Re^2: Sparse arrays?

Update: If you didn't catch it, I was being sarcastic on the "wasted" space. It's itsy-bitsy by perl standards.

Exactly! I was missing the forest for the trees. Take a close look at the memory addresses being used here. I swapped the undef elements for numeric 1 just to see a real SvIV. I think what actually happens here is that the ARRAY pointer array is fully allocated out to the limit of @ary. The unused elements appear to be null pointers or at least all share a single empty Sv struct with no allocated svpv structures. So actually... that array with 1001 elements appears to have consumed enough memory for five elements plus an additional 994 32 bit pointers for a whopping 3K of "wasted" space. If I'm wrong on what this actually looks like then that'd be really good to know.

My interpretation is that arrays are sparse by default when there are uninitialized elements. Getting a sparse array to work nicely under iteration operators appears to require some additional footwork. This looks like the sort of thing you'd want to restrict to knowledgable perl users since it doesn't quite work with the normal perl idioms and is highly likely to confuse (but is a useful thing to have). If anyone knows of a different way to go directly from real element to real element that'd really keen.

@ary[1,100,500,1000] = (1) x 4;
# Iterate a sparse array
exists($ary[$_]) and print "$_\n" for 0 .. $#ary;
# Iterate a normal array
print "$_\n" for @ary
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Elt No. 994  0x4284
SV = NULL(0x0) at 0x4284
  REFCNT = 2147483634
  FLAGS = (READONLY)
Elt No. 995  0x4284
SV = NULL(0x0) at 0x4284
  REFCNT = 2147483634
  FLAGS = (READONLY)
Elt No. 996  0x4284
SV = NULL(0x0) at 0x4284
  REFCNT = 2147483634
  FLAGS = (READONLY)
Elt No. 997  0x4284
SV = NULL(0x0) at 0x4284
  REFCNT = 2147483634
  FLAGS = (READONLY)
Elt No. 998  0x4284
SV = NULL(0x0) at 0x4284
  REFCNT = 2147483634
  FLAGS = (READONLY)
Elt No. 999  0x706c
SV = IV(0x12094) at 0x706c
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 1
[download]

Fun Fun Fun in the Fluffy Chair

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Re: Re^2: Sparse arrays? by paulbort (Hermit) on Dec 18, 2002 at 16:47 UTC
If anyone knows of a different way to go directly from real element to real element that'd really keen. Easy-peasy: `foreach my $element (@sparse) { if ( defined $element ) { # YOUR CODE HERE # } }` [download] defined() should be fast enough that calling it even a million times is no big deal. -- Spring: Forces, Coiled Again!	[reply] [d/l]
Re^4: Sparse arrays? by diotalevi (Canon) on Dec 18, 2002 at 16:52 UTC
That is less powerful than the example I posted and gains you nothing. Three tested states are uninitialied, undefined and defined. My snippet replaces your defined() test with an exists() test which diferentiates between non-existant elements and merely undefined. The "different" way I was thinking of would probably use something other than for() since for() also returns uninitialized elements. It's not sparse aware. You know - something like each() for arrays. Fun Fun Fun in the Fluffy Chair	[reply]