How to merge two arrays in a zipper like fashion in Bash?

Assuming both arrays are the same size,

unset result
for (( i=0; i<${#array1[*]}; ++i)); do
    result+=( "${array1[$i]}" "${array2[$i]}" )
done

I've found more common the case where I want to zip two arrays into two columns. This isn't as natively Zsh as the "RTLinuxSW" answer, but for this case, I use paste.

% tabs 16
% paste <(print -l $array1) <(print -l $array2)
one     two
three   four
five    six
seven   eight

And then can shove that into another array to get the intended output:

% array3=( `!!`«tab»«tab» )
% print $array3
one two three four five six seven eight

This is based on RTLinuxSW's answer, with the improvement from Paused until further notice's comment, which adds support for sparse and associative arrays.

for index in "${!array1[@]}"; do  # Also, quote indices
    result+=( "${array1[$index]}" "${array2[$index]}" )
done

After:

$ echo "${result[@]}"
one two three four five six seven eight
$ declare -p result
declare -a result=([0]="one" [1]="two" [2]="three" [3]="four" [4]="five" [5]="six" [6]="seven" [7]="eight")

This assumes the indices of the two arrays are identical.

< Update >This below solution is designed to work with data delimited by newlines: each value to be loaded into the array on a separate line in each file. Works perfectly as written, but if your data is organized differently, please see @Socowi 's alternative using paste with printf in the comments. Much thanks to @Socowi for both raising the issue & offering a workaround for data delimited in other ways! < / Update >

Here's another solution to interleave data from (2) arrays which are populated with data delimited by newlines in separate files. This solution uses paste, echo & xargs:

Array Data: I feed files to arrays because I like to disaggregate data from code. Following files with each value delimited by a newline will be consumed by readarray:

test1.txt:

one
three
five
seven

test2.txt:

two
four
six
eight

Put it all together:

#!/bin/bash

readarray arrayTest1 < /root/test1.txt
readarray arrayTest2 < /root/test2.txt

paste <( echo "${arrayTest1[*]}" ) <( echo "${arrayTest2[*]}" ) | xargs

Output:

one two three four five six seven eight