Logical and comparison operators

Before we introduce the corresponding Perl operators here are the basics of Boolean algebra.

Boolean algebra is a variant of algebra where instead of numbers are truth values 0 and 1, where 1 is called truth, and 0 is called false.

Like in a normal algebra there are operations like +, * etc, the basic ones are called NOT, AND and OR. As you already know in Boolean algebra we have only Truth and False values. That means that not only they can be used in different operations, but also the result of those operations is either Truth or False. Let's look at them one by one.

Truth and False

There are no Truth and False values in Perl. In Perl Truth is everything that is not False where False is everything that converts to 0: 0 itself, '' (empty string), undef for example.

NOT

NOT operator is a unary operator, which means it operates on one value. In Perl NOT operator is !. NOT truth table:

x	!x
0	1
1	0

Let's see what are the results of using this operator on various values. In the following example we add 0 to False values so they are not converted to empty strings by say function.

say !0;
say !1 + 0;
say !'string that converts to 1' + 0;
say !'';

AND

AND operator is a binary operator, which means it operates on two values. In Perl AND operator is &&. AND truth table:

x	y	&&
0	0	0
1	0	0
0	1	0
1	1	1

Let's see what are the results of using this operator on various values. In the following example we add 0 to False values so they are not converted to empty strings by say function.

say 1 && 1;
say 0 && 1;
say 0 && 0;
say 'string' && 1;

OR

OR operator is also a binary operator, which means it operates on two values. In Perl OR operator is ||. OR truth table:

x	y	||
0	0	0
1	0	1
0	1	1
1	1	1

Let's see what are the results of using this operator on various values. In the following example we add 0 to False values so they are not converted to empty strings by say function.

say 1 || 1;
say 0 || 1;
say 0 || 0;
say 'string' || 0;

Priority

As in a normal algebra the operators in Boolean algebra have their priority, where different operators are evaluated earlier than others. Order of Boolean operators:

! && ||

Combinations

NOT, AND and OR can be combined altogether. You can use also brackets to change the order of logical flow:

say (1 || 0) && 1

Exercise

Fix the following statement by introducing brackets so it prints empty string instead of 1.

say  !1 || 1 && 1

Comparison operators

Comparison operators also return True and False but are used with numbers and strings. Because Perl does not distinguish between numbers and strings there are two separate groups of comparison for numbers and strings.

==	!=	<	<=	>	>=
eq	ne	lt	le	gt	ge

Let's try this example:

say 1 == 1;
say 10 > 2;
say 3 <= 3;

say 'foo' ne 'bar';