Check out this excellent meditation by bobf.

If you enable warnings, perl will most times tell you quite clearly what you are doing wrong:

1 == 1; #is True.
1 == 2; #is False.
'a' == 'a'; #is True.
Argument "a" isn't numeric in numeric eq (==) at (eval 6) line 1, 
Argument "a" isn't numeric in numeric eq (==) at (eval 6) line 1, 

'a' == 'b'; #is True.
Argument "b" isn't numeric in numeric eq (==) at (eval 7) line 1, 
Argument "a" isn't numeric in numeric eq (==) at (eval 7) line 1, 


'1' == '1'; #is True.
'1' == '2'; #is False.

'1' == '1foo'; #is True.
Argument "1foo" isn't numeric in numeric eq (==) at (eval 10) line 2,

'1' == 'foo1'; #is False.
Argument "foo1" isn't numeric in numeric eq (==) at (eval 11) line 1,
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Mantadin, thank you. I am aware of this, however I am wondering why '1' == '1foo' evals to true while '1' eq 'foo1' does not.

When converting strings to numbers, Perl only looks at the start of the string. '1foo' becomes 1 and 'foo1' becomes 0.

--
<http://dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

because strings are automatically converted to numbers when needed. '1foo' is converted to 1 and 'foo1' is converted to 0.

Others have already explained how the conversion works, but if you want to see for yourself the number into which a string is coerced, add 0 to the string.

print(0+'a',    "\n");  # 0
print(0+'b',    "\n");  # 0
print(0+'1',    "\n");  # 1
print(0+'2',    "\n");  # 2
print(0+'1foo', "\n");  # 1
print(0+'foo1', "\n");  # 0
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That's why you get warnings if the conversion fails ("foo" isn't numeric) or one of the values is undef.

The reason why perl has both stringwise and numerical comparisons,, is because you're not really supposed to know whether a scalar contains a string or a number. A value "1" is thought of as being equivalent to just 1. Other languages that have only one equality test operator, like Javascript, VB, and PHP, can cause bugs because it tries to infer what you mean from the type of variable. Which could be different from what you think. Ditto with the concatenation operator, . vs. +.

p.s. In the CB, jkva seems to have had some doubt on why == is called a "binary operator". The reason is because == has 2 arguments, plain and simple (bi == 2). Likewise, ! is an "unary operator" (un == 1), and ?: is a "ternary operator" (ter == 3).

== is the numeric equality operator. It compares its operands as numbers. Therefore any strings are given a numeric value before the comparison takes place. Perl does this by looking for numbers at the start of the string. If none are found then the string is given the value 0. This is why 'a' and 'b' appear to be equal (they are both converted to zero).

For string comparisons, use 'eq'.

Perl numifies strings when a number is required. A string that matches /(\d+)\D/ for example simply becomes the integer contained in $1, or 0 if ! defined $1. '1foo' becomes 1. 'foo1' becomes 0.

'==' ne 'eq';  # but
'==' == 'eq';
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#!/usr/bin/perl

use strict;
no warnings;

use Scalar::Util qw/ dualvar /;

my $foo = dualvar 1, "foo";
my $bar = dualvar 2, "foo";

print "Using dualvar$/";
print '$foo', ($foo eq $bar ? " eq " : " ne "), '$bar', $/;
print '$foo', ($foo == $bar ? " == " : " != "), '$bar', $/;

__END__
Using dualvar
$foo eq $bar
$foo != $bar
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Using overload could result in similarly confusing comparisons. This isn't as contrived as it sounds:

#!/usr/bin/perl

use strict;
use warnings;

use URI;

my $foo = URI->new('http://www.perlmonks.org');
my $bar = URI->new('http://www.perlmonks.org');

print "Using URI$/";
print '$foo', ($foo eq $bar ? " eq " : " ne "), '$bar', $/;
print '$foo', ($foo == $bar ? " == " : " != "), '$bar', $/;

__END__
Using URI
$foo eq $bar
$foo != $bar
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URI defines an overload for stringification, but not for numification. In numerical context, objects evaluate to their location in memory. And the two different objects are stored in different locations, so == returns false.

So be careful with == and eq :)

   
#!/usr/bin/perl -w
if (1 == 1)
{
  print "triue\n";
}
 if(1 == 2)
{
  print"false\n";
}
if ('a' eq 'b')
{
  print "trie\n";
}
 if('1'eq '1foo')
{
  print "true\n";
}
if('1' eq 'foo1')
{
  print "false\n";
}
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