Assigning script options/arguments to a variable and manually loop through - bash

Question

I'm using a variation of the BashFAQ manual loop and I want to convert it to a function and assign the arguments to local variable but I can't figure out the correct syntax. Here's what I have:
function parseArguments() {
local arguments=( "$#" )
while :; do
case ${1:-} in
-d|--debug)
set -o xtrace
# [...more stuff...]
;;
-p|--prompt)
IsPromptEnabled=true
;;
--)
shift
break
;;
-?*)
error ${LINENO} "\"${1:-}\" is an unknown option" 1
;;
*)
break
esac
shift
done
}
parseArguments "$#"
This works fine as is until I try to replace $1 in the loop with the value from arguments. ${arguments[0]} and every other variation I can think of fails, I'd like to understand why (and figure out the solution).

Answer 1

You should loop over the array
function parseArguments() {
local arguments=( "$#" )
for a in "${arguments[#]}"
do
case ${a:-} in
-d|--debug)
set -o xtrace
# [...more stuff...]
;;
-p|--prompt)
IsPromptEnabled=true
;;
--)
# shift
break
;;
-?*)
error ${LINENO} "\"${a:-}\" is an unknown option" 1
;;
*)
break
esac
shift
done
}
parseArguments "$#"

Answer 2

It's because you set the arguments array to the initial list of arguments.
For example, if you call parseArguments -d -p, then you start with $1 set to "-d" and $2 set to "-p". So arguments gets set to ("-d" "-p") as expected.
On the first time through the loop, it recognizes ${1:-} as "-p", sets IsPromptEnabled=true, and shifts that argument away.
So at the end of the first time through the loop, $1 is "-d" and $2 is unset. But arguments hasn't changed, it's still set to ("-d" "-p"). The shift doesn't update it, and it never gets reassigned, so it's always going to have the full list of arguments.
Even after "-d" gets recognized, acted on, and shifted away (making the actual argument list empty), arguments will still be set to ("-d" "-p").
If you want arguments to hold the non-option arguments, you should move the assignment to it after the option-parsing loop, so it gets set after all the options have been shifted away.