Logical operators are symbols or words used to connect two or more expressions. They can be used, although not always, to test the relationship between two expressions, in conjonction with conditionals (
Their counterparts in electronic circuit are the logic gates symboles such as
OR, which are sometimes used to “physically” represent logical operators.
In Ruby, as well as in most programming language, logical operators are
Ruby also have
not which can respectively be used in lieu of the operators above. Note however that their precedence are lower than their mathematical symboles counterpart.
How to Use Logical Operators
The AND operator
&& evaluates both expressions to the left and to the right. Both have to evaluate to true in order for the entire expression to be evaluated to true.
irb :001 > (4 == 4) && (5 == 5) => true irb :002 > (4 == 5) && (5 == 5) => false irb :002 > (4 == 5) && (5 == 6) => false
The OR operator
|| first evaluates the expressions to the left. If it evaluate to true, the entire expression evaluates to true, whatever is on the right. If it evaluates to false, the OR operator then evaluates the expression on the right. One of the two expression has to evaluate to true for the entire expression to evaluate to true.
irb :001 > (4 == 4) || (5 == 5) => true irb :002 > (4 == 5) || (5 == 5) => true irb :002 > (4 == 5) || (5 == 6) => false
In fronte of a boolean expression, it change the boolean value to its opposite.
irb :001 > !(4 == 4) => false
Note that the value in parentheses are evaluated first, and the result of the evaluation is then flipped by
Order of Precedence in Ruby
When evaluating multiple expressions, Ruby follows an order of precedence. From highest order of precegence to lowest, it goes like the following:
- Logical AND:
- Logical OR:
This is important: statements are not necessarily executed from left to right.
if x && y || z # do something end
x && y is executed first. If that statement is true, the condition is met and the program will enter inside
if..end. If it’s false, then
z will be evaluated. The code on the next line will be executed if
z is true, it won’t otherwise.