for loop and appending over list of arrays

Question

I am new to Bash scripting and presently find myself dealing with a small problem in working with for loops, arrays and variable assignment/substitution which I do not know how to solve. Since I am quite literally new, I do not even know how to look this up. Although anyone could say this, I stress that this isn't homework or anything of the sort - I just need to reorganize a humongous amount of data for later processing and thought I'd do it in a somewhat smart way while learning some Bash.

Background: I have a directory which itself contains three subdirectories - Folder1, Folder2, Folder3. Each subdirectory has one data.txt file. Each data.txt file features three columns with a fixed number of rows.

Goal: I want to launch a Bash script from the main directory which iterates over the subdirectories and carries out the following:

1. it enters and accesses the data.txt;
2. it reads each column into the corresponding array arr1, arr2, arr3 (these are emptied/reset when entering the next subdirectory);
3. it reads arr2[0] into earr1, arr2[1] into earr2, ..., arr2[j-1] into earr$j. A similar structure is followed for the data in arr3. The earr$j keep the data from past iterations and constitute my desired output arrays. With every sweep of a new directory, data are appended to these.

It sounds overly complicated - but that's the structure of the data I have to work with. For the most part, I have figured out a working solution (granted, probably not the best one).

Problem: For some reason I am not able to identify, I am getting all kinds of errors with every attempt at appending as earr$j+=(${arr2[j-1]}) (and all sorts of variations I have tried out) in a loop over j. Below is the snippet giving me trouble. I would appreciate if someone could point out how what the problem is and, if possible, provide a working example of a solution.

The for-loop in question:

for j in $(seq 1 $number_of_lists); do echo "(${arr2[j-1]})"; earr$j+=(${arr2[j-1]}); echo "(${arr3[j-1]})"; done

In particular, I have trouble with the earr$j+=(${arr2[j-1]}) part. This is my error output:

./next.sh: line 33: syntax error near unexpected token "${arr2[j-1]}'' and ./next.sh: line 33: earr$j+=('${arr2[j-1]}').

Thank you in advance for any suggestions.