Ruby Operator Precedence

In the previous chapter of Ruby Essentials we looked at Ruby operators and expressions. An equally important area or understand is operator precedence. This is essentially the order in which the Ruby interpreter evaluates expressions comprising more than one operator.

An Example of Ruby Operator Precedence

When humans evaluate expressions, they usually do so starting at the left of the expression and working towards the right. For example, working from left to right we get a result of 300 from the following expression:

10 + 20 * 10 = 300

This is because we, as humans, add 10 to 20, resulting in 30 and then multiply that by 10 to arrive at 300. Ask Ruby to perform the same calculation and you get a very different answer:

irb(main):003:0> 10 + 20 * 10
=> 210

This is a direct result of operator precedence. Ruby has a set of rules that tell it in which order operators should be evaluated in an expression. Clearly, Ruby considers the multiplication operator (*) to be of a higher precedence than the addition (+) operator.

Overriding Operator Precedence

The precedence built into Ruby can be overided by surrounding the lower priority section of an expression is parentheses. For example:

(10 + 20) * 10
=> 300

In the above example, the expression fragment enclosed in parentheses is evaluated before the hight precedence multiplications.

Operator Precedence Table

The following table provides a reference when you need to to know the operator precedence used by Ruby. The table lists all operators from highest precendence to lowest.

Ruby operators (highest to lowest precedence)

Method	Operator	Description
Yes	[ ] [ ]=	Element reference, element set
Yes	**	Exponentiation (raise to the power)
Yes	! ~ + -	Not, complement, unary plus and minus (method names for the last two are +@ and -@)
Yes	* / %	Multiply, divide, and modulo
Yes	+ -	Addition and subtraction
Yes	>> <<	Right and left bitwise shift
Yes	&	Bitwise `AND'
Yes	^ |	Bitwise exclusive `OR' and regular `OR'
Yes	<= < > >=	Comparison operators
Yes	<=> == === != =~ !~	Equality and pattern match operators (!= and !~ may not be defined as methods)
	&&	Logical `AND'
	||	Logical `AND'
	.. ...	Range (inclusive and exclusive)
	? :	Ternary if-then-else
	= %= { /= -= += |= &= >>= <<= *= &&= ||= **=	Assignment
	defined?	Check if specified symbol defined
	not	Logical negation
	or and	Logical composition
	if unless while until	Expression modifiers
	begin/end	Block expression

Operators with a Yes in the method column are actually methods, and as such may be overridden.