Re: the basic datatypes, three

in reply to the basic datatypes, three

%hash=(jan=>31,feb=>28,mar=>31,apr=>30);
print %hash;

output:
jan31apr30feb28mar31
Can anyone tell me why my order of output is like this ?

Comment on Re: the basic datatypes, three

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Re^2: the basic datatypes, three by Thilosophy (Curate) on Mar 01, 2005 at 10:06 UTC
Can anyone tell me why my order of output is like this ? Unlikely. For all practical purposes you cannot rely on the hash elements to be stored in any particular order. It depends on how many elements the hash has, on the value of the keys, and in recent Perl versions can be additionally randomized to prevent certain types of attacks. The order of the elements as returned by the `keys`, `each` and `values` functions is not the order of insertion or follows the alphabet. The order produced by these three functions is however internally consistent (meaning you can iterate over keys and values separately and they will match). You can sort the keys (for example) alphabetically if you need to do this, but you cannot recover the order of insertion. This may seem inconvenient, but it is (a fundamental) part of how a hash works. It is not made to retrieve elements in a certain order, but it is very fast at single-element lookups. Testing if an element exists is for example much more efficient in a hash than in an array.	[reply] [d/l] [select]

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Re^2: the basic datatypes, three
by Thilosophy (Curate) on Mar 01, 2005 at 10:06 UTC

Can anyone tell me why my order of output is like this ?

For all practical purposes you cannot rely on the hash elements to be stored in any particular order. It depends on how many elements the hash has, on the value of the keys, and in recent Perl versions can be additionally randomized to prevent certain types of attacks.

The order of the elements as returned by the keys, each and values functions is not the order of insertion or follows the alphabet. The order produced by these three functions is however internally consistent (meaning you can iterate over keys and values separately and they will match).

You can sort the keys (for example) alphabetically if you need to do this, but you cannot recover the order of insertion.

This may seem inconvenient, but it is (a fundamental) part of how a hash works. It is not made to retrieve elements in a certain order, but it is very fast at single-element lookups. Testing if an element exists is for example much more efficient in a hash than in an array.

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