How can I read 2 consecutive lines of a text file and save them as temporary variables

Question

I have files with an ID, model, and date. the files have a format similar to 10000_9999-99_10-01-2011.zip (where 10000 is the ID, 9999-99 is the model, and of course 10-01-2011 is the date).

I would like to modify the dates of these files, but maintain the interval between sessions with the same ID. For example, if 2 sessions had the dates 1/1/2011 and 2/1/2011, and I wanted to update the last session date to 8/1/2012, the first session would have the date 7/1/2012.

Currently my code looks like this:

@echo off
setlocal enabledelayedexpansion
del filedates.txt
FOR /F "tokens=1,2,3,4,5 delims=_" %%a in (filenames.txt) do @echo %%c >>filedates.txt
FOR /F "tokens=1,2,3 delims=-" %%a in (filedates.txt) do (
  echo %%a%%b
)

---

The output is similar to this (YearMonth):

201107
201109
201204
etc..

---

I was looking to read a line in filedates.txt, store this date as a variable, then read the next line, and store this as another variable. That way the two variables could be compared to see which is greater, and the process would continue.

Answer 1

One straightforward way to do read two lines at a time is to process the lines in the input file one by one with a for /f loop, and "react" only on even numbered lines. Here's a sample code:

@echo off
setlocal enabledelayedexpansion
set evenflag=1
for /f "tokens=*" %%x in (filedates.txt) do set x1=!x2! && set x2=%%x && (
set /a evenflag^^=1 && if !evenflag!==1 (
    rem Do something with !x1! and !x2!
))

Here the variable evenflag is a boolean flag that indicates whether the current line number is even (1 for even lines, 0 to odd lines). x1 and x2 hold the currently read pair of lines.