What is The ||= Idiom in Ruby

Introduction

In this blog, we will see what is assignment and abbreviated assignment and then we will learn about Idiom in Ruby with code implementation.

An abbreviated assignment is a shorthand expression that updates the value of a variable by performing other operations (such as addition) on the variable's current value. It is used to express something more concisely. 

So, now let us learn about '||=' abbreviated assignment (Idiom in Ruby).

Abbreviated Assignment

The equal operator '=' is used in Ruby as an assignment operator. This is applicable for variables and objects because strings, floats, and integers are all objects in Ruby, and we're always assigning objects. 

A combination of an assignment and another operation is an abbreviated assignment. It's most commonly used to increase the value of a variable: 

x += 9

'+=' isn't an actual Ruby operator(it is a pseudo-operator), and the expression above is just a shorthand for x = x + 9

Note - Pseudo-Operators are not true operators themselves; they are simply shorthand for expressions that use other operators.

An abbreviated assignment works when there is a single Ivalue on the left and a single value on the right. When the lvalue is a constant, it should not be used since it will reassign the constant, resulting in a warning. When the lvalue is an attribute, the abbreviated assignment can be used.

Note - 

• An lvalue is something present on the left hand side of an assignment operator
• Values on the right hand side of an assignment operator are commonly referred to as rvalues.
   

To learn more about the abbreviated assignment with various pseudo-operators and code implementation, you can click here.
 

Now, let us dive deeper into the Idiom in Ruby: 

Idiom in Ruby

'||=' assign passd value if the lvalue is nil or false, and no assignment is actually made if the lvalue of '||=' is neither nil nor false.

Example 1: 

a = a
a ||= 3
# '||=' sets value 3 to variable 

You can also try this code with Online Ruby Compiler

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In this example, a is not assigned with any value hence it is being set to 3. If it would have been assigned with any value, ||= idiom would have not assigned the new value to a. That’s the power of ||= idiom. 

Interesting right?

Let us see one more example: 

Example 2: 

b = 6
b = b || 3
# no assignment is actually made

You can also try this code with Online Ruby Compiler

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Note - Contrary to popular belief, a ||= b is equivalent to a || a = b instead of being equivalent to a = a || b which lead to the concept Idiom in Ruby as a ||= b behave more like a || a = b than a = a || b.

• a || a = b value is set to variable a only if a is logically false (that is, if it is nil or false) 
• a = a || b sets a to something on each run. If the || comparison's left side is true, the right side need not be validated. 
   

Now with this background detail, we understood ||= Idiom in Ruby.

Code Implementation

# understanding Idiom in Ruby
# ||= Idiom in Ruby with nil lvalue
a = nil
a ||= 3
puts a

# ||= Idiom in Ruby with non nil or false lvalue 
b = 6
b ||= 3
puts b

# ||= Idiom in Ruby with true lvalue 
c = true
c ||= 9
puts c

You can also try this code with Online Ruby Compiler

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Output

Explanation