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Re: Hash keys not DWIMming

by sflitman (Hermit)
on Oct 07, 2010 at 05:21 UTC ( #863907=note: print w/ replies, xml ) Need Help??


in reply to Hash keys not DWIMming

I agree with Dave that line 6 has a problem, the solution being to put the 512_x64 in single quotes like you did for the hash definition. This code works fine:

#!/usr/bin/perl use warnings; use strict; my %h = ( '512_x64' => '24', 'world' => 'hello'); print $h{world}, "\n"; # line 5 print $h{'512_x64'}, "\n"; # line 6
So you can use a key that starts with a digit!
HTH,
SSF


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Re^2: Hash keys not DWIMming
by syphilis (Canon) on Oct 07, 2010 at 07:04 UTC
    the solution being to put the 512_x64 in single quotes

    Yes, I know that works. It's just that I thought steps had been taken to avoid the need for quoting hash keys .... that's obviously not so and we're left with what looks like a dog's breakfast when it comes to deciding if a key needs to be quoted or not.

    After spending a number of minutes looking at this, the best rule of practice I can come up with is "if the name of the key matches /^[0-9_\.]/ then quote it" - otherwise there's too many "ifs and buts" involved in trying to determine whether it will work without quotes.

    Cheers,
    Rob
      It's just that I thought steps had been taken to avoid the need for quoting hash keys

      From "Learning Perl (5th)", p 95:

      ...the keys are always converted to strings. So, if you used the numeric expression 50/20 as the key, it would be turned into the three-character string "2.5"...

      And on p. 254, in a section titled "Unquoted Hash Keys":

      Perl offers many shortcuts that can help programmers. Here's a handy one: you may omit the quote marks on some hash keys.

      Of course, you can't omit the quote marks on just any key, since a hash key may be any arbitrary string. But keys are often simple. If the hash key is made up of nothing but letters, digits, and underscores without starting with a digit, you may be able to omit the quote marks. This kind of simple string without quote marks is called a bareword, since it stands alone without quotes.

      ...But beware: if there's anything inside the curly braces besides a bareword, Perl will interpret it as an expression.

      It's just that I thought steps had been taken to avoid the need for quoting hash keys .... that's obviously not so and we're left with what looks like a dog's breakfast when it comes to deciding if a key needs to be quoted or not.

      The rule seems pretty straightforward to me. If the hash key is a valid identifier, it doesn't need quoted. Otherwise it does. Has always worked fine for my usage and always does what I want.

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