zsh

to reverse keys <=> values

In zsh, where the primary syntax for defining a hash is hash=(k1 v1 k2 v2...) like in perl (newer versions also support the awkward ksh93/bash syntax for compatibility though with variations when it comes to quoting the keys)

keys=("${(@k)hash}")
values=("${(@v)hash}")

typeset -A reversed
reversed=("${(@)values:^keys}") # array zipping operator

or using a loop:

for k v ("${(@kv}hash}") reversed[$v]=$k

The @ and double quotes is to preserve empty keys and values (note that bash associative arrays don't support empty keys). As the expansion of elements in associative arrays is in no particular order, if several elements of $hash have the same value (which will end up being a key in $reversed), you can't tell which key will be used as the value in $reversed.

for your loop

You'd use the R hash subscript flag to get elements based on value instead of key, combined with e for exact (as opposed to wildcard) match, and then get the keys for those elements with the k parameter expansion flag:

for value ("${(@u)hash}")
  print -r "elements with '$value' as value: ${(@k)hash[(Re)$value]}"

your perl approach

zsh (contrary to ksh93) doesn't support arrays of arrays, but its variables can contain the NUL byte, so you could use that to separate elements if the elements don't otherwise contain NUL bytes, or use the ${(q)var} / ${(Q)${(z)var}} to encode/decode a list using quoting.

typeset -A seen
for k v ("${(@kv)hash}")
  seen[$v]+=" ${(q)k}"

for k v ("${(@kv)seen}")
  print -r "elements with '$k' as value: ${(Q@)${(z)v}}"

ksh93

ksh93 was the first shell to introduce associative arrays in 1993. The syntax for assigning values as a whole means it's very difficult to do it programmatically contrary to zsh, but at least it's somewhat justified in ksh93 in that ksh93 supports complex nested data structures.

In particular, here ksh93 supports arrays as values for hash elements, so you can do:

typeset -A seen
for k in "${!hash[@]}"; do
  seen[${hash[$k]}]+=("$k")
done

for k in "${!seen[@]}"; do
  print -r "elements with '$k' as value ${x[$k][@]}"
done

bash

bash added support for associative arrays decades later, copied the ksh93 syntax, but not the other advanced data structures, and doesn't have any of the advanced parameter expansion operators of zsh.

In bash, you could use the quoted list approach mentioned in the zsh using printf %q or with newer versions ${var@Q}.

typeset -A seen
for k in "${!hash[@]}"; do
  printf -v quoted_k %q "$k"
  seen[${hash[$k]}]+=" $quoted_k"
done

for k in "${!seen[@]}"; do
  eval "elements=(${seen[$k]})"
  echo -E "elements with '$k' as value: ${elements[@]}"
done

As noted earlier however, bash associative arrays don't support the empty value as a key, so it won't work if some of $hash's values are empty. You could choose to replace the empty string with some place holder like <EMPTY> or prefix the key with some character that you'd later strip for display.

The problem with your partly working solution is you store the single value from the sum of one column into the entire array every time, which deletes the previous values.

The solution is to stuff the values into the array one by one, rather than using the mapfile command which requires all the values on one input line:

FILE_NAME="$1"
COLUMN_NAME="$2"

alpha=( $(awk -F"|" 'NR==1{for(i=1;i<=NF;i++){if ($i ~ /'$COLUMN_NAME'/){print i;}}}' $FILE_NAME) )

declare -a array=()

n=0

for each in "${alpha[@]}"
do
        array[n]="$(awk -F'"?\\|"?' '{T+=$('$each')} END { printf "%.2f\n", T }' $FILE_NAME)"
        ((n=n+1))
done

echo "${array[@]}"

Running this gives:

$ bash test.sh demo.txt 'SAL|COL|PER|TAG'
3.58 40.68 6.52 6.88