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How do I use a hash as a set?

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Contributed by vroom on Jan 19, 2000 at 00:23 UTC
Q&A  > hashes

Answer: How do I use a hash as a set?
contributed by swiftone

Using the answer above, here are some "quick" (to the programmer) ways to initalize a set:

Using a hash slice

foreach (@fruit{'apple','banana','plum'}){$_=1}
This method works best when your set will not change, because you (pretty much) have to hardcode this string in.
That is:
#This will NOT work as intended $value="'apple','banana','plum'"; foreach (@fruit{$value}){$_=1} #doesn't work!

Often you will not know the set you want at write-time, so you will want to be able to push and pop the set from an array. Converting an array into a set can be done like:

@array=("apple","banana"); push @array, "plum"; foreach (@array){$fruit{$_}=1}
These foreach loops can obfuscate your code, so a simple
foreach (@array){$fruit{$_}=1} # initialize fruit set
can greatly increase readability of code while not complicating your programming efforts
Answer: How do I use a hash as a set?
contributed by vroom

You simply have a hash where each element in the set represents a key and its corresponding value is 1 or some other true value.

my (%fruits,%vegetables); $fruit{apple}=1; $fruit{orange}=1; $fruit{banana}=1; $vegetable{corn}=1; $vegetable{carrot}=1; #test whether apple is in the set fruit if($fruit{apple}){ #test for inclusion in a set print "apple is a fruit"; }
Answer: How do I use a hash as a set?
contributed by ariels

No need to store an actual 1 value in your hash. This works just fine:

  my %fruit;
  @fruit{qw/apple orange banana/} = ();
  warn "I know how to defend myself\n"
    if exists $fruit{banana};
Note that the actual values stored here are undef. But in any case we just want to know if the key is there or not.
Answer: How do I use a hash as a set?
contributed by CharlesClarkson

The repeat operator (x) can be used with hash slices to get an easy to read set representation:

my @fruit_set = qw/apple banana orange mango/; my @vegie_set = qw/onion tomato lettuce eggplant/; my (%fruit, %vegetable); @fruit{@fruit_set} = (1) x @fruit_set; @vegetable{@vegie_set} = (1) x @vegie_set;

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