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in reply to Do people find warning for undef with string compare useful?

I find it useful. IMHO you should always check for undef and never compare undef with anything. If you expect your variable to act like 0 or empty string - just assign 0 or empty string.
  • Comment on Re: Do people find warning for undef with string compare useful?

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Re^2: Do people find warning for undef with string compare useful?
by perl-diddler (Chaplain) on Jun 01, 2013 at 14:59 UTC
    What if get two paired variables, and want to check that both are initialized, or both are empty or null strings?

    I don't necessarily want it to act like anything (0 or empty string) -- I just want it to compare as "not equal" to something that is defined, or "equal" to something that isn't -- with the caveat, that undef == undef, is only equal in a certain context -- i.e. they are both uninitialized.

    I would want it to compare different if I compared undef against 0 or the empty string, so assigning either of those other values would miss the point entirely.

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      What if get two paired variables, and want to check that both are initialized, or both are empty or null strings?

      If '0' isn't a valid value 

          if( ! $one == ! $two ) {

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      If '0' is a valid value but '' is not 

          if( ! length $one == ! length $two ) {

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      If '' is also a valid value 

          if( defined $one == defined $two ) {

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      That middle case isn't sufficient on rather old versions of Perl because it used to be that length(undef) was '0' with a warning. Now length(undef) is undef so you'll only get the warning if you treat the length as a number.

      Some people replace '!' with '!!' because they use '!!' as an idiom for "I just care about whether it is true or not" or "convert to Boolean".

      [ Update: I've seen people use something like 

          unless( $one xor $two ) {

      [download]

      But I find it way too easy to end up making mistakes when you start using the ultra-low-precendence Boolean operators for things other than flow control (I've found quite a few such mistakes made by some of the most experienced Perl programmers I've worked with), so I don't really like that approach. (I also don't like 'unless' but I won't go into that here.) ]

      - tye        

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[d/l]
[select]
      Then maybe you want 3-value-logic ?

      http://blogs.perl.org/users/ovid/2013/02/three-value-logic-in-perl.html

      http://en.wikipedia.org/wiki/Three-valued_logic
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      For those very rare occasions, disable the warning locally.

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

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