# True and false
# List of true and false values
use feature qw( say );
# Numbers are true if they're not equal to 0.
say 0 ? 'true' : 'false'; # false
say 1 ? 'true' : 'false'; # true
say 2 ? 'true' : 'false'; # true
say -1 ? 'true' : 'false'; # true
say 1-1 ? 'true' : 'false'; # false
say 0e7 ? 'true' : 'false'; # false
say -0.00 ? 'true' : 'false'; # false
# Strings are true if they're not empty.
say 'a' ? 'true' : 'false'; # true
say 'false' ? 'true' : 'false'; # true
say '' ? 'true' : 'false'; # false
# Even if a string would be treated as 0 in numeric context, it's true if nonempty.
# The only exception is the string "0", which is false.
# To force numeric context add 0 to the string
say '0' ? 'true' : 'false'; # false
say '0.0' ? 'true' : 'false'; # true
say '0e0' ? 'true' : 'false'; # true
say '0 but true' ? 'true' : 'false'; # true
say '0 whargarbl' ? 'true' : 'false'; # true
say 0+'0 argarbl' ? 'true' : 'false'; # false
# Things that become numbers in scalar context are treated as numbers.
my @c = ();
my @d = (0);
say @c ? 'true' : 'false'; # false
say @d ? 'true' : 'false'; # true
# Anything undefined is false.
say undef ? 'true' : 'false'; # false
# References are always true, even if they point at something false
my @c = ();
my $d = 0;
say \@c ? 'true' : 'false'; # true
say \$d ? 'true' : 'false'; # true
say \0 ? 'true' : 'false'; # true
say \'' ? 'true' : 'false'; # true
# Syntax
- undef # False
- '' # Defined, False
- 0 # Defined, Has Length, False
- '0' # Defined, Has Length, False
# Remarks
Perl does not have a boolean data type, nor does it have any true
and false
keywords like many other languages. However, every scalar value will evaluate to true or false when evaluated in a boolean context (the condition in an if statement or a while loop, for example).
# The following values are considered false:
''
, the empty string. This is what the built-in comparison operators return (e.g.0 == 1
)0
, the number 0, even if you write it as 000 or 0.0'0'
, the string that contains a single 0 digitundef
, the undefined value- Objects that use overloading (opens new window) to numify/stringify into false values, such as
JSON::false
(opens new window)
# All other values are true:
- any non-zero number such as
1
,3.14
,'NaN'
or'Inf'
(opens new window) - any string that is numerically 0 but not literally the string
'0'
, such as'00'
,'0e0'
,"0\n"
and"abc"
.
If you are intentionally returning a true numerically 0 value, prefer'0E0'
(used by well known modules) or'0 but true'
(used by Perl functions) - any other string that is not empty, such as
' '
,'false'
- all references, even if they reference false values, such as
\''
,[]
, or{}
- an array or hash of false values