autovivify non-sequential array/list in Python like Perl

Question

In perl I can have what I think are called sparse arrays.

my @a;
$a[4321] = "blah";

and it just does what I want. I don't know how to do this in Python without an error IndexError: list assignment index out of range. What is the (simplest) way to do this in Python?

In Python, you'd probably want a dictionary - the keys are not required to have any relationship to each other. — jasonharper
– jasonharper, Commented Oct 29, 2022 at 0:23
Perl arrays aren't sparse, but as you note they do auto-extend on assignment. You could do something similar in python, but it's not a pythonic thing to do. — craigb
– craigb, Commented Oct 29, 2022 at 0:24
@jasonharper if I used a dictionary I'd need to sort the keys numerically afterwards. Hmm. — Lucky
– Lucky, Commented Oct 29, 2022 at 0:40
@craigb I'm not a pythonic person, but I can live with being unnecessarily C-ish. I was hoping to avoid exactly that approach though. — Lucky
– Lucky, Commented Oct 29, 2022 at 0:42

ikegami · Accepted Answer · 2022-10-29 02:21:33Z

1

That array is not sparse. It has 4322 elements.

In Python, the following would create a similar construct:

a = [ None for i in range( 4321 ) ] + [ "blah" ]

If you want to set an element of an array that might be beyond the end of an existing array, @OmnipotentEntity proposed this function.

def set_at( xs, idx, x, default=None ):
   if len( xs ) <= idx:
      xs.extend( [ default ] * ( idx - len( xs ) + 1 ) )
   xs[ idx ] = x

a = [ ]
set_at( a, 4321, "blah" )

If you truly want something sparse, you can use a dictionary with integer keys.

a = { }
a[ 4321 ] = "blah"

edited Oct 29, 2022 at 2:21

answered Oct 29, 2022 at 2:14

ikegami

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3 Comments

ikegami Over a year ago

My Python knowledge is limited. Let me know if there's a better way.

Lucky Over a year ago

I think that is pythonic, but that would clobber any elements I sprinkled in prior to 4321. In perl I can just do $a[1234] = "blah"; $a[4321] ="foo" without wiping the value at 1234.

ikegami Over a year ago

Re "but that would clobber any elements I sprinkled in prior to 4321", I was adding an solution to that as you posted your comment :) See update.

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