How to create array-variable from function argument input -- Remove Array Blanks function

Question

After seeing this question, I decided to put together a function to remove array empty elements in case it saves someone a few seconds.

Is there any way to return (or export) a dynamically named input array-variable as the output of a function?

Ideally

User calls: removeArrayBlanks "newArrayName" "arrayItem1" "" "arrayItem2"...
The function unsets the old array and creates: ${newArrayName[@]}, which expands to "arrayItem1" "arrayItem2" without any blank items or non-sequential index numbers

Also, does anyone have any optimizations/suggestions to the function? I've included the function below. Thanks!

removeArrayBlanks() {
    # Usage: Provide array as input, store output as array. 
    # Example 1: mapfile -t lastArray < <(removeArrayBlanks "hate" "" "empty" "array" "items")
    # Example 2: mapfile -t lastArray < <(removeArrayBlanks "${inputArray[@]}")
    [[ $# -lt 2 ]] && echo "Usage: Provide array as an argument." && exit 1 # Ensure input is array
    tempArray=( "$@" ) # Rebuild array from inputs
    for i in "${!tempArray[@]}"; do
        [[ ! -z "${tempArray[$i]}" ]] && finalArray+=( "${tempArray[$i]}" ) # Add non-empty strings
    done && printf '%s\n' "${finalArray[@]}" && unset tempArray && unset finalArray
}

Unless you declare it local, variables assigned in a function are visible in the main script, so you don't need to return it. — Barmar, Aug 02 '21 at 20:58
do you want to remove the 'blanks' from the original array, or create a brand new array with no 'blanks'? — markp-fuso, Aug 02 '21 at 21:01
@Barmar ty, was have a hard time getting the new array to be called `$1` from user input. I couldn't find an easy way to 'update' the input array, without forcing the user to have to call a new array name. — jmtornetta, Aug 02 '21 at 21:06
@markp-fus trying to just remove blanks from input array. And, ideally, not have to rename it or declare a new array. — jmtornetta, Aug 02 '21 at 21:07
@Jetchisel it is kinda like a double negative, i hear ya. Was trying to find the best way possible to take non-empty values and put them into a new , fresh array without non-sequential index numbers or empty items. — jmtornetta, Aug 02 '21 at 21:08
@jmtornetta : I took the liberty to remove the _sh_ tag, since the question is not related to `sh` (after all, _sh_ does not have arrays). — user1934428, Aug 03 '21 at 05:55

markp-fuso · Accepted Answer · 2021-08-02T21:17:20.703

2

Assumptions:

objective is to remove array elements that contain empty strings (aka 'blanks') from current array
function should be able to work for any array (name) passed to the function
OP is running, or can upgrade to/above, bash 4.3+ (needed for nameref support)

NOTE: to 'remove' an array element just unset the array reference (eg, unset array[index#1]

One idea using a nameref in the function and the unset command:

function removeArrayBlanks() {

declare -n localarray="${1}"                                  # define nameref; remains a local array variable

for i in "${!localarray[@]}"                                  # loop through indices
do
    [[ -z "${localarray[${i}]}" ]] && unset localarray[${i}]  # remove entries that contain empty strings
done
}

In operation:

$ myarray=( a "" 1 "" A )
$ typeset -p myarray
declare -a myarray=([0]="a" [1]="" [2]="1" [3]="" [4]="A")

$ removeArrayBlanks myarray
$ typeset -p myarray
declare -a myarray=([0]="a" [2]="1" [4]="A")

# verify localarray not known by parent
$ typeset -p localarray
-bash: typeset: localarray: not found

edited Aug 02 '21 at 21:17

answered Aug 02 '21 at 21:10

markp-fuso

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Thanks @markp-fuso! I didn't realize that the variable would be updated in the current context/shell from which the function was called. I figured I would have had to 'export' the new array somehow. – jmtornetta Aug 02 '21 at 21:24
Ahh, I see...I suppose I wasn't wrong? I just didn't realized that this was possible: `declare -n` "Give each name the nameref attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name, except those using or changing the -n attribute itself, are performed on the variable referenced by name's value." – jmtornetta Aug 02 '21 at 21:27
keep in mind a function call is (in essence) a macro call (think `replace '' with ''`) and not a sub-process call; net result is anything you do in the function is actually applied in the 'parent' process; one exception is if you define a variable as 'local' to the function in which case the 'parent' does not see it; in this case the `nameref' is a 'local' variable that also doubles as a **reference** (and not a copy) of the passed in array name – markp-fuso Aug 02 '21 at 21:31
I opted to populate the array first and then remove/unset the empty strings; you could still implement your logic ... filter out the empty strings before assiging values to the array ... via a function, but also use a `nameref` to eliminate the overhead of creating/printing/trashing 'temporary' arrays ... your choice – markp-fuso Aug 02 '21 at 21:34
Yes, adding in the nameref was one thing I didn't realize. The other thing I am seeing that I missed was not expanding out the array as function argument. That is, I was doing `removeArrayBlanks ${inputArray[@]}` which I gather would cause the array name to be lost, because the array is expanded before function can do stuff to it. – jmtornetta Aug 02 '21 at 21:40

How to create array-variable from function argument input -- Remove Array Blanks function

Ideally

1 Answers1