I was stuck debugging a larger program but finally located the cause of the misunderstanding. The code takes one element off an array then process the rest of the array using while ( $next = pop @x) { Process element in $next }; I noticed that in some circumstances an element was being dropped. After some digging I found that the behaviour of shift and pop are not reversibly consistent in this context. The code works as expected using shift but pop stops when the element value 0 is processed.

Can anyone explain why pop behaves differently from shift in this context ?

In the demonstrator code below the array x delivers 2 elements but all the rest deliver 3 elements.

This is perl 5, version 22, subversion 1 (v5.22.1) built for darwin-thread-multi-2level on a MacBookPro 
 
use strict;

my @a = ( 0,1,2,3 ) ; my @b = ( 1,2,3,4 );
my @x = ( 0,1,2,3 ) ; my @y = ( 1,2,3,4 );
my @r = reverse @a;

my $next = 0 ; my $first = 0;

my $first = shift @a ;
print "a $#a shift ",$first,"->";
while ( $next = shift @a )
                    { print ",",$next }
print "\n";

$first = shift @b ;
print "b $#b shift ",$first,"->";
while ( $next = shift @b )
                    { print ",",$next }
print "\n";

$first = pop @x ;
print "x $#x pop ",$first,"->";
while ( $next = pop @x )
                    { print ",",$next }
print "\n";

$first = pop @y ;
print "y $#y pop ",$first,"->";
while ( $next = pop @y )
                    { print ",",$next }
print "\n";

$first = pop @r ;
print "r $#r pop ",$first,"->";
while ( $next = pop @r )
                    { print ",",$next }
print "\n";

Gives --- >>>>> See line starting x
$ perl test.pl 
a 2 shift 0->,1,2,3
b 2 shift 1->,2,3,4
x 2 pop 3->,2,1
y 2 pop 4->,3,2,1
r 2 pop 0->,1,2,3

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In reply to pop and shift not reversible by gannett

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