Need to remove an array from another array

Discussion in 'Perl Misc' started by M.L., Oct 7, 2008.

  1. M.L.

    M.L. Guest

    Given an array: @planets = qw(mercury venus earth mars jupiter saturn uranus
    neptune pluto);
    and @removes = qw(venus mars pluto);

    I'd like to know how to grep the @removes from @planets such that
    @planets = qw(mercury earth jupiter saturn uranus neptune);

    I tried
    @planets = grep (!/$removes[0..$#removes]/, @planets);
    but that didn't work and I'm out of ideas. Any assistance appreciated.
    Thanks.
     
    M.L., Oct 7, 2008
    #1
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  2. A standard way would be to 'upgrade' the @removes array to ans hash
    and then use the existance of a key in the has as the condition for a
    grep:

    %removes = map { $_ => 1 } @removes;
    @planets = grep { !$remove{$_} } @planets;

    //Makholm
     
    Peter Makholm, Oct 7, 2008
    #2
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  3. perldoc -q intersection:
    "How do I compute the difference of two arrays? How do I compute the
    intersection of two arrays?"

    jue
     
    Jürgen Exner, Oct 8, 2008
    #3
  4. M.L.

    sln Guest

    Why don't you just say it in English, do you really need examples?

    The most important array comparison is thier similarity, not the difference!!!
    Similarity first, difference by default.

    sln
     
    sln, Oct 8, 2008
    #4
  5. M.L.

    M.L. Guest

    Given an array: @planets = qw(mercury venus earth mars jupiter saturn
    I found your solution to be not only elegant, but the easiest to understand.
    After making a slight correction ($remove{$_} -> $removes{$_} ), your
    solution worked like a charm. Thank you so much.
     
    M.L., Oct 8, 2008
    #5
  6. M.L.

    M.L. Guest

    Given an array: @planets = qw(mercury venus earth mars jupiter saturn
    Thank you for your perldoc reference. While it provided a little more
    horsepower than I needed, I found it very enlightening. I'll keep it in my
    Perl notes file.
     
    M.L., Oct 8, 2008
    #6
  7. Coming to think of it, your spec via example is rather incomplete:
    - what about if @planets contains multiple entries with the same value,
    e.g. 'mars'? Are you supposed to remove all occurences of 'mars' from
    @planets, even if @removes contains only one entry 'mars'? And if only
    one is to be removed, then which one? Any? The first?
    -probably less critical: what about if @removes contain an element that
    is not part of @planets? Error? Or just ignore?

    It appears to me you are using the arrays to represent mathematical
    sets: elements are unique and sequence doesn't matter. If this is true,
    then a hash would be a much better data structure.

    jue
     
    Jürgen Exner, Oct 8, 2008
    #7
  8. M.L.

    sln Guest

    It just makes me sick listening to crap like this. I wan't to puke in your face.

    sln
     
    sln, Oct 8, 2008
    #8
  9. M.L.

    M.L. Guest

    Given an array: @planets = qw(mercury venus earth mars jupiter saturn
    Nothing really complex about my setup. I created @planets as an easy to
    understand analogue representing the unique form variables on my HTML form.
    I deliberately created the @removes array of variables as a means to get rid
    of any unneeded form variables before they were printed out for an email
    response.

    For my email response problem it would be necessary to get rid of *any*
    duplicate entries in @planets that were specified in @removes since I
    wouldn't want the recipient to ever see them. Since I created @removes for a
    specific duty, I would never allow it to have duplicate entries. Even if it
    did, I've already used Google to learn how to get rid of duplicate entries
    from an array. So I'm all set for now.
     
    M.L., Oct 8, 2008
    #9
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