comment on

If you implement your own substr (swab2 below) then you get yet another behavior---it always returns '\0B'---but you can see the order things are being called in. The substr's are evaluated left-to-right and are always working on the original string---it seems to do all of the substr's first, then do the assignments. In that case, you'd get:

  char 0: A ^ B ^ A ^ B = \0
  char 1: B ^ A ^ B     = A

which is what you're seeing. On the other hand, if you use assignments to guarantee the order things are evaluated in (swab3 below), you get the results you'd expect.

I think you may simply be running into a case where Perl's order of operations is undefined. It can evaluate xor's, substr's, and assignments in any order it feels like, and it doesn't always feel like doing it the same way.

Here's the code I played with, and the results.

#!/usr/bin/perl -slw

sub mysubstr : lvalue 
{
  print "mysubstr(@_)";
  print "Returning value '",substr($_[0],$_[1],$_[2]),"' as lvalue.";
  substr($_[0],$_[1],$_[2]);
}

sub swab{
  my $s = "swab$ARGV[0]";
  goto &$s;
}

sub swab1{
    substr( $_[0], $_[1], 1 ) ^=
    substr( $_[0], $_[2], 1 ) ^=
    substr( $_[0], $_[1], 1 ) ^=
    substr( $_[0], $_[2], 1 );
}

sub swab2{
    mysubstr( $_[0], $_[1], 1, "1" ) ^=
    mysubstr( $_[0], $_[2], 1, "2" ) ^=
    mysubstr( $_[0], $_[1], 1, "3" ) ^=
    mysubstr( $_[0], $_[2], 1, "4" );
}

sub swab3{
  my($c1,$c2);
  print "   Char $_[2]: ",$c2 = substr($_[0],$_[2],1);
  print "   Char $_[1]: ",$c1 = substr($_[0],$_[1],1);
  print "        1 ^ 2: ",$c1 = $c1 ^ $c2;
  print "    2 ^ 1 ^ 2: ",$c2 = $c2 ^ $c1;
  print "1 ^ 2 ^ 2 ^ 2: ",$c1 = $c1 ^ $c2;
  substr($_[0],$_[1],1)=$c1;
  substr($_[0],$_[2],1)=$c2;
}

my $t;

$t ='AB';
swab( $t, 0, 1 );
print "'$t'";
print "'$t'";
print "length: ",length($t);

$t ='AB';
swab( $t, 0, 1 );
print "'$t'";
print "'$t'";
print "length: ",length($t);
[download]

produces:

$ perl /tmp/t55 1            
'BA'
'BA'
length: 2
'A'
'A'
length: 2
$ perl /tmp/t55 2
mysubstr(AB 0 1 1)
Returning value 'A' as lvalue.
mysubstr(AB 1 1 2)
Returning value 'B' as lvalue.
mysubstr(AB 0 1 3)
Returning value 'A' as lvalue.
mysubstr(AB 1 1 4)
Returning value 'B' as lvalue.
'B'
'B'
length: 2
mysubstr(AB 0 1 1)
Returning value 'A' as lvalue.
mysubstr(AB 1 1 2)
Returning value 'B' as lvalue.
mysubstr(AB 0 1 3)
Returning value 'A' as lvalue.
mysubstr(AB 1 1 4)
Returning value 'B' as lvalue.
'B'
'B'
length: 2
$ perl /tmp/t55 3
   Char 1: B
   Char 0: A
        1 ^ 2: 
    2 ^ 1 ^ 2: A
1 ^ 2 ^ 2 ^ 2: B
'BA'
'BA'
length: 2
   Char 1: B
   Char 0: A
        1 ^ 2: 
    2 ^ 1 ^ 2: A
1 ^ 2 ^ 2 ^ 2: B
'BA'
'BA'
length: 2

In reply to Re: An obscure side effect? by sgifford
in thread An obscure side effect? by BrowserUk

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		`&`
	<		`<`
	>		`>`
	[		`[`
	]		`]`

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.