Iterative file name in bash

I want to iterate over different file names. The file names are XY1.txt, XY2.txt…XY500.txt. Each file that is taken as input generates 3 output files which I want to rename according to the current index. For XY1.txt, output1.txt becomes 1_1.txt, output2.txt becomes 1_2.txt and so on. But I do not know how to change the filename as per index. I looked it up but I was not finding what I needed on this website or elsewhere. I am new to bash. The below pseudo code is what I was able to write until now. Please help me out

    for i in {1..500} ; 
         filename = "XY"+str(i) + ".txt"  ;
         do ./CompiledCodeFile  -filename -use_option 4; 
         mv output1.txt str(i)+"_1.txt"   ;
         mv output2.txt str(i)+"_2.txt"   ;
         mv output3.txt str(i)+"_3.txt"   ;
    done

>Solution :

^{edit: corrected a little error in the code.}

#!/bin/bash

for i in {1..500}
do
    filename="XY$i.txt"
    ./CompiledCodeFile  "$filename" -use_option 4
     mv output1.txt "${i}_1.txt"
     mv output2.txt "${i}_2.txt"
     mv output3.txt "${i}_3.txt"
done

notes:

• $i and ${i}, what’s the difference?

They’re the same thing, but sometimes you need to use the ${} for differentiating the variable name from the following letters (here the _)

• Why use double quotes for expanding variables, for example "$filename"?

That’s mandatory