Subtract 1 from FOR loop index in Windows batch - windows

Question

In Windows batch, I have a for loop like so:
for /l %%a in (0,1,337) do (
for /F "tokens=*" %%b IN ("tile%%a.jpg") DO set size=%%~zb
if !size! GTR 0 (
echo Size is greater than 0
) ELSE (
)
)
I know this code doesn't make much sense right now, but I'm going to develop it further. I just want to know how to subtract 1 from %%a in the ELSE statement. Basically I want to be able to "redo" a loop number when the IF isn't true, if that makes sense. Thanks.

Answer 1

You can't modify the value of a loop variable. You can only modify the value of an environment variable.
But why using for /L %%a in (0,1,337) do at all?
Better would be for example:
#echo off
for %%A in (tile*.jpg) do (
if %%~zA == 0 (
echo File size of %%A is 0 bytes.
) else (
echo File size of %%A is greater than 0.
)
)
This loop processes simply all tile*.jpg in current directory.
But this loop can't be used if files with 0 bytes are deleted in current directory. Processing the list of tile*.jpg files in current directory and change the files list in the same loop is no good idea because simply not working. The solution is using command DIR to get first the list of all files matching the file name pattern and next process the output of DIR line by line using FOR.
#echo off
setlocal EnableExtensions DisableDelayedExpansion
for /F "delims=" %%A in ('dir /A-D /B /OS tile*.jpg 2^>nul') do (
if %%~zA == 0 (
echo File size of %%A is 0 bytes.
) else (
echo First file with more than 0 bytes is: %%A
goto ExitLoop
)
)
:ExitLoop
endlocal
The command DIR is executed to output the list of files matching the pattern tile*.jpg with ignoring directories which by chance would be matched also by this wildcard pattern because of option /A-D in bare format (only file name) because option /B in order sorted by file size because of option /OS from smallest to largest file.
2^>nul redirects the error message output by command DIR to handle STDERR on not finding any file matching the wildcard pattern to device NUL to suppress this error message. The redirection operator > must be escaped here with caret character ^ to be interpreted as literal character on parsing the FOR command line and interpreted as redirection operator on execution of DIR command line by FOR.
The loop is immediately exited once a file with more than 0 bytes is found as all further files have surely also more than 0 bytes.
One more loop can be used after label ExitLoop which should be renamed to something more suitable in this case for example to renumber the remaining files using command REN when first loop deletes files with 0 bytes.
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.
dir /?
echo /?
endlocal /?
for /?
goto /?
if /?
setlocal /?
See also the Microsoft article Using command redirection operators for an explanation of 2>nul.

Answer 2

You cannot modify the loop variable %%a. Only the loop itself can modify it.
If you want to calculate a new value you can do:
set /A NEW_VALUE=%%a-1
echo %NEW_VALUE% (prove that NewValue is now 1 smaller than %%a)

Answer 3

You cannot modify a for variable reference like %%a, but you can store its value into a standard environment variable (like index) and modify this. For this to work you need to enable and use delayed expansion, because the variable is modified and read within the same block of code, namely the loop body, so read it like !index!; using normal expansion like %index% returned the value present before the loop has even started:
#echo off
setlocal EnableDelayedExpansion
for /L %%a in (0,1,337) do (
set /A "index=%%a-1"
echo %%a - 1 = !index!
)
endlocal
A nice alternative that avoids need of delayed expansion is to use an embedded for /F loop that gets the output of the subtraction and iterates once only per iteration of the surrounding for /L loop, like this:
#echo off
for /L %%a in (0,1,337) do (
for /F %%b in ('set /A "%%a-1"') do (
echo %%a - 1 = %%b
)
)
This works because the for /F loop executes the set /A command in cmd context, in which it returns the resulting value -- in contrast to the aforementioned approach, where set /A is executed in batch-file context, in which it does not output anything.