Case-statements

Objectives:

1. Distinguish a case statement from other patterns of flow control.

2. Identify when to use a case statement.

3. Write a case statement

What is a case Statement?

A case statement is a powerful tool to test for certain conditions. They are used to run multiple conditions against one value. There are three basic steps to creating a case statement. First, we'll need a value. Second, we'll want one or more conditions to compare to the value. Third, we'll add the code we want to run if that condition is met. Let's walk through these steps in more detail below.

Why Use a case Statement?

In the previous lessons, we've learned about using if, elsif, and else statements to enact flow control in our programs. Let's look at an example of using if statements that would benefit from being refactored to use a case statement instead.

Let's say we have a program that sets a name variable equal to a person's name. Our program needs to execute certain code depending on what that person's name is.

name = "Alice"

if name == "Alice"
  puts "Hello, Alice!"
elsif name == "The White Rabbit"
  puts "Don't be late, White Rabbit"
elsif name == "The Mad Hatter"
  puts "Welcome to the tea party, Mad Hatter"
elsif name == "The Queen of Hearts"
  puts "Please don't chop off my head!"
else
  puts "Whoooo are you?"
end 

Above we are using many if and elsif statements to check if the value of our name variable matches a particular string by using the comparative operator (==) in each one.

Top-Tip: Remember that the assignment operator (=) is distinct from the comparative operator (==).

Using if and elsif statements in this manner creates "code smell"—a piece of code that is needlessly complex or difficult to read. Not only are we using a lot of if statements, but we are being repetitive in our use of the comparative operator (==). We can eliminate this "code odor" by refactoring our flow control to use a case statement instead. The case statement will allow us to run multiple conditions against the same value, meaning that we can check the name variable against a variety of conditions without repeating our use of the comparative operator (==) in each one.

Let's take a look:

case name 

  when "Alice"
    puts "Hello, Alice!"
  when "The White Rabbit"
    puts "Don't be late, White Rabbit"
  when "The Mad Hatter"
    puts "Welcome to the tea party, Mad Hatter"
  when "The Queen of Hearts"
    puts "Please don't chop off my head!"
  else 
    puts "Whoooo are you?"
end

Writing a case Statement

Now that we understand when to use a case statement in place of a series of if and elsif statements, let's look at how to build a case statement from scratch.

Step 1: Create a Value

A case statement starts with the case keyword followed by a value to test.

Step 2: Create the Conditions

Next, the when keyword is followed by a condition.

case greeting
  when "unfriendly_greeting"
    #...
  when "friendly_greeting"
    #...
end

Step 3: Add the Code

The functionality that we wish to happen when the condition is met is placed on an indented line directly under the when line. Let's define the behavior:

greeting = "friendly_greeting"

case greeting
  when "unfriendly_greeting"
    puts "What do you want!?"
  when "friendly_greeting"
    puts "Hi! How are you?"
end

Advanced: How does it work?

Under the hood, case statements actually evaluate their when conditionals by implicitly using the "case equality operator"; the case equality operator is otherwise represented by === ("threequals") sign. While case can be used to replace the comparison operator in a situation like the first example in this reading, it's doing something slightly different. Read more about === here.

Similar to the comparison operations above, the when statement evaluates to a boolean value by using the case value at the start of the case statement and the value following the when keyword. If this when condition evaluates to false, then the indented code beneath that condition is skipped; if it evaluates to true, then the indented code beneath it is executed.

In the above case, Ruby compares the case value to the two when conditions; "friendly_greeting" === "unfriendly_greeting" is false, so puts "What do you want!?" is not run; however, "friendly_greeting" === "friendly_greeting" is true, so puts "Hi! How are you?" is run.

It is not necessary at this point to understand the distinction between the comparative operator (==) and the case comparison operator (===). Just realize that there is a distinction, even though the usages relevant to you right now will be similar.

Example 1: Weather

In this example, we set the current_weather to "raining". Next, we use when statements to describe a list of possible matches. Since current_weather === "raining" we'd expect this code to put "grab an umbrella".

current_weather = "raining"

case current_weather
  when "sunny"
    puts "grab some sunscreen!"
  when "raining"
    puts "grab an umbrella"
  when "snowing"
    puts "bundle up"
end

Example 2: Grades

This example requires a basic understanding of gets.chomp. It allows us to get a user's input, and use it in our code. Read more on what it does here.

Here, we are prompting the user to input a student's grade. Based on that grade, the program then prints out the string associated with the matching condition. If the user enters "A", then grade = "A". Since grade === "A", Ruby will print Good job, Homestar! to the screen.

print "Enter your grade: "
grade = gets.chomp

case grade
  when "A"
    puts "Good job, Homestar!"
  when "B"
    puts "You can totally do better!"
  when "C"
    puts "Find a mentor to help you!"
  else
    puts "You're just making that up!"
end

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