Defining and incrementing a variable in bash

Question

I've something like this:

declare -r constant1=0
declare -r constant2=0

for xx in 1 2 3 4 5 6 7 8 9 10
do  
    constant1=$((constant1 + 1))
    for yy in 8 7 3 9 3 9 2 8 4 4
    do
        constant2=$((constant2 + 1))
        if [ $constant1 == $constant2 ]; then
            something here bla bla
        fi
    done
done

But it's giving me an error, namely:

line 6: constant1: readonly variable

How do I go about fixing this?

Basically, I want the " do something here bla bla" to run whenever constant1 = constant2. that is, [xx,yy] = [1,8],[2,7], etc

Answer 1

If you want to modify constant1 and constant2, you shouldn't declare them as read-only (that's what declare -r does). And if you want to operate of pairs of variables, you should consider using arrays:

array1=(1 2 3 4 5 6 7 8 9 10)
array2=(8 7 3 9 3 9 2 8 4 4)

for i in ${!array1[@]}
do
    echo "${array1[i]}" "${array2[i]}"
done