Map unknown associative array element

How can I map an unknown value alongside other known values in bash using an associative array, so that:

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"
map[unknown]="?"

for KEY in "${array[@]}"; do
   echo ${map[$KEY]}
done

outputs:

1
1
2
?
?
2

I do not know in which index my element is located, nor the length of the array to be mapped. I have tried doing map[${array[@]}]="?" before setting the other values, but this does not work

Default value

See Parameter Expansion in bash man page:

man -Pless\ +/parameter:-word bash

  ${parameter:-word}
       Use  Default  Values.  If parameter is unset or null, the expan‐
       sion of word is substituted.  Otherwise, the value of  parameter
       is substituted.

So

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map
map["foo"]="1"
map["bar"]="2"

for key in "${array[@]}";do
    printf '%-20s -> %s\n' "$key" "${map["$key"]:-?}"
done

Will produce:

foo                  -> 1
foo                  -> 1
bar                  -> 2
something else       -> ?
also something else  -> ?
bar                  -> 2

Default to another variable

But, if you really want to use map[unknown], you could:

#!/bin/bash

array=("foo" "foo" "bar" "something else" "also something else" "bar")

declare -A map="( ["foo"]="1"  ["bar"]="2"  ["unknown"]="?" )"

for key in "${array[@]}";do
    printf '%-20s -> %s\n' "$key" "${map["$key"]:-${map["unknown"]}}"
done

This will produce same output!

Note: you could even use "default to variable, with default":

echo "${map["$key"]:-${map["unknown"]:-?}}"

This will print

• content of map["$key"], or if not exist,
  • content of map["unknown"], or if not exist,
    • a single interrogation mark: ?.

You could map your array, using printf

array=("foo" "foo" "bar" "something else" "also something else" "bar")
declare -A map=( ["foo"]="1"  ["bar"]="2"  ["unknown"]="?" )

printf -v mapStr '"${map["%s"]:-${map["unknown"]}}" ' "${array[@]}"
declare -a "mappedArray=($mapStr)"
echo "${mappedArray[*]@Q}"

'1' '1' '2' '?' '?' '2'

Then

printf -v mapStr '%-20s -> %%s\n' "${array[@]}"
printf "$mapStr" "${mappedArray[@]}"

foo                  -> 1
foo                  -> 1
bar                  -> 2
something else       -> ?
also something else  -> ?
bar                  -> 2

Same into a function:

mapA2Array () { 
    if [[ $1 == -A ]]; then
        local -n _map=$2
        shift 2
    else
        local -n _map=map
    fi
    local _resArry=$1
    shift
    local _mapStr
    printf -v _mapStr '"${_map["%s"]:-${_map["unknown"]}}" ' "$@"
    declare -ga "$_resArry=($_mapStr)"
}

declare -A map="( ["foo"]="1"  ["bar"]="2"  ["unknown"]="?" )"

mapA2Array tstResult foo foo bar 'something else' 'also something else' bar
declare -p tstResult

declare -a tstResult=([0]="1" [1]="1" [2]="2" [3]="?" [4]="?" [5]="2")

declare -A otherMap=( ["foo"]="Hello world."  ["bar"]="42"  ["unknown"]="?" )
mapA2Array -A otherMap tstResult foo foo bar 'something else' 'also something else' bar
declare -p tstResult

tstResult=([0]="Hello world." [1]="Hello world." [2]="42" [3]="?" [4]="?" [5]="42")

But using this way could lead to security issues!
See Warning: limitation at end of Array elements in sed operation