True and false

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 digit
undef, the undefined value
Objects that use overloading to numify/stringify into false values, such as JSON::false

All other values are true:

any non-zero number such as 1, 3.14, 'NaN' or 'Inf'
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

The following operators are commonly treated to return a boolean in scalar context:

@a returns whether the array is empty or not
%h returns whether the hash is empty or not
grep returns whether any matching items were found or not
@a = LIST and (LIST) = LIST return whether the right-hand side LIST produced any scalars or not

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