Expansion of bash array when using scp - bash

Question

I am trying to retrieve several files using scp.
I already know the paths of the files to get on the remote, so I decided to add them in an array:
declare -a array
array+=("path/to/file1")
array+=("path/to/file2")
array+=("path/to/file3")
scp "$USER#$HOST:${array[#]}" .
outputs:
path/to/file1
cp: cannot stat `path/to/file2': No such file or directory
cp: cannot stat `path/to/file3': No such file or directory
Only the first file gets copied. The scp command only takes the first file into account, then cp is invoked for the remaining files.
Something as simple as this makes it work:
declare -a array
array+=("path/to/file1")
array+=("path/to/file2")
array+=("path/to/file3")
string="${array[#]"
scp "$USER#$HOST:$string" .
outputs:
path/to/file1
path/to/file2
path/to/file3
When I launch my script with bash -x, it shows that with the array, the command is not properly quoted:
+ scp $USER#$HOST:path/to/file1 path/to/file2 path/to/file3 .
Contrary to the string version:
+ scp '$USER#$HOST:path/to/file1 path/to/file2 path/to/file3' .
What exactly is causing this ? And is there a way to make the array version work, or should I use a string every time I want to use scp ? (which could be quite inconvenient with special characters)

Answer 1

Expanding an array with # results in multiple arguments:
$ array=(foo bar baz)
$ printf '<%s>\n' "${array[#]}"
<foo>
<bar>
<baz>
Expanding it with * results in a single argument separated by the first character in $IFS:
$ array=(foo bar baz)
$ printf '<%s>\n' "${array[*]}"
<foo bar baz>
99% of commands expect one filename per argument, but scp for historical reasons uses multiple filenames per arguments. In this case, you can therefore use
scp "$USER#$HOST:${array[*]}" .
though you'll likely want to escape the filenames as well, again for historical scp reasons:
scp "$USER#$HOST:${array[*]#Q}" .

Answer 2

We can substitute printf for scp to see what it actually expands to:
$ printf '>%s<\n' "$USER#$HOST:${array[#]}"
>user#host:path/to/file1<
>path/to/file2<
>path/to/file3<
Which is not what you want: you need to attach the user&host to each array element.
The ${var/pattern/string} expansion can be used here:
$ printf '>%s<\n' "${array[#]/#/$USER#$HOST:}"
>user#host:path/to/file1<
>user#host:path/to/file2<
>user#host:path/to/file3<
This is a tricky one:
we have the "var" as array[#] -- that expands to each array element
the pattern is #, which means "the empty string anchored at the start of the string
and the replacement string is the user&host.
Expanding the files array into a brace expression for the remote host:
$ remote_files=$( IFS=","; printf '%s#%s:{%s}' "$USER" "$HOST" "${array[*]}" )
$ printf '>%s<\n' "$remote_files"
>user#host:{path/to/file1,path/to/file2,path/to/file3}<