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Re: Inverting a hash safely

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Shawn H. Corey
August 4, 2009 06:03
Re: Inverting a hash safely
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Ed Avis wrote:
> Shawn H. Corey <shawnhcorey <at>> writes:
>> But then again I never have to invert a hash; when I populate it, I 
>> would populate its inverse as well.  I would build both data structures 
>> at the same time, inserting only the data I need, where I need it.
> That's often a good approach.  But in the particular case I was thinking of,
> there was some (programmer-maintained, not user-maintained) configuration data
> in a hash:
>     my %lookup = (Frob => 55, Boo => 66, Grick => 67);
> Of course it would be silly to write a %lookup_reverse hash by hand and then
> worry about keeping the two consistent.  Better to write it once and invert it.

This assumes that programmers never make mistakes.  This source of 
input, and like all input, should be validated before it is used.  And 
while you're validating it, that is, while you're going through the hash 
one item at a time, you can build your inverse, just like I said.

> Similarly, if you are making a subroutine that takes a hash as input, it is
> inconvenient to require your caller to pass both the hash and its inverse.
> Indeed, it creates all sorts of opportunities for bugs when somehow the hash
> and inverse-hash you are passed aren't consistent with each other.  (Generally
> anything that can go wrong will go wrong, sooner or later.)

The bugs show up because of the lack of validation.  Or are you thinking 
that some software upstream causes bugs and you're responsible for 
catching them?  It's a better idea to unit test _all_ the software so it 
does not create bugs.

> Or if your routine gives a hash as output (such as many XML processing or
> database modules), it would be seen as a bit weird to return both a hash and
> an inverse-hash.  It's a question of taste, but I would prefer to just get one
> hash as the return value, and invert it if I need to.

Why on earth would you want to invert an XML file?  Sounds like someone 
is being clever.  Remember:  clever == hard to understand == costly to 

As for inverting the return from a database, I would get the database to 
do it.  It can take advantage of relationships inside the database that 
are lost when the data is extracted.

> So I don't think it is fair to say that inverting a hash is never needed,
> although you can certainly minimize the need for it if you build your own
> data structures carefully.

To be specific, I said I never had to invert a hash since I build all 
the data structures I need while validating the input.  And no, I don't 
pass large data structures around.  Either I need only one set at a 
time, in which case they're globals, or I put them in an object and pass 
that around.

Just my 0.00000002 million dollars worth,

Programming is as much about organization and communication
as it is about coding.

My favourite four-letter word is "Done!"

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