Difference between $var = 500 and $var = '500'

Question

In Perl, what is the difference between

$status = 500;

and

$status = '500';

Answer 1

Not much. They both assign five hundred to $status. The internal format used will be different initially (IV vs PV,UTF8=0), but that's of no importance to Perl.

However, there are things that behave different based on the choice of storage format even though they shouldn't. Based on the choice of storage format,

• JSON decides whether to use quotes or not.
• DBI guesses the SQL type it should use for a parameter.
• The bitwise operators (&, | and ^) guess whether their operands are strings or not.
• open and other file-related builtins encode the file name using UTF-8 or not. (Bug!)
• Some mathematical operations return negative zero or not.

Answer 2

As already @ikegami told not much. But remember than here is MUCH difference between

$ perl -E '$v=0500; say $v'

prints 320 (decimal value of 0500 octal number), and

$ perl -E '$v="0500"; say $v'

what prints

0500

and

$ perl -E '$v=0900; say $v'

what dies with error:

Illegal octal digit '9' at -e line 1, at end of line
Execution of -e aborted due to compilation errors.

And

perl -E '$v="0300";say $v+1'

prints

301

but

perl -E '$v="0300";say ++$v'

prints

0301

similar with 0x\d+, e.g:

$v = 0x900;
$v = "0x900";

Answer 3

There is only a difference if you then use $var with one of the few operators that has different flavors when operating on a string or a number:

$string = '500';
$number = 500;
print $string & '000', "\n";
print $number & '000', "\n";

output:

000
0

Answer 4

To provide a bit more context on the "not much" responses, here is a representation of the internal data structures of the two values via the Devel::Peek module:

user@foo ~ $ perl -MDevel::Peek -e 'print Dump 500; print Dump "500"'
SV = IV(0x7f8e8302c280) at 0x7f8e8302c288
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 500
SV = PV(0x7f8e83004e98) at 0x7f8e8302c2d0
  REFCNT = 1
  FLAGS = (PADTMP,POK,READONLY,pPOK)
  PV = 0x7f8e82c1b4e0 "500"\0
  CUR = 3
  LEN = 16

Here is a dump of Perl doing what you mean:

user@foo ~ $ perl -MDevel::Peek -e 'print Dump ("500" + 1)'
SV = IV(0x7f88b202c268) at 0x7f88b202c270
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 501

Answer 5

The first is a number (the integer between 499 and 501). The second is a string (the characters '5', '0', and '0'). It's not true that there's no difference between them. It's not true that one will be converted immediately to the other. It is true that strings are converted to numbers when necessary, and vice-versa, and the conversion is mostly transparent, but not completely.

The answer When does the difference between a string and a number matter in Perl 5 covers some of the cases where they're not equivalent:

• Bitwise operators treat numbers numerically (operating on the bits of the binary representation of each number), but they treat strings character-wise (operating on the bits of each character of each string).
• The JSON module will output a string as a string (with quotes) even if it's numeric, but it will output a number as a number.
• A very small or very large number might stringify differently than you expect, whereas a string is already a string and doesn't need to be stringified. That is, if $x = 1000000000000000 and $y = "1000000000000000" then $x might stringify to 1e+15. Since using a variable as a hash key is stringifying, that means that $hash{$x} and $hash{$y} may be different hash slots.
• The smart-match (~~) and given/when operators treat number arguments differently from numeric strings. Best to avoid those operators anyway.

Answer 6

There are different internally:)

($_ ^ $_) ne '0' ? print "$_ is string\n" : print "$_ is numeric\n" for (500, '500');

output:

500 is numeric
500 is string

Answer 7

I think this perfectly demonstrates what is going on.

$ perl -MDevel::Peek -e 'my ($a, $b) = (500, "500");print Dump $a; print Dump $b; $a.""; $b+0; print Dump $a; print Dump $b'
SV = IV(0x8cca90) at 0x8ccaa0
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 500
SV = PV(0x8acc20) at 0x8ccad0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x8c5da0 "500"\0
  CUR = 3
  LEN = 16
SV = PVIV(0x8c0f88) at 0x8ccaa0
  REFCNT = 1
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 500
  PV = 0x8d3660 "500"\0
  CUR = 3
  LEN = 16
SV = PVIV(0x8c0fa0) at 0x8ccad0
  REFCNT = 1
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 500
  PV = 0x8c5da0 "500"\0
  CUR = 3
  LEN = 16

Each scalar (SV) can have string (PV) and or numeric (IV) representation. Once you use variable with only string representation in any numeric operation and one with only numeric representation in any string operation they have both representations. To be correct, there can be also another number representation, the floating point representation (NV) so there are three possible representation of scalar value.

Answer 8

Many answers already to this question but i'll give it a shot for the confused newbie:

my $foo = 500;
my $bar = '500';

As they are, for practical pourposes they are the "same". The interesting part is when you use operators. For example:

print $foo + 0;

output: 500

The '+' operator sees a number at its left and a number at its right, both decimals, hence the answer is 500 + 0 => 500

print $bar + 0;

output: 500

Same output, the operator sees a string that looks like a decimal integer at its left, and a zero at its right, hence 500 + 0 => 500

But where are the differences? It depends on the operator used. Operators decide what's going to happen. For example:

my $foo = '128hello';
print $foo + 0;
output: 128

In this case it behaves like atoi() in C. It takes biggest numeric part starting from the left and uses it as a number. If there are no numbers it uses it as a 0.

How to deal with this in conditionals?

my $foo = '0900';
my $bar = 900;
if( $foo == $bar)
{print "ok!"}
else
{print "not ok!"}

output: ok!

== compares the numerical value in both variables. if you use warnings it will complain about using == with strings but it will still try to coerce.

my $foo = '0900';
my $bar = 900;
if( $foo eq $bar)
{print "ok!"}
else
{print "not ok!"}

output: not ok!

eq compares strings for equality.

Answer 9

You can try "^" operator.

my $str  = '500';
my $num  = 500;

if ($num ^ $num)
{
    print 'haha\n';
}

if ($str ^ $str)
{
    print 'hehe\n';
}

$str ^ $str is different from $num ^ $num so you will get "hehe". ps, "^" will change the arguments, so you should do

my $temp = $str;
if ($temp ^ $temp )
{
    print 'hehe\n';
}

. I usually use this operator to tell the difference between num and str in perl.