hi

in this discussion : Re^6: Using an array element as a loop iterator I was quite disappointed to find out that shift in list context can lead to an endless loop:

This will never stop: 

while ( ($x) = shift @a ) {
  ...
}

[download]

After some meditation I consider this at least a design flaw, there is only an empty list to assign to the LHS when @a is exhausted, so the condition should be false.

($x is of course undef then, but while has to check the list assignment not the scalar!)

To make things worse my tests revealed that the "equivalent" splice doesn't have the same behavior:

  DB<170> @a=1..2
 => (1, 2)

  DB<171> ( ($x) = shift @a ) ? $x : "false"
 => 1

  DB<172> ( ($x) = shift @a ) ? $x : "false"
 => 2

  DB<173> ( ($x) = shift @a ) ? $x : "false"
 => undef                   # true!

[download]

but

  DB<174> @a=1..2
 => (1, 2)

  DB<175> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => 1

  DB<176> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => 2

  DB<177> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => "false"

[download]

according to the docs: 

The following equivalences hold (assuming "$[ == 0 and $#a >= $i" )

...
                   shift(@a)           splice(@a,0,1)

[download]

OK one might argue that $i means 0 here but I still can't understand if there is a good reason justifying this breach of analogy.

Can someone shed light on this?

I suppose it's too late to fix that in newer Perl versions w/o breaking compability ... (I used 5.10)

Cheers Rolf

( addicted to the Perl Programming Language)

In reply to shift in list context buggy? by LanX

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