Create list or arrays in Windows Batch

Can I declare a list or array in a batch file like this:

set list = "A B C D"

And then I need to write these to a file, with the spaces between:

A

B

C 

D

Yes, you may use both ways. If you just want to separate the elements and show they in separated lines, a list is simpler:

set list=A B C D

A list of values separated by space may be easily processed by for command:

(for %%a in (%list%) do (
   echo %%a
   echo/
)) > theFile.txt

You may also create an array this way:

setlocal EnableDelayedExpansion
set n=0
for %%a in (A B C D) do (
   set vector[!n!]=%%a
   set /A n+=1
)

and show the array elements this way:

(for /L %%i in (0,1,3) do (
   echo !vector[%%i]!
   echo/
)) > theFile.txt

For further details about array management in Batch files, see: Arrays, linked lists and other data structures in cmd.exe (batch) script

ATTENTION! You must know that all characters included in set command are inserted in the variable name (at left of equal sign), or in the variable value. For example, this command:

set list = "A B C D"

create a variable called list (list-space) with the value "A B C D" (space, quote, A, etc). For this reason, it is a good idea to never insert spaces in set commands. If you need to enclose the value in quotes, you must enclose both the variable name and its value:

set "list=A B C D"

PS - You should NOT use ECHO. in order to left blank lines! An alternative is ECHO/. For further details about this point, see: http://www.dostips.com/forum/viewtopic.php?f=3&t=774

Sometimes the array element may be very long, at that time you can create an array in this way:

set list=a
set list=%list%;b 
set list=%list%;c 
set list=%list%;d

Then show it:

@echo off
for %%a in (%list%) do ( 
 echo %%a
 echo/
)

@echo off
setlocal
set "list=a b c d"
(
 for %%i in (%list%) do (
  echo(%%i
  echo(
 )
)>file.txt

You don't need - actually, can't "declare" variables in batch. Assigning a value to a variable creates it, and assigning an empty string deletes it. Any variable name that doesn't have an assigned value HAS a value of an empty string. ALL variables are strings - WITHOUT exception. There ARE operations that appear to perform (integer) mathematical functions, but they operate by converting back and forth from strings.

Batch is sensitive to spaces in variable names, so your assignment as posted would assign the string "A B C D" - including the quotes, to the variable "list " - NOT including the quotes, but including the space. The syntax set "var=string" is used to assign the value string to var whereas set var=string will do the same thing. Almost. In the first case, any stray trailing spaces after the closing quote are EXCLUDED from the value assigned, in the second, they are INCLUDED. Spaces are a little hard to see when printed.

ECHO echoes strings. Clasically, it is followed by a space - one of the default separators used by batch (the others are TAB, COMMA, SEMICOLON - any of these do just as well BUT TABS often get transformed to a space-squence by text-editors and the others have grown quirks of their own over the years.) Other characters following the O in ECHO have been found to do precisely what the documented SPACE should do. DOT is common. Open-parenthesis ( is probably the most useful since the command

ECHO.%emptyvalue%

will produce a report of the ECHO state (ECHO is on/off) whereas

ECHO(%emptyvalue%

will produce an empty line.

The problem with ECHO( is that the result "looks" unbalanced.