Logical Operators

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 (if, else…).

Their counterparts in electronic circuit are the logic gates symboles such as AND and OR, which are sometimes used to “physically” represent logical operators.

In Ruby, as well as in most programming language, logical operators are &&, || and !.

Ruby also have and, or and 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

Logical AND

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

Logical OR

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

NOT Operator

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:

  • Comparison: <=, <, >, >=
  • Equality: ==, !=
  • Logical AND: &&
  • Logical OR: ||

This is important: statements are not necessarily executed from left to right.

if x && y || z
  # do something

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.

Using parentheses to group expressions together when possible helps with readability and helps the computer to understand our intention more accurately. Parentheses are evaluated in normal algebraic order.