China DOS Union

I was learning the set command and used the following batch script to test the problem of variable reference, but the result could not be displayed correctly. Please help me solve it.

The batch script is as follows:
@echo off
set num=iamyxj
set a=0
set /a a+=1
echo %num:~%a%,1%
echo %num:~1,1%
pause

I want to use the reference of variable a to display any character of variable num. I found that variable %a% cannot be parsed. Even if I set the variable directly to 1, it cannot be displayed correctly, but the following sentence can display the first character normally.

I can't figure it out. Hope the expert can explain.

This belongs to the problem of variable delay. You can change it to the following code:

@echo off

setlocal enabledelayedexpansion

set num=iamyxj

set a=0

set /a a+=1

echo !num:~%a%,1!

echo %num:~1,1%

pause

For more information, please refer to: Under what circumstances should variable delay be used?

I read the article about variable delay. Brother willsort said like this.

The behavior of the command interpreter expanding environment variables is roughly as follows: First, read a complete statement of the command line. After some preliminary preprocessing, before the command is interpreted and executed, it will match the string enclosed by percent signs. If a matching environment variable is found in the environment space, its value will replace the original string and the percent sign itself. If no match is obtained, an empty string will replace it. This process is the "expansion" of the environment variable, which still belongs to the preprocessing category of the command line.

But the batch processing I mentioned above is not a single statement. I first set the variables nul and a, and then when reading the next sentence, should I be able to find this variable from the environment space? Why do I also need to use the! number to delay referencing this variable?

Can the moderator give us some more examples for newbies?

echo %num:~%a%,1%

This will match the string enclosed by percent signs. This will match the part between the first two %% first, which is %num:~%!

Because you are using variable nesting variables, you must use!, and when using!, you must enable delayed variables.

Actually, besides the method of brother NAMEJM on the second floor, you can also use the internal command CALL

@echo off
set num=iamyxj
set a=0
set /a a+=1
call echo %%num:~%a%,1%% This form can be okay
echo %num:~1,1%
pause

The person upstairs is also unable to use it.

I think the person upstairs should first ask for the test results of others before speaking.

```
@echo on&setlocal enabledelayedexpansion
set num=iamyxj
set a=12345
set b=0
set /a b+=1
call echo %%num:~!a:~%b%,1!,1%%
echo %num:~1,1%
pause
```

Both can be used to apply variable twice.

Just started learning batch processing under CMD, keep it for later use.

Multiple variable nested test experiment, version: Yunyun version ~:)

@echo off

cls



set num=Redtek 2006 bbs.cn-dos.net

set a=12345

set b=654321



echo ● Variable test:

echo.

echo ● Want to get ^%num:~5,1^%, that is: take the value of variable num starting from offset 5, take 1 character, the result: k

echo 　　According to the above principle, test variable nested variable value taking without using delayed variables.

echo.

echo ● Now we set that the ~5 in ^%num:~5,1^% should come from another variable b indirectly.

echo 　　So, ^%num:~5,1^% should (pseudocode) be expressed as: ^%num:~b,1^%, but there are multiple values in variable b,

echo 　　We only want to take the first element from offset 0 in variable b, which is 5. We have started nested variables.

echo 　　So we need to nest the statement to get variable b (^%b:~1,1^%) in the variable value taking statement of num again.

echo.

echo 　　That is: taking a certain character in variable num should be determined by another variable b from where to start taking.

echo 　　　　But, we also need to nest variables, that is:

echo 　　　　The character value in variable b that determines which character in variable num to take is determined, so we need to decompose b again,

echo 　　　　In this way, it is a three-level nested variable relationship, and they influence and nest variables layer by layer, haha...

echo.

echo ● So, we use multi-level test to display the whole process of value taking evolution ~:)

echo.

echo 　　The following are the results of four "asymptotic" evaluation processes, and finally it takes 3 Calls to find out these nested variables ~:)

echo.



echo 　　%%%%%%%%num:~%%%%b:~%a:~0,1%,1%%%%,1%%%%%%%%



call echo 　　%%%%%%%%num:~%%%%b:~%a:~0,1%,1%%%%,1%%%%%%%%



call call echo 　　%%%%%%%%num:~%%%%b:~%a:~0,1%,1%%%%,1%%%%%%%%



call call call echo 　　The final value is: %%%%%%%%num:~%%%%b:~%a:~0,1%,1%%%%,1%%%%%%%%





rem  In theory, we can Call it n times, so we can nest variables randomly in variables ~:)

rem  The above is an experimental process of "Yunyun" to have a deeper understanding of nested variables ~:)

The test output value results are as follows:

　　%%%%num:~%%b:~1,1%%,1%%%%

　　%%num:~%b:~1,1%,1%%

　　%num:~5,1%

　　The final value is: k

The above experimental variable nesting requires 4 "actions" to completely "transform" and display the actual desired value ~:)

Simple principle explanation: The character value taken from variable num is determined by another variable b, and the orientation of a certain character value in variable b is determined by variable a again.

Rule: %% two percent signs are used for output, which represents one % sign,
　　　　If you want to output %% two percent signs, you need to use four percent signs.
　　　　Using the above principle, use different quantities of % percent signs to determine the level of variable nesting,
　　　　The more levels, the double use quantity of percent signs also increases accordingly, and this principle can be used to complete extremely complex nesting.

　　　　Disadvantage: The more complex the nesting, the more difficult it is to read clearly, and the more difficult it is to maintain the code.
　　　　　　　So, another advantage of using delayed variables is that the code can be very clear ~:)　　

[ Last edited by redtek on 2006-11-21 at 09:53 AM ]

After testing, Brother 9527's code on the 7th floor is correct and the correct result is obtained. Add 5 points