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Ruby Case Examples: Ranges, Strings and Regexp

Use the case statement. Match values, and ranges of values, with this statement.
Case. A case-statement tests values. It matches the value of a variable against ranges and other values. It yields a specified value. It influences control flow.
Ranges, expressions. Case is an alternative syntax form to the if-statement. We use ranges, strings and numbers with cases. We can assign or return the case's result.
Case example. We use a case in a method. The method test() returns the value from a case-statement evaluation. It returns the strings Low, Medium or High based on the parameter.

Ranges: We use numeric ranges in the when-statements. To specify a range, separate two numbers with two periods.

Ruby program that uses case-statement # Test method. def test(quality) # Return a case. return case quality when 0..2 then "Low" when 3..5 then "Medium" when 6..8 then "High" else "Invalid" end end # Call test method. print test(0), "\n" print test(4), "\n" print test(8), "\n" print test(-1), "\n" Output Low Medium High Invalid
Else statement. Case optionally has an else-statement. If it appears, this must come at the end. If no other ranges or values match, the else-statement is reached.

Tip: In a C-like language, the else is equivalent to a "default" case, which matches unmatched values.

Numbers. A case requires no ranges. In this example, we use simple number "when" statements. For example, we return 400 when the value equals 4.

Note: The case statement evaluates and its result is stored in the "result" variable's memory location.

Expression: We use the case as an expression in this program, one that is evaluated to return a value.

Tip: In Ruby, expressions and statements are often interchangeable. This makes constructs like "case" useful in many contexts.

Ruby program that uses case, no ranges value = 5 # Use case with numbers, no ranges. result = case value when 4 then 400 when 5 then 500 when 6 then 600 else 0 end # Display input and output. puts value puts result Output 5 500
String. A string can be used in a case. In this example, we use a string in a case expression, and then assign an integer to the result. The case evaluates to the value 2.

Also: This case expression uses an else-statement. If the value of "name" is not matched, the value 3 is returned by this expression.

Ruby program that uses string case # The value. name = "sam" # Assign to result of string case expression. id = case name when "edna" then 0 when "fred" then 1 when "sam