Re: Hidden costs of PHP arrays?

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On Wed, 2009-01-28 at 10:38 +1100, Clancy wrote:
> PHP arrays permit extremely concise programming; for example if I have all my contacts in
> an array $contacts, I can write:
> 
> $my_phone_no = $contacts['clancy']['phone'];
> 
> However it is clear that there must be a lot going on behind the scenes to achieve this
> simple result, as it requires some sort of search procedure.
> 
> Is it possible to give any indication of the overheads and memory costs that are involved
> in such a statement, and of how well the search procedure is implemented?
> 
> Also what the relative virtues of defining the same set of fields for every contact, as
> against either defining only the fields which actually hold values, as in the following
> examples?
> 
> a:
> $contacts['clancy']['home_address'] = 'jkjkjk';
> $contacts['clancy']['home_phone'] = 0123 4567;
> $contacts['clancy'][' office_address''] = '';
> $contacts['clancy']['office_phone'] = '';
> $contacts['joe']['home_address'] = '';
> $contacts['joe']['home_phone'] = '';
> $contacts['joe']['office_address'] = 'jsfvkl';
> $contacts['joe']['office_phone'] = 'jsfvkl';
> 
> b;
> $contacts['clancy']['home_phone'] = 0123 4567;
> $contacts['clancy']['home_address'] = 'jkjkjk';
> $contacts['joe']['office_address'] = 'jsfvkl';
> $contacts['joe']['office_phone'] = 'jsfvkl';
> 
> And is there any advantage in always assigning the keys in the same order?

Lookup is O( lg n ). Since your examples above are nested 2 levels deep
then it's actually 2 * O( lg n ) which is O( lg n ). But really, the
variable itself, $contacts is probably also looked up and it is also
O( lg n ) and of course 3 * O( lg n ) is still O( lg n ). Moving
along... for arbitrary depth paths, you'd be talking O( m lg n ) but for
realistic cases you'll not get deep enough for that to matter much.
Either way, if you are looping over an array and always accessing X
levels deep, you might want to create a temporary variable that is level
X - 1 (unless that's not a possible option).

Cheers,
Rob.
-- 
http://www.interjinn.com
Application and Templating Framework for PHP


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