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map and undef - odd behaviour with an array of arrays

by Melly (Hermit)
on Dec 22, 2012 at 08:26 UTC ( #1009983=perlquestion: print w/ replies, xml ) Need Help??
Melly has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monkees

The following one liner prints, rather unexpectedly, 'still defined' - any ideas as to what's going on?

perl -e "$x[0]=[0];map{undef $_}(@x);print 'still defined' if defined +@x;"
map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2 -$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk

Comment on map and undef - odd behaviour with an array of arrays
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Re: map and undef - odd behaviour with an array of arrays
by eyepopslikeamosquito (Canon) on Dec 22, 2012 at 08:38 UTC

    Running:

    use Data::Dumper; $x[0]=[0];map{undef $_}(@x);print "still defined\n" if defined @x; print Dumper(\@x);
    produces:
    defined(@array) is deprecated at undef1.pl line 2. (Maybe you should just omit the defined()?) still defined $VAR1 = [ undef ];
    I trust that clarifies what is going on. That is, your array contains a single element with the value undef. Note that an array containing a single element with an undef value is not undef, not even false in scalar context. To be false in scalar context, the array needs to be empty.

    Oh, and please heed the deprecated warning and replace "if defined @x" with simply "if @x" (see defined for why).

      But why did map{undef $_}(@x) behave differently than undef @x?

      Edit to add - oh, I'm being stupid - it's undefining each element rather than @x. When I was testing this, for some reason I concluded that perl -e "@x=qw(a);map{undef $_}(@x);print 1 if defined @x;" didn't print 1 (but it does).

      One of those days...

      map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2 -$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
      Tom Melly, pm (at) cursingmaggot (stop) co (stop) uk

        But why did map{undef $_}(@x) behave differently than undef @x?
        "undef @x" empties the array while "map{undef $_}@x" sets each element in the array to the value undef. Since your array had one element in it, that element was not deleted, rather its value was set to undef.

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