http://www.perlmonks.org?node_id=852410


in reply to Numification of strings

While a variable is a string and if it has neither been assigned a value nor numified its value will be '', I am not sure on this but I think '' or undef or 0 can be interchanging depending on the context that the null variable scalar is used under (i.e when a scalar which's null is numified its value becomes 0).

Isn't this behaviour mainly hiding ugly bugs?
This behavior seems consistent and indeed in/of use, for instance, consider when quickly wanting to start a counter: 

C:\Documents and Settings>perl -
print "$hash{count} \n";           
print -$hash{count},"\n";
print --$hash{count},"\n";
print ++$hash{count},"\n";
__END__

0
-1
0
C:\Documents and Settings>

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Update: did s/' '/''/, thanks Jenda for noticing the unintended typo as '' and ' ' are not the same thing. Added technical corrections too..

Excellence is an Endeavor of Persistence. A Year-Old Monk :D .

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Re^2: Numification of strings
by Jenda (Abbot) on Aug 02, 2010 at 13:32 UTC

    No, it will not be ' ', it will be ''. That's not the same thing.

    And actually if a variable has not been assigned, then it's value is undef, which is not a string, but its stringified value is '' an numified value is 0. There is no such thing as an "undefined string" or "string variable"! It is a scalar variable whose value may be undef and/or string and/or integer and/or float and/or reference.

    Jenda
    Enoch was right!
    Enjoy the last years of Rome.

[reply]
[d/l]
      well always? :)

             dualvar NUM, STRING
           Returns a scalar that has the value NUM in a numeric contex
+t and
           the value STRING in a string context.
 
               $foo = dualvar 10, "Hello";
               $num = $foo + 2;                    # 12
               $str = $foo . " world";             # Hello world

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      Cheers Rolf

[reply]
[d/l]

        And what does this disagree with? I never said the integer and string values of a scalar variable have to be in any way related.

        Jenda
        Enoch was right!
        Enjoy the last years of Rome.

[reply]
Re^2: Numification of strings
by LanX (Saint) on Aug 02, 2010 at 11:28 UTC
    >This behavior seems consistent and indeed in/of use, for instance, consider when quickly wanting to start a counter:

    Sure that's useful, but it's only about converting undef or (maybe sometimes) the empty string ''.

    Cheers Rolf

[reply]
      "but it's only about converting undef"

      Lest you or anyone else read that as generally true, I repeat, for emphasis re warnings (or for brevity here, -w) and re 'consistent':

      perl -we "$x="LanX"; print ($x * 3);"
Argument "LanX" isn't numeric in multiplication (*) at -e line 1.
0
perl -we "$x="LanX"; print ($x % 3);"
Argument "LanX" isn't numeric in modulus (%) at -e line 1.
0
perl -we "$x="LanX"; print (3^$x);"
Argument "LanX" isn't numeric in bitwise xor (^) at -e line 1.
3
perl -we "$x="LanX"; print (3**$x);"
Argument "LanX" isn't numeric in exponentiation (**) at -e line 1.
1

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      but

      perl -we "$x='7LanX'; print (3^$x);"
Argument "7LanX" isn't numeric in bitwise xor (^) at -e line 1.
4
perl -we "$x='7LanX'; print ($x*3);"
Argument "7LanX" isn't numeric in multiplication (*) at -e line 1.
21
perl -we "$x='7LanX'; print (3**$x);"
Argument "7LanX" isn't numeric in exponentiation (**) at -e line 1.
2187

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      and where it's useful -- using a string as a number because it's being used in an operation which is arithmetic:

      perl -we "$x=\"5\"; print ($x*3);"  # 2
15
perl -we "$x='5'; print ($x*3);"
15

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      2   Interesting (or not enough coffee yet?): doze balked at doublequotes without escapes here, but passed 'em cheerfully above. WTF?

[reply]
[d/l]
[select]
        > > "but it's only about converting undef"

        converting undef is differently handled with ++, enabling warnings doesn't help here!

        see Re^3: Numification of strings

        Cheers Rolf

[reply]
      wow incrementing with ++ is not orthogonal to +1!

        DB<33> my $y;$y++
  
  DB<34> my $y;$y+1
Use of uninitialized value $y in addition (+) at (eval 38)[/usr/share/
+perl/5.10/perl5db.pl:638] line 2.

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      from perlop

              The auto-increment operator has a little extra builtin magic t
+o it.
 
...(<i>special magic for incrementing strings, ie <c>++($x='x9') eq 'y
+0'</i>)...
 
       "undef" is always treated as numeric, and in particular is chan
+ged to 0
       before incrementing (so that a post-increment of an undef value
+ will
       return 0 rather than "undef").

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      Cheers Rolf

[reply]
[d/l]
[select]
        Orthogonal means unrelated, independent. For example, if the size of a fish is orthogonal to its coloring, knowing the colour of a fish sheds no information about its size and vice-versa.
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