yahoo.com"
) else if "%num%"=="1" (
   set "location=msn.com"
) else (
   set "location=google.com"
)
start "Chrome" chrome.exe --new-window "%location%"
timeout /t 30 /nobreak >NUL
goto main

以下是Ed Heal建议的批处理代码：

@echo off
:main
set location=
set /A num=%random% %% 10
if "%num%"=="0" set "location=yahoo.com" & goto OpenSite
if "%num%"=="1" set "location=msn.com"   & goto OpenSite
set "location=google.com"
:OpenSite
start "Chrome" chrome.exe --new-window "%location%"
timeout /t 30 /nobreak >NUL
goto main

有关代码的说明，请参见使用Windows批处理文件的单行多命令set "location=..." & goto OpenSite。

这个额外的信息是给rudicangiotti的，因为他在Middas的回答下面发表了评论。

这是没有必要的if，并else只用一个命令就可以使用括号。

因此，代码块

if "%num%"=="0" (
   set "location=yahoo.com"
) else if "%num%"=="1" (
   set "location=msn.com"
) else (
   set "location=google.com"
)

被解析像

if "%num%"=="0" (
   set "location=yahoo.com"
) else (
   if "%num%"=="1" (
      set "location=msn.com"
   ) else (
      set "location=google.com"
   )
)

也许这对于C / C ++来说更容易理解，它不需要else与命令位于同一行if或属于匹配项的右括号if。

完整的可编译C / C ++示例代码：

#ifdef __cplusplus
#include <cstdio>    /* printf */
#include <cstdlib>   /* rand   */
#include <cstring>   /* strcmp */
#else
#include <stdio.h>   /* printf */
#include <stdlib.h>  /* rand   */
#include <string.h>  /* strcmp */
#endif

int main (int argc, char* argv[])
{
   const char* sLocation;
   int iNum = rand() % 10;

   if(iNum == 0) sLocation = "yahoo.com";
   else if(iNum == 1) sLocation = "msn.com";
   else sLocation = "google.com";
   printf("Number is %d and location is \"%s\".\n",iNum,sLocation);

   /* Some not really useful code to avoid warnings. */
   if(argc > 1)
   {
      if(!strcmp(argv[1],"/?"))
      {
         printf("There is no help for this small demo application.\n");
      }
   }
   return 0;
}

同样在C / C ++中，没有else if语句的ElseIf关键字作为Visual Basic中的关键字或#elif预处理器的指令。

因此，以上条件块也可以写成：

/* Variant 1: Same usage of brackets and indents like in first batch example. */
if(iNum == 0) {
   sLocation = "yahoo.com";
} else if(iNum == 1) {
   sLocation = "msn.com";
} else {
   sLocation = "google.com";
}

/* Variant 2: Same usage of brackets like in first batch example,
   but this time with indents as it would be 100% correct according
   to processing. It is not possible to use this syntax in batch
   files because the second if must be on same line as first else. */
if(iNum == 0) {
   sLocation = "yahoo.com";
} else
   if(iNum == 1) {
      sLocation = "msn.com";
   } else {
      sLocation = "google.com";
   }

/* Variant 3: Same usage of brackets like in second batch example,
   but without omitting not necessary brackets for first else block. */
if(iNum == 0) {
   sLocation = "yahoo.com";
} else {
   if(iNum == 1) {
      sLocation = "msn.com";
   } else {
      sLocation = "google.com";
   }
}

/* Variant 4: One more variant not possible in batch file,
   but using a very common style for C/C++ programmers. */
if(iNum == 0)
{
   sLocation = "yahoo.com";
}
else if(iNum == 1)
{
   sLocation = "msn.com";
}
else
{
   sLocation = "google.com";
}

/* Variant 5: This is variant 3 in coding style of variant 4. */
if(iNum == 0)
{
   sLocation = "yahoo.com";
}
else
{
   if(iNum == 1)
   {
      sLocation = "msn.com";
   }
   else
   {
      sLocation = "google.com";
   }
}

可以使用C / C ++编写此代码块，并且可以使用很多其他变体来考虑方括号和缩进的不同样式。

对于批处理文件编码而言，真正令人感兴趣的是变体1到3，其中，变体2显示了如果在批处理文件中可能的话，变体1的外观将是什么样子。但是没有人用方括号else if3所示的方括号和缩进块，而是在else if最后插入了多个块if，else因此将其定位在最右边。