Deleting specific values from an array in ksh - arrays

Question

I have a customized .profile that I use in ksh and below is a function that I created to skip back and forth from directories with overly complicated or long names.
As you can see, the pathnames are stored in an array (BOOKMARKS[]) to keep track of them and reference them at a later time. I want to be able to delete certain values from the array, using a case statement (or OPTARG if necessary) so that I can just type bmk -d # to remove the path at the associated index.
I have fiddled around with array +A and -A, but it just wound up screwing up my array (what is left in the commented out code may not be pretty...I didn't proofread it).
Any suggestions/tips on how to create that functionality? Thanks!
# To bookmark the current directory you are in for easy navigation back and forth from multiple non-aliased directories
# Use like 'bmk' (sets the current directory to a bookmark number) to go back to this directory, i.e. type 'bmk 3' (for the 3rd)
# To find out what directories are linked to which numbers, type 'bmk -l' (lowercase L)
# For every new directory bookmarked, the number will increase so the first time you run 'bmk' it will be 1 then 2,3,4...etc. for every consecutive run therea
fter
# TODO: finish -d (delete bookmark entry) function
make_bookmark()
{
if [[ $# -eq 0 ]]; then
BOOKMARKS[${COUNTER}]=${PWD}
(( COUNTER=COUNTER+1 ))
else
case $1 in
-l) NUM_OF_ELEMENTS=${#BOOKMARKS[*]}
while [[ ${COUNTER} -lt ${NUM_OF_ELEMENTS} ]]
do
(( ACTUAL_NUM=i+1 ))
echo ${ACTUAL_NUM}":"${BOOKMARKS[${i}]}
(( COUNTER=COUNTER+1 ))
done
break ;;
#-d) ACTUAL_NUM=$2
#(( REMOVE=${ACTUAL_NUM}-1 ))
#echo "Removing path ${BOOKMARKS[${REMOVE}]} from 'bmk'..."
#NUM_OF_ELEMENTS=${#BOOKMARKS[*]}
#while [[ ${NUM_OF_ELEMENTS} -gt 0 ]]
#do
#if [[ ${NUM_OF_ELEMENTS} -ne ${ACTUAL_NUM} ]]; then
# TEMP_ARR=$(echo "${BOOKMARKS[*]}")
# (( NUM_OF_ELEMENTS=${NUM_OF_ELEMENTS}-1 ))
#fi
#echo $TEMP_ARR
#done
#break
#for VALUE in ${TEMP_ARR}
#do
# set +A BOOKMARK ${TEMP_ARR}
#done
#echo ${BOOKMARK[*]}
#break ;;
*) (( INDEX=$1-1 ))
cd ${BOOKMARKS[${INDEX}]}
break ;;
esac
fi
}

Answer 1

Arrays in the Korn shell (and Bash and others) are sparse, so if you use unset to delete members of the array, you won't be able to use the size of the array as an index to the last member and other limitations.
Here are some useful snippets (the second for loop is something you might be able to put to use right away):
array=(1 2 3)
unset array[2]
echo ${array[2]} # null
indices=(${!array[#]}) # create an array of the indices of "array"
size=${#indices[#]} # the size of "array" is the number of indices into it
size=${#array[#]} # same
echo ${array[#]: -1} # you can use slices to get array elements, -1 is the last one, etc.
for element in ${array[#]}; do # iterate over the array without an index
for index in ${indices[#]} # iterate over the array WITH an index
do
echo "Index: ${index}, Element: ${array[index]}"
done
for index in ${!array[#]} # iterate over the array WITH an index, directly
That last one can eliminate the need for a counter.
Here are a couple more handy techniques:
array+=("new element") # append a new element without referring to an index
((counter++)) # shorter than ((counter=counter+1)) or ((counter+=1))
if [[ $var == 3 ]] # you can use the more "natural" comparison operators inside double square brackets
while [[ $var < 11 ]] # another example
echo ${array[${index}-1] # math inside an array subscript
This all assumes ksh93, some things may not work in earlier versions.

Answer 2

you can use unset. eg to delete array element 1
unset array[0]
to delete entire array
unset array

Answer 3

A few caveats regarding the previous answer:
First: I see this error all the time. When you provide an array element to "unset", you have to quote it. Consider:
$ echo foo > ./a2
$ ls a[2]
a2
$ a2="Do not delete this"
$ a=(this is not an array)
$ unset -v a[2]
$ echo "a2=${a2-UNSET}, a[]=${a[#]}"
a2=UNSET a[]=this is not an array
What happened? Globbing. You obviously wanted to delete element 2 of a[], but shell syntax being what it is, the shell first checked the current directory for a file that matched the glob pattern "a[2]". If it finds a match, it replaces the glob pattern with that filename, and you wind up making a decision about which variable to delete based on what files exist in your current directory.
This is profoundly stupid. But it's not something anyone has bothered to fix, apparently, and the error turns up in all kinds of documentation and example code from the last 3 decades.
Next is a related problem: it's easy to insert elements in your associative array with any key you like. But it's harder to remove these elements:
typeset -A assoc
key="foo] bar"
assoc[$key]=3 #No problem!
unset -v "assoc[$key]" #Problem!
In bash you can do this:
unset -v "assoc[\$key]"
In Korn Shell, you have to do this:
unset -v "assoc[foo\]\ bar]"
So it gets a bit more complicated in the case where your keys contain syntax characters.