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Table of contents
1.
Introduction
2.
Parallel Assignment
2.1.
Equal number of lvalues and rvalues
2.2.
One lvalue, multiple rvalues
2.3.
Multiple lvalues, single array rvalue
2.4.
Different numbers of lvalues and rvalues
2.5.
The splat operator
2.6.
Parentheses in parallel assignment
3.
Code Implementation
4.
Frequently Asked Questions
4.1.
What is an assignment operator?
4.2.
What are Expression and Operator?
4.3.
What is a splat operator?
4.4.
What is rvalue?
4.5.
How many different forms of assignment expressions are there in Ruby?
5.
Conclusion
Last Updated: Mar 27, 2024
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Parallel Assignment in Ruby

Author Sanchit Kumar
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Introduction

Ruby

In this blog, we will see what is assignment and parallel assignment and then we will learn about different cases of parallel assignments with examples.

Any assignment expression with multiple lvalues, rvalues, or both are referred to as a parallel assignment. Parallel assignments are useful to write assignment expressions concisely instead of assigning value to different variable in separate lines we can simply assign values in one single line.

So to understand how to write in an efficient way let us learn about parallel assignments in detail.

Parallel Assignment

A parallel assignment is any assignment expression having more than one lvalue/ rvalue or both. More than one lvalues or rvalues are separated from one another with commas. The majority of parallel assignment expressions are easy and self-evident. However, there are some cases that are more complicated.

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

Equal number of lvalues and rvalues

When there are an equal number of lvalues and rvalues, it is a simplest parallel assignment.

For example: x, y, z = 3, 6, 9  # x=3; y=6; z=9

In this example, the first rvalue is assigned to the first lvalue , the second rvalue to the second lvalue, and so on.

One lvalue, multiple rvalues

Ruby constructs an array to hold the rvalues and assigns it to the lvalue when there is only one lvalue and many rvalues.

For example: x = 3, 6, 9  # x = [3,6,9]

You can add a * before the lvalue to keep the semantics and return value of this assignment.

If you wish to avoid the numerous rvalues being aggregated into a single array, put a comma after the lvalue. Ruby will treat it as if there are several lvalues even if there is no lvalue following that comma.

For example: x, = 2, 4, 6 # x = 2; other values are discarded

Multiple lvalues, single array rvalue

Ruby tries to expand the rvalue into a list of values to assign when there are multiple lvalues and only a single rvalue. Ruby expands an array so that each element becomes its own rvalue if the rvalue is an array.

For example: x, y, z = [2, 4, 6] # Same as x,y,z = 2,4,6

Different numbers of lvalues and rvalues

If there are more number of lvalues than rvalues and there is no splat operator, the first rvalue is assigned to the first lvalue, the second rvalue to the second lvalue, and so on until all rvalues have been assigned. Then, for each of the remaining lvalues, nil is assigned, overwriting any previous value.

For example: 

x, y, z = 1, 3 # x=1; y=3; z=nil

Suppose there are more rvalues than lvalues and there is no splat operator, then rvalues are assigned to each of the lvalues in sequence, and the leftover rvalues are discarded.

For example:  x, y = 1, 3, 9 # x=1; y=3; 9 is not assigned anywhere

The splat operator

Asterisks before rvalues indicate that they are arrays (or objects that resemble arrays) and that the values of each of their elements should be rvalues. The assignment continues after the array elements in the original rvalue list replace the array.

For example:  x, y, z = 3, *[6,9] # Same as x,y,z = 3,6,9

Note - 

  • With Ruby 1.9 in a parallel assignment, the list of rvalues may have any number of splats which can appear at any position in the list
  • Double splat is not legal on a nested array: x,y = **[[3,4]] # SyntaxError!

 

An asterisk before lvalue indicates that all additional rvalues should be collected into an array and assigned to this lvalue and value assigned is always an array, and it may have zero, one, or more elements.

