Example of delayed expansion in batch file

Question

Can someone give me an example of where a batch script would act differently with or without delayed expansion? Are there any situations where you would NOT want to use delayed expansion? Thanks.

Answer 1

Look at the following examples...

Example 1: The following code DOESN'T use delayed expansion, so the variables in the for loop are expanded only one time. This means that %Count% will always expand to 0 in each iteration of the loop, no matter what we do to it with the set command:

@echo off set COUNT=0  for %%v in (1 2 3 4) do (   set /A COUNT=%COUNT% + 1   echo Count = %COUNT% ) pause 

So this script will output:

Count = 0 Count = 0 Count = 0 Count = 0 

This is not how this loop is supposed to work.

Example 2: On the other hand, if we use delayed expansion, we have the following script, which will run as expected.

setlocal ENABLEDELAYEDEXPANSION set COUNT=0  for %%v in (1 2 3 4) do (   set /A COUNT=!COUNT! + 1   echo Count = !COUNT! )  pause 

and, as expected, it will output:

Count = 1 Count = 2 Count = 3 Count = 4 

When you use the ENABLEDELAYEDEXPANSION, and expand a variable using ! instead of %, the variable is re-expanded each time, and everything works as it's supposed to.

Answer 2

I wanted to add a great example on how "EnableDelayedExpansion" (EDE) can be useful outside of the ubiquitous FOR loop examples.

Here is a line of earthquake data that I wish to parse (I call it it 1line.txt)

ak_11574812 2015.04.29.193822 62.9525 -148.8849 1.0 9.5 1 49km S of Cantwell, Alaska

The problem I ran into was that last segment of this line does not always start at the same column number. So I needed to create a flexible SET command that will accurately pluck out the last segment of this line.

ECHO OFF setlocal enableDelayedExpansion set where=72 set /p line=<1line.txt set locate=!line:~%where%,28! echo %locate% 

EDE allows me to place a variable (where) inside another variable (line). EDE will translate the variable bracketed by % first, then process the variable bracketed by ! and (in this case) push out the results into the "locate" variable.