Side effects of comparison on numeric string's type

[p5pRT]

Migrated from rt.perl.org#1316 (status was 'resolved')

Searchable as RT1316$

[p5pRT]

From merriam@world.std.com

Hello again,

1. Thanks for the answer to "$a+++$b" issue. All it needs is
documentation. Thanks again.

2. It's cool that ++ works on serial number strings. There is
a wierd type switching here. If I make an implicit numeric conversion
for use in an expression, the state of the variable is changed and
causes
headaches at the next ++ operator. Here's an example,

$x = "0000"; # ok, a string.
print "x = $x "; # "0000"
$x++;
print "x = $x "; # "0001"
$x++;
print "x = $x "; # "0002"
$y = $x > 20; # side effect here...
print "x = $x "; # "0002" still.
$x++; # trigger to effect here...
print "x = $x "; # "3"
$x++;
print "x = $x "; # "4"
print "\nOutput was 0000 0001 0002 0002 3 4\n\n";

and the variants.

$x = "0000"; # ok, a string.
print $x;
$y = ($x) > 20; # side effect here...
print "x = $x ";
$x++; # trigger to effect here...
print "x = $x ";
print "\nOutput was 0000 0000 1\n\n";

$x = "0000"; # ok, a string.
print "x = $x ";
$z = $x;
$y = $z > 20; # Of course, $x doesn't change.
print "x = $x ";
$x++;
print "x = $x ";
print "\nOutput was 0000 0000 0001\n\n";

Why? It doesn't seem like desirable behavior.

Thank you again for your patience,

Charles Merriam
merriam@​world.std.com
408 773 8824

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[p5pRT]

From [Unknown Contact. See original ticket]

On Sat, 04 Sep 1999 11​:23​:53 -0700, C M <merriam@​world.std.com> wrote​:

2. It's cool that ++ works on serial number strings. There is
a wierd type switching here. If I make an implicit numeric conversion
for use in an expression, the state of the variable is changed and
causes
headaches at the next ++ operator. Here's an example,

That's documented behavior (see perlop.pod)​:

| The auto-increment operator has a little extra builtin magic to it. If
| you increment a variable that is numeric, or that has ever been used in
| a numeric context, you get a normal increment. If, however, the
| variable has been used in only string contexts since it was set, and has
| a value that is not null and matches the pattern `/^[a-zA-Z]*[0-9]*$/',
| the increment is done as a string, preserving each character within its
| range, with carry​:

$x = "0000"; # ok, a string.
print "x = $x "; # "0000"
$x++;
print "x = $x "; # "0001"
$x++;
print "x = $x "; # "0002"
$y = $x > 20; # side effect here...

Yes, using $x in numeric context. Maybe you want

  $y = $x gt "0020"; # no side effect here...

print "x = $x "; # "0002" still.
$x++; # trigger to effect here...
print "x = $x "; # "3"
$x++;
print "x = $x "; # "4"
print "\nOutput was 0000 0001 0002 0002 3 4\n\n";

and the variants.

[snip]

Why? It doesn't seem like desirable behavior.

I would agree. I don't like this behavior much, but it has been in Perl
for a very long time and cannot be changed. This is something one must
simply accept and move on. Besides, there are people who find that the
usefulness of the feature does compensate for it's negative attributes
(action at a distance, reliance on internal caching behavior etc.).

-Jan