Re^2: Homework question list

[runrig]

Smart people use hash slices. Really smart people use 'undef'd hash slices to save memory.

undef'd hash slices save time not memory.

Update: Also, as pointed out to me in a previous post, only use this method if you don't care about the values of the hash, as the values do not necessarily get set to undef.

[TomDLux]

I notice that in five years no one has filled in the details of the concise implementation.

# given two arrays you want to compare for common elements, @a1 & @a2 
+...
my %common;
undef @common{@a1};
delete @common{@a2};
my @common = keys %common;
#for example:
  DB<1> @a1 = qw(a b c d e)

  DB<2> @a2 = qw(a c e )

  DB<3> undef @common{@a1}

  DB<4> x \%common
0  HASH(0x8408418)
   'a' => undef
   'b' => undef
   'c' => undef
   'd' => undef
   'e' => undef
  DB<5> delete @common{@a2}

  DB<6> x \%common
0  HASH(0x8408418)
   'b' => undef
   'd' => undef
  DB<7> print join " ", keys %common
b d