Put every other value into a seperate array - arrays

Question

I have a basic array at the moment and it works as expected. But what I want to do is put every other value into a seperate array. I just need the 100 and 200 to be in array 1, and 1000 and 2000 to be in array 2. How do I do that?
$ cat scriptx.sh
while read line
do
my_array=("${my_array[#]}" $line)
done
echo ${my_array[#]}
$ ./scriptx.sh
100
1000
200
2000
100 1000 200 2000

Answer 1

For an arbitrary number of arrays n_arrays, you can use a counter of lines read so far, dynamically generate array names, such as array_0, array_1 etc., and append new lines to the right arrays based on counter % n_arrays. Obviously, n_arrays set to 2 gives an answer to the question.
#!/bin/bash
set -euo pipefail
declare -ri n_arrays=2 # number of arrays to use
declare -i i counter=0 # number of lines read so far
for ((i = 0; i < n_arrays; ++i)); do
declare -a "array_${i}" # optional; needed with set -u
done
while IFS= read -r line; do
declare -n my_array="array_$((counter++ % n_arrays))"
my_array+=("$line") # append line to the right array
done
for ((i = 0; i < n_arrays; ++i)); do
declare -n my_array="array_${i}"
echo "${my_array[#]#A}" # print a declare+assign command
done

Answer 2

Something along the lines of:
#!/bin/bash
while IFS= read -r line; do
array1+=("$line")
if IFS= read -r line; then
array2+=("$line")
fi
done
echo "array1: ${array1[*]}"
echo "array2: ${array2[*]}"

Answer 3

You could do something like this:
#!/bin/bash
i=0
while read line; do
if (( i++ % 2 )); then
odd_indexes+=($line)
else
even_indexes+=($line)
fi
done
echo "even_indexes: ${even_indexes[*]}"
echo "odd_indexes: ${odd_indexes[*]}"
Assuming that your input data is:
100
1000
200
2000
This script will output:
even_indexes: 100 200
odd_indexes: 1000 2000
But you could also do something like this if you don't mind reading the input file twice:
#!/bin/bash
mapfile -t odd_indexes < <(sed -n 'n;p' data.txt)
mapfile -t even_indexes < <(sed -n 'p;n' data.txt)
echo "even_indexes: ${even_indexes[*]}"
echo "odd_indexes: ${odd_indexes[*]}"
If you're reading from stdin you would need to write that out to a temporary file and use that file in the above script.

Answer 4

One idea:
$ cat runme
#!/bin/bash
cnt=${1:-2} # 1st (optional) arg states number of arrays; if not supplied default is '2'
for ((i=1;i<=cnt;i++))
do
unset array_"$i" # make sure arrays do not exist
done
n=0
while read -r line
do
(( ndx = n++%cnt +1 )) # calculate array index
declare -n curr_array=array_"${ndx}" # use nameref to dynamically reference correct array
curr_array+=( "${line}" ) # add line to array
done
echo ""
for ((i=1;i<=cnt;i++)) # display contents of our arrays
do
typeset -p array_"$i"
done
Taking for a test drive:
$ runme
100
1000
200
2000
declare -a array_1=([0]="100" [1]="200")
declare -a array_2=([0]="1000" [1]="2000")
$ runme 2
100
1000
200
2000
declare -a array_1=([0]="100" [1]="200")
declare -a array_2=([0]="1000" [1]="2000")
$ runme 3
1 a
2 b
3 c
4 d
5 e
declare -a array_1=([0]="1 a" [1]="4 d")
declare -a array_2=([0]="2 b" [1]="5 e")
declare -a array_3=([0]="3 c")