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Re^2: Learning Programming, desperately need to know what information is contained in scalar variables

by Anonymous Monk
on Jul 20, 2020 at 15:50 UTC ( #11119549=note: print w/replies, xml ) Need Help??


in reply to Re: Learning Programming, desperately need to know what information is contained in scalar variables
in thread Learning Programming, desperately need to know what information is contained in scalar variables

That's awesome, thank you very much Dave. I have another question about this piece of code say "Catastrophic crypto fail!" if $alice eq $bob; what value is the expression $alice eq $bob producing?
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Re^3: Learning Programming, desperately need to know what information is contained in scalar variables
by Lotus1 (Vicar) on Jul 20, 2020 at 16:28 UTC
      So it's returning true because both $alice and $bob are undef?

        Evaluating two undef values for string equality yields a true result. undef is temporarily promoted to "" (the empty string) in string context. If warnings are enabled, warnings are emitted.

        c:\@Work\Perl\monks>perl -le "use strict; use warnings; ;; use Data::Dump qw(dd); ;; my ($value, $alice, $bob); ;; $value = $alice eq $bob; dd 'result of comparison of undefined values', $value; " Use of uninitialized value in string eq at -e line 1. Use of uninitialized value in string eq at -e line 1. ("result of comparison of undefined values", 1)


        Give a man a fish:  <%-{-{-{-<

        My response is 'undef'

Re^3: Learning Programming, desperately need to know what information is contained in scalar variables
by karlgoethebier (Abbot) on Jul 20, 2020 at 18:32 UTC
    «...awesome...»

    Sure. It is the canonical answer. But now comes practice. Which is just another word for repetition. Remember: «No Intuition Without Repetition» (Dr Wisenheimer)

    «The Crux of the Biscuit is the Apostrophe»

    perl -MCrypt::CBC -E 'say Crypt::CBC->new(-key=>'kgb',-cipher=>"Blowfish")->decrypt_hex($ENV{KARL});'Help

Re^3: Learning Programming, desperately need to know what information is contained in scalar variables
by Anonymous Monk on Jul 20, 2020 at 16:56 UTC
    The expression $alice eq $bob is returning true because both $alice and $bob are undef, that's both contain the same information hence they are equal. Am I right?

      Perl's operators are monomorphic, and its data is polymorphic. Look at this:

      print "Yes\n" if 1 == "1"; # Yes print "Yes\n" if "0" eq 0; # Yes

      In the first test, the number 1 is compared numerically to the string "1". Perl converts that string to a number using rules defined in perldata, and perlop.

      In the second test, the string "0" is compared to the number 0 using stringwise comparison. So the number 0 is promoted to a string for the comparison.

      If you compare undef to undef numerically, undef is treated as 0. If you compare undef to undef stringwise, undef is treated as empty string. So undef == undef produces the same result (true) as 0 == 0. And undef eq undef produces the same value (a Boolean true value) as '' eq '' (empty string is equal to empty string).

      To put it even more plainly:

      • '' eq '' # true: Empty string is equal to empty string.
      • undef, when treated as a string acts like an empty string.
      • The eq operator treats its operands like strings.
      • If $foo and $bar are undef, then $foo eq $foo becomes an empty string to empty string comparison.

      Dave

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