Subtract a specific column of a file from another for even lines only

Question

I have two long files that have the repeating general structure,
File1:
AA 100
BB 10
File2:
AA 100
BB 1

I want to subtract column 2 from even lines of File2 from File1, but maintain/print the other information so it remains.
Desired output:
AA 100
BB 9

For which I have written the awk:
awk '{a=$2;getline<f;$2-=a}!(NR % 2)' f=File1 File2

This produces:
BB 9
Which does the subtraction and prints the result fine, but I'm having trouble adding in the other data.

How can I also print the other data alongside this subtraction?

Answer 1

$ paste file1 file2 | awk '{print $1, $2 - (NR%2 ? 0 : $4)}'
AA 100
BB 9

or if you wanted an awk-only solution:

$ awk 'NR==FNR{if (!(NR%2)) a[NR]=$2; next} {$2 -= a[FNR]} 1' file2 file1
AA 100
BB 9

If you were considering using getline for this, please read http://awk.freeshell.org/AllAboutGetline so you're aware of the issues around doing so.

Answer 2

Something like this?

awk '{a=$2;getline<f;(NR%2)?$2:$2-=a;print}' f=File1 File2