Re: [Help-bash] Comparing arrays

[Greg Wooledge]

On Sun, Mar 31, 2013 at 03:34:37PM -0400, Jerry wrote:
> Using Bash-4.x, I have created two arrays.
> 
> declare -a New_Files  # Contains a list of files
> declare -a Old_Files  # Contents of a directory
> 
> Now, what I am attempting to do is check the list of existing files
> "Old_Files" against the list of "New_Files". If any of the files in
> "Old_Files" does not exist in the "New_Files" list, it is to be
> deleted.

The most efficient approach is to use an associative array:

New_Files=(...)
Old_Files=(...)

declare -A New_Seen
for file in "address@hidden"; do
  New_Seen["$file"]=1
done

for file in "address@hidden"; do
  if [[ ${New_Seen["$file"]} ]]; then
     : put code to delete "$file"
  fi
done

If the associative array implementation is optimally efficient, the
lookup of each file in the New_Seen AA should be done in O(1) instead
of O(n) time.  I'm not sure how efficient Bash's implementation
actually is.  Nevertheless, this is the algorithm of choice.