How does a scalar know what it is?

[Tom]

In C, I can just read the K&R book and know
how everything is pretty much coded at the
machine level. At least to some extent.

But how does Perl know when you type

print $ref

for example that it is not an integer, string or what have
you?

And how does

print $num; 2 or "Two"

again at the machine level, how does perl know
what it has?

[Jürgen Exner]

Re: How does a scalar know what it is?

Tom wrote:[color=blue]
> In C, I can just read the K&R book and know
> how everything is pretty much coded at the
> machine level. At least to some extent.
>
> But how does Perl know when you type
>
> print $ref
>
> for example that it is not an integer, string or what have
> you?
>
> And how does
>
> print $num; 2 or "Two"
>
> again at the machine level, how does perl know
> what it has?[/color]

Wrong way of thinking. Every scalar has a numerical, string, and boolean
attribute (probably I forgot some). AFAIR a length attribute is also stored
with each scalar and probably some others.
Which one is being used depends on the context and perl will compute the
value if it is not known yet.

Example:

my $foo = "1234";
# the _string_ attribute of $foo contains the four characters '1', '2', '3',
'4'
# the numerical attribute is not defined

my $bar = $foo + 1;
# the _numerical_ attribute of $foo is now defined as 1234
# and the _numerical_ attribute of $bar contains the number 1235

jue

[Joe Smith]

Re: How does a scalar know what it is?

Tom wrote:[color=blue]
> In C, I can just read the K&R book and know
> how everything is pretty much coded at the
> machine level. At least to some extent.
>
> But how does Perl know when you type
>
> print $ref
>
> for example that it is not an integer, string or what have
> you?[/color]

A scalar variable (SV) is a struct with several flags. The flags
indicate which elements in the struct have valid values.

IV = Integer Value
NV = Numeric (floating point) Value
PV = Printable string (with LEN=bytes allocated, CUR=current size)
RV = Reference

See "perldoc Devel::Peek" for more details.

#!/usr/bin/perl
use Devel::Peek;
$a = '1'.'23'; Dump $a; # String
$a -= 1; Dump $a; # Integer (not string)
$_ = "a=$a"; Dump $a; # String (and integer)
$a *= 1.3; Dump $a; # Float
$_ = "a=$a"; Dump $a; # String (and float)
$a = \$_; Dump $a; # Scalar ref
$a = \@INC; Dump $a; # Array ref
$a = \%INC; Dump $a; # Hash ref;
$a = sub {$_}; Dump $a; # Code ref