Re: Ignoring values in a hash slice assignment

An update:

After working through the great responses I got, and testing various options, here's where I am:

My first solution (using undef as key values for fields I didn't want) wasn't doing what I thought it was. Although Perl warned (when warnings were turned on) about assigning to an uninitialized value in a hash slice, it also quite happily initialized those keys to an empty string (like it does with other undefined scalars in similar situations). So both of my solutions were really doing the same thing: assigning to a dummy key (named '' or 'DUMMY', respectively). The only problem was that I wasn't deleting the dummy key in the first solution. :-)
There were no real surprises with using an array slice with my hash slice (as suggested by almost everyone), but there was one significant downside: I was replacing two lines of very readable code with up to five lines of much less readable code, and I feared for those who would have to maintain this after me.
So, unless someone comes up with a better idea, I will be using my second solution. I think that in general a combination of the various answers I received might be a better solution, but for this project, simplicity wins.

Thanks to everyone who I haven't thanked yet (petral and seattlejohn) and I'm still open to more suggestions. :-)

Impossible Robot

Comment on Re: Ignoring values in a hash slice assignment

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Re2: Ignoring values in a hash slice assignment by dragonchild (Archbishop) on Mar 01, 2002 at 17:45 UTC
Ok. You're just not thinking it through. You say that you have a list of some 70 fields, some of which may be needed and others not. Given how you presented your original question, you're storing this list of fieldnames in an array, putting 'DUMMY' for the fields you wish to ignore. That, my friend, is a maintainer's nightmare. You don't ever say what the field is you're ignoring You don't say WHY you're ignoring that field It isn't easy to change which fields you're ignoring without researching into what fields exist It also isn't easy to change which order the fields exist in. So, we need some way of abstracting out which fields are there. Then, we need a way of identifying which fields we actually want. Lastly, we need a way of making sure that this information (which will become quite large) doesn't impact our main program. Enter "Modular Design". Let's create a module where we store some data about what fields we want. Let's also create a few functions that allow us to talk about various aspects (or attributes) of each field. Now, you don't need a full-blown OO implementation of this. That would be too much work and too unwieldy to manage. However, you can do a very nice Exporter implementation of this. Something along the lines of: Read more... (2 kB)	[reply] [d/l] [select]

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Re2: Ignoring values in a hash slice assignment
by dragonchild (Archbishop) on Mar 01, 2002 at 17:45 UTC

That, my friend, is a maintainer's nightmare.

You don't ever say what the field is you're ignoring
You don't say WHY you're ignoring that field
It isn't easy to change which fields you're ignoring without researching into what fields exist
It also isn't easy to change which order the fields exist in.

So, we need some way of abstracting out which fields are there. Then, we need a way of identifying which fields we actually want. Lastly, we need a way of making sure that this information (which will become quite large) doesn't impact our main program.

Enter "Modular Design". Let's create a module where we store some data about what fields we want. Let's also create a few functions that allow us to talk about various aspects (or attributes) of each field.

Now, you don't need a full-blown OO implementation of this. That would be too much work and too unwieldy to manage. However, you can do a very nice Exporter implementation of this. Something along the lines of: