##Objectives:
- Distinguish a
case
statement from other patterns of flow control. - Identify when to use a
case
statement. - Write 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.
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"โa 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
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.
A case statement starts with the case
keyword followed by a value to test.
case greeting
# ...
end
Next, the when
keyword is followed by a condition.
case greeting
when "unfriendly_greeting"
#...
when "friendly_greeting"
#...
end
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
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.
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
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
Please open a GitHub issue or pull-request. Provide a detailed description that explains the issue you have found or the change you are proposing. Then "@" mention your instructor on the issue or pull-request, and send them a link via Connect.
PHRG `case` Statements