How to reverse string list in batch - windows

Question

I have a problem with reversing string list in a batch script. Let say I have a list L=string1,string2,string3 I would like to obtain reversed list L=string3,string2,string1. Any ideas??

Answer 1

You may also use this shorter/simpler approach:
#echo off
setlocal EnableDelayedExpansion
set "L=string1,string2,string3"
echo Input =%L%
set "revL="
set "str=%L:,=" & set "revL=,!str!!revL!" & set "str=%"
set "revL=%str%%revL%"
echo Output=%revL%
This method use the same procedure of the other answers, but in less lines. If you want to know what happens here, remove the #echo off line and run it. ;)

Answer 2

Without knowing what your input looks like, this might be a first attempt:
#ECHO OFF
SETLOCAL EnableDelayedExpansion
SET var=abc,def,ghi
SET rev=
:LOOP
IF NOT "!var!"=="" (
FOR /F "delims=, tokens=1,*" %%F IN ("!var!") DO (
SET rev=%%F,!rev!
SET var=%%G
)
) ELSE (
SET rev=!rev:~0,-1!
GOTO ENDLOOP
)
GOTO LOOP
:ENDLOOP
ECHO reversed list is: !rev!
EDIT: As requested, here is an explanation how it works:
var is your starting list of strings separated by commas.
rev will be the reversed string. At the beginning this string is empty.
Now let's take a look at the loop:
In each iteration, we are separating our string into two parts: %%F and %%G. %%F will be everything before the first comma and %%G will be the rest of the string: FOR /F "delims=, tokens=1,*" %%F IN ("!var!"). delims=, means that we are using comma as delimiter. tokens=1,* means that the first found substring will be stored in %%F while the rest will be stored in %%G (%%F is defined for the first token so Windows command interpreter will put every token afterwards in G, H, I, and so on - as we are using *, everything will land in %%G). Finally, we take the first token of our string (%%F) and append ,!rev! to it. Then we set the remaining string list to everything behind the first comma (%%G).
In the first iteration, this loop does the following (pseudo code):
var=abc,def,ghi
rev=
split the string into %%F=abc and %%G=def,ghi
set rev to %%F,rev //means abc,
set var to var but without the first token //means def,ghi
In the second iteration:
var=def,ghi
rev=abc,
split the string into %%F=def and %%G=ghi
set rev to %%F,rev //means def,abc
set var to var but without the first token //means ghi
In the third iteration:
var=ghi
rev=def,abc
split the string into %%F=ghi %%G=
set rev to %%F,rev //means ghi,def,abc,
set var to var but without the first token //means empty string
Now, after jumping back to :LOOP, the if condition is no longer fulfilled as !var! has shrunk from formerly abc,def,ghi to now an empty string. So IF NOT !var!=="" becomes false and we are jumping to the ELSE clause.
There is one problem left: as we are constructing our reversed string by pre-appending the first token from the original list AND a comma, we will end up with a comma at the end of the reversed string list: ghi,def,abc,
SET rev=!rev:~0,-1! fixes this. It takes a "substring" from our string, starting at index 0 and finishing at "end-1". So this line simply removes the last , at the end of our string. Then we are jumping to :ENDLOOP.

Answer 3

Here is a batch file code assuming L=string1,string2,string3 is assigned to an environment variable:
#echo off
setlocal EnableExtensions DisableDelayedExpansion
set "ListLine=L=string1,string2,string3"
for /F "tokens=1* delims==" %%I in ("%ListLine%") do (
set "LineBegin=%%I"
set "ListItems=%%J"
)
set "ReversedItems="
for %%I in (%ListItems%) do call set "ReversedItems=%%I,%%ReversedItems%%"
set "ListLine=%LineBegin%=%ReversedItems:~0,-1%"
echo %ListLine%
endlocal
Windows command interpreter interprets a comma in list of strings in a simple FOR loop like a space character as it can be seen on running this batch file without #echo off from within a command prompt window. Therefore the second FOR loop runs first with string1 assigned to loop variable I, second with string2 and third with string3.
Command CALL is used to do a double processing of the command SET to avoid the requirement to use delayed environment variable expansion as explained by help for command SET output on running set /? in a command prompt window.
For understanding the used commands and how they work, open a command prompt window, execute there the following commands, and read entirely all help pages displayed for each command very carefully.
call /?
echo /?
endlocal /?
for /?
set /?
setlocal /?

Answer 4

Aacini, definitely has the fastest code out of all of the answers. This is some longer code that uses a similar SET trick.
#echo off
setlocal EnableDelayedExpansion
set i=1
set "x=abc,def,ghi"
set "x!i!=%x:,=" & set /A i+=1 & set "x!i!=%"
FOR /L %%G IN (%i%,-1,1) DO (
IF "%i%"=="%%G" (
set "reverse=!x%%G!"
) ELSE (
set "reverse=!reverse!,!x%%G!"
)
)
echo %reverse%
pause
Just some quick timed testing of all 4 answers. First one uses the original string 3 characters in each of the 3 comma separated fields. The second one uses 3 characters in 9 comma separated fields. Each time I tested running each one 100 times and calculated the average. The differences are negligible.
Average of 100 tries using 3x3
Aacini 0.39254
Squashman 0.39851
Michael 0.3999
Mofi 0.40434
Average 100 tries using 3x9
Aacini 0.39925
Squashman 0.40278
Michael 0.41457
Mofi 0.43397