Parse array based on variable and nth character

You can use this search function using printf and awk:

srch() {
    printf "%s\n" "${array[@]}" | awk -v s="$1" 'substr($1, 1, 2) == s{
    print substr($1, 3)}' ;
}

Then use it as:

srch 75
01

srch 73
02

srch 84
03

srch 99
04

Since bash arrays are sparse, even in older versions of bash that don't have associative arrays (mapping arbitrary strings as keys), you could have a regular array that has keys only for numeric indexes that you wish to map. Consider the following code, which takes your input array and generates an output array of that form:

array=( 7501 7302 8403 9904 )

replacements=( )                    # create an empty array to map source to dest
for arg in "${array[@]}"; do        # for each entry in our array...
  replacements[${arg:0:2}]=${arg:2} # map the first two characters to the remainder.
done

This will create an array that looks like (if you ran declare -p replacements after the above code to dump a description of the replacements variable):

# "declare -p replacements" will then print this description of the new array generated...
# ...by the code given above:
declare -a replacements='([73]="02" [75]="01" [84]="03" [99]="04")'

You can then trivially look up any entry in it as a constant-time operation that requires no external commands:

$ echo "${replacements[73]}"
02

...or iterate through the keys and associated values independently:

for key in "${!replacements[@]}"; do
  value=${replacements[$key]}
  echo "Key $key has value $value"
done

...which will emit:

Key 73 has value 02
Key 75 has value 01
Key 84 has value 03
Key 99 has value 04

Notes/References:

• See the bash-hackers wiki on parameter expansion for understanding of the syntax used to slice the elements (${arg:0:2} and ${arg:2}).
• See BashFAQ #5 or the BashGuide on arrays for more details on the syntax used above.