Shell Scripting - Array iteration not working correctly - Nested for loop

Question

So, I have two arrays:

arr1=("A" "C" "E")
arr2=("B" "D" "F")

I have nested loops like this:

 for i in $(seq 1 $numberOfYears); 
    do
        echo "$i"
        for j in "${arr1[@]}";
            do
                echo ${arr1[j]} <---Keeps printing "A" 
                echo ${arr2[j]} <---Keeps printing "B" 
            done
    done

New to programmatic shell scripting. What would cause the inner for loop to correctly iterate through array1 when you reference the element like this: $j but not like this ${arr1[j]} and ${arr2[j]} ..? In all of my reading/searching, this should correctly iterate through both arrays.

My expected results:

Expected

 1ABCDEF
 2ABCDEF
 3ABCDEF

Actual

 1ABABAB
 2ABABAB
 3ABABAB

If I change ${arr1[j]} to $j it works fine, but I need to get the elements of arr2 as well, so I have to get it like ${arr2[j]}.

Maybe associative array would work better? declare -A arr=([A]=B [C]=D [E]=F); for k in "${!arr[@]}"; do echo $k ${arr[$k]} ; done. — choroba
– choroba, Commented Oct 31, 2018 at 14:45

Tom Fenech · Accepted Answer · 2018-10-31 14:50:33Z

2

You're looping over the values of the array.

You can loop over the keys if you want, using "${!arr1[@]}" (add the !):

for i in $(seq 1 $numberOfYears); 
do
    echo "$i"
    for j in "${!arr1[@]}";
    do
        echo ${arr1[j]}
        echo ${arr2[j]}
    done
done

Otherwise, you are expanding parameters like ${arr1["A"]}, and since these keys are not defined, you get the first element of the array.

As an aside, you can also use a different style of loop to iterate over numeric keys:

for (( i = 0; i <= numberOfYears; ++i ))
do
  echo "$i"
  for (( j = 0; j < ${#arr1[@]}; ++j ))
  do
    echo "${arr1[j]}"
    echo "${arr2[j]}"
  done
done

edited Oct 31, 2018 at 14:50

answered Oct 31, 2018 at 14:36

Tom Fenech

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