For example: 

x,*y = 3, 6, 9 # x=3; y=[6,9]
*x,y = 3, 6 # x=[3]; y=6
x,*y = 3 # x=3; y=[]


Note - 

  • With Ruby 1.9, one splat operator on the left hand side of a parallel assignment is allowed, though it can be placed anywhere in the list
  • In a parallel assignment expression splats may appear on both sides

For example:

x, y, *z = 1, *[2,3,4] # x=1; y=2; z=[3,4]

Parentheses in parallel assignment

Parentheses can be used for sub-assignments in a parallel assignment on the left-hand side. A group of two or more lvalues is originally handled as a single lvalue if it is enclosed in parentheses. The rules of the parallel assignment are applied recursively after determining the corresponding rvalue—that rvalue is assigned to the group of lvalues that was in parentheses.

For example: 

x,(y,z) = a, b # effectively two assignments
#x = a
#y,z = b # itself a parallel assignment


Note - The array of rvalues is the return value of a parallel assignment expression (after being augmented by any splat operators).

Check out this article - Balanced Parentheses

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Code Implementation

#Parallel Assignment in Ruby

#Parallel Assignment in Ruby with equal number of lvalues and rvalues
x, y, z = 3, 6, 9  # x=3; y=6; z=9
puts x, y, z

#Parallel Assignment in Ruby with One lvalue, multiple rvalues
x = 13, 16, 19  # x = [13,16,19]
puts x

#Parallel Assignment in Ruby with One lvalue
#(treated as more than one), multiple rvalues
x, = 2, 4, 6 # x = 2; other values are discarded
puts x

#Parallel Assignment in Ruby with Multiple lvalues, single array rvalue
x, y, z = [2, 4, 6] # Same as x,y,z = 2,4,6
puts x, y, z

#Parallel Assignment in Ruby with more number of lvalues than rvalues
x, y, z = 1, 3 # x=1; y=3; z=nil
puts x, y, z

#Parallel Assignment in Ruby with more number of rvalues than lvalues
x, y = 1, 3, 9 # x=1; y=3; 9 is not assigned anywhere
puts x, y

#Understanding Splat Operator
x,*y = 3, 6, 9 # x=3; y=[6,9]
puts x, y

#Understanding parentheses in Parallel Assignment
x,(y,z) = 4, 8 # z=nil
puts x, y, z

Output

3
6
9
13
16
19
2
2
4
6
1
3

1
3
3
6
9
4
8

Frequently Asked Questions

What is an assignment operator?

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

What are Expression and Operator?

An expression is a section of Ruby code that the Ruby interpreter can evaluate to produce a value. An operator is a token or a particular symbol in the Ruby language representing an operation (such as addition or comparison) performed on one or more operands.

What is a splat operator?

When an asterisk precedes a rvalue, it indicates that the value is an array (or an array-like object) with rvalues as its elements. The array elements in the original rvalue list are replaced by the array elements in the array elements, and assignment proceeds as explained above.
For example: x, y, z = 4, *[2,3] # Same as x,y,z = 4,2,3

What is rvalue?

Values on the right-hand side of an assignment operator are commonly referred to as rvalues.

How many different forms of assignment expressions are there in Ruby?

There are three different forms of assignment expressions in Ruby which are simple assignment (x = 1), abbreviated assignment (x += 1) and parallel assignment (x,y,z = 1,2,3).

Conclusion

This article extensively discussed the Parallel Assignment in Ruby. We started with a brief introduction about the simple assignment and the parallel assignment and learnt how to use it with code implementation.

After reading about the Parallel Assignment in Ruby, are you not feeling excited to read/explore more articles on the topics related to Ruby? Don't worry; Coding Ninjas has you covered. To learn, see History of RubyExpressions and Operators in Ruby8 reasons why Ruby should be your first languageRuby vs PythonDocumentation of RubyOfficial Ruby FAQ, and Learn Ruby with the Edgecase Ruby Koans.

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