One should never assume any particular visitation order in a plain perl hash structure. This includes how they're printed, how the each iterator works, how the for will walk the hash, nothing.

The internal structure is well-documented for those who are hacking the interpreter, but really none of the concern of a perl script that uses it. Perl's interpreter is free to rearrange at will, to ensure either memory efficiency or lookup efficiency. Some versions of perl will even hash differently each time the script is run, to thwart attempts to "attack" the interpreter by generating very unbalanced hashes.

There is a specialized version of the hash available, that will keep a second structure around internally to remember what order you inserted things, or how you liked things ordered. This is a subclass, a special case, and useful if your script must have that knowledge or power.

Update: It seems like some people are downvoting, perhaps assuming that I didn't read the question because I'm talking about visitation, and the question talks about bulk insertion. I phrased this intentionally; they're the same thing as far as dealing with a data structure goes. Don't rely on a quirk of the parser to populate hashes in a particular order, either. Just because the ( x => y ) syntax on the right-hand-side is implemented as a list, and today's interpreter happens to walk that list in order to populate the hash, and any duplicate values of x will get written multiple times, does not mean that making these assumptions are a good coding practice. You're inserting into a hash. If you expect a lot of overwrites, and this is important to you, express the appropriate order of insertions to manage these redundant overwrites carefully. While it seems unlikely that the interpreter will break this assumption tomorrow, that's more likely due to the fact that the perl hackers know there are a million poorly written scripts that do make this assumption out there. As a benefit, it makes your code more literate, more self-explanatory and clear.

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[ e d @ h a l l e y . c c ]

In reply to Re: Bulk hash population order by halley
in thread Bulk hash population order by JakeIII

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