Ruby Constants, What Are They?

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Publish date:

October 10, 2022

Updated on:

June 13, 2024

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Ruby Constants, What Are They?


When programmers worldwide mention programming languages that provide flexibility and easy use, Ruby is near the top of the list. Ruby is one of the top programming languages that has gained so much traction since it was first launched, due to its easy access and use with features like Ruby constants. It’s only suitable to agree that the future of Ruby already looks very promising.

Yukihiro Matsumoto created Ruby in 1995 to serve as a scripting language that combines both code reuse and object-oriented programming to give developers a better shot at faster development. It's a high-level programming language that provides a coding syntax that allows developers to feel like they're coding in English, with little to no interaction with actual machine language. This is possible because, as a scripting language, Ruby is written to a text file, then parsed by an interpreter, turning it into code.

Ruby programming language code to improve web performance.

The flexibility of Ruby and its open-source code — which allows programmers to change parts of the language — is another reason it's widely loved. Ruby is used across different applications in the development world. Developers can use Ruby for web development, its most common application, with the use of development frameworks. Developers can also use Ruby for web servers, static site generation, data processing, DevOps, and much more.

Like every other programming language, Ruby has certain features that make it work as a developing tool. Some of these features are unique — and in this article, the spotlight is on Ruby constants.

What are Ruby Constants?

Regardless of the programming language, coding requires you to assign values to specific elements to be used across the code. This is where variables come in. Variables are present in every programming language and are essential to the Ruby coding system. Ruby constants are a special type of variable. As the name implies, it's a variable whose value is expected to remain the same throughout the program.

Before going further into constants, it’s crucial to understand Ruby variables.

What are Ruby Variables?

In programming, a variable is a name representing a program's value. Variables are used to store different values — including character strings, numeric values, and memory addresses — for use in any part of the program when they're required. It isn’t any different in the Ruby programming language. Ruby supports 5 types of variables in every program:

  • Local Variables: Represented by starting with a lowercase letter (e.g., number) or an underscore (_number).
  • Instance Variables: Defined by starting with the “@” character, these variables are shared in all the methods for the particular object in the variable.
  • Class Variable: Represented by starting with “@@,” class variables share the values between a class, its subclasses, and its instances.
  • Global Variables: Defined by starting with a “$” sign, global variables are accessible anywhere within the Ruby program.
  • Ruby Constants: A special type of variable beginning with an uppercase letter as its representation (e.g., Name).

What Makes Ruby Constants Special?

Although constants are a type of variable in Ruby, certain features distinguish them. The prominent feature that makes constants unique is that you can easily change a variable's value at any time in the program. In contrast, the value assigned to constants remains the same throughout the program. This is the same across every programming language.

However, Ruby constants have a slight variation. In Ruby, you can change the value assigned to any constant in the code, but by doing this, Ruby will give a warning. Unlike other programming languages that will not run the code or give error messages before effecting changes, Ruby lets you change constant values with a slap on the hand. But it notifies you of the activity with a line like "Warning: already initialized constant," although it'll reflect the value of the constant you wrote. That makes it special, as it is one of the pros of Ruby's development.

These constants can be defined by starting with an uppercase letter as the first letter or making the whole constant all caps. That is, writing Rate = (any value you assign) or RATE = (any value you assign). This distinguishes it from other program listings.

Ruby Constants and Methods

Another special feature of Ruby constants is that they can’t be defined inside a method. In Ruby, a method works similarly to a function in other programming languages. These methods are used to group one or more repeatable statements into one unit.

A method works as a set of expressions that returns a value. Methods let you have subroutines that can be called from other program areas. So, if you define a constant inside a method like this, it'll fail with a syntax error.

However, there are certain ways to bypass this. Metaprogramming tricks can give you a chance to use your constant inside a select number of methods, including const_get (returns the value for a constant), remove_cont (removes a constant), and many more.

An overview of Ruby programming language code on a web developer's laptop.

Ruby Literals

It's impossible to go on without mentioning literal when talking about Ruby constants. Literals are used every time an object is typed into the Ruby code. Literals are any constant value that can be assigned to a variable or constant in the code. They're special syntaxes that create a specific type of object used in the program.

There are different types of Ruby Literals, and some of them include:

  • Booleans and nil (Boolean constants true, false, nil)
  • Numbers or integers
  • Symbol
  • Range (prints all the possible values in the given boundaries)
  • Array (a collection of objects created with special brackets [ and ] )
  • Hashes (used as perfect keys)

Ruby Constants and Global Variables

Constants and global variables are two different types of variables across different programming languages. Despite their different classifications and uses, they have quite a number of similarities. Their differences, however, depend on what programming language they use. In the Ruby programming language, constants and global variables have a range of similarities and differences.

In Ruby, constants and global variables are very similar. They both run outside of a method, assigning values to literals on the code. Constants also have the visibility of a global variable. This means you can use a constant anywhere in a Ruby program without paying too much attention to the scope. However, a significant difference between them is the ability to change values. You can change the value of a global variable in Ruby easily. However, as it has already been established for a Ruby constant, its value will only change, accompanied by a warning from Ruby.

So, a Ruby constant is like a global variable with a warning for value changes. Also, assigning a value to a global variable changes the global status of that variable. Another difference between a constant and a global variable in Ruby is that constants can be defined by modules and classes — unlike global variables — so they can have qualified names.

Ruby constants are a unique kind of constant in the programming world. The ability of the constant to be somewhat dynamic is what makes it special. Regardless of how you might be using the Ruby constant in your programs, it's going to be a different experience whether you’re using Ruby or Ruby on Rails.


Q1. How do you use a constant in Ruby?

You have to first declare a constant to use it. There are 2 ways to do this. Your first option is to begin the first letter of the constant with an uppercase letter, and there you have it. On the other hand, you can decide to type the whole constant in uppercase. These are the simple ways to use a constant in Ruby.

The simplicity of the entire programming and scripting language known as Ruby is one of the reasons it's everybody's favorite. The simplicity also extends to the Ruby Constant.

Q2. Are constants global in Ruby?

In the Ruby programming language, constants and global variables are two different kinds of variables. A constant and a global variable have a number of differences, the major one being the ability to easily change the value of a global variable without getting a warning.

Despite the differences, constants and global variables have shared commonalities. One of them is the visibility of a global variable in Ruby. Yes, constants are global on Ruby. You can use a constant anywhere on Ruby without bothering too much scope.

Q3. What are variables in Ruby?

Variables are used to store a value on Ruby and are instrumental in helping you recall these values when needed during the program. There are 5 types of variables in Ruby. They include local variables, instance variables, class variables, global variables, and constants. Like every other programming language, Ruby enables the use of variables to be able to use values all through the code. These variables are used as a means of storage for names, strings, characters, and many more as values in a code.


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Peace Akinwale

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I'm Peace Akinwale, a freelance B2B writer specializing in SaaS. With over 5 years of experience and a HubSpot Academy certification, I excel in crafting engaging SEO content that drives results for your business.

I have worked with diverse clients like, HackerNoon, FanDuel, and Bays Planet Foundation. The experience has improved my understanding of search intent, research, data-led and journalism-rich content. This ability allows me to write compelling pieces that provide practical value to readers and improve your company's visibility on SERPs.

Currently focused on martech, my writing experience spans various industries, including tech, software, finance, lifestyle, and more. I'm adaptable and can tackle any subject matter in SaaS, bringing a fresh perspective to your content strategy.

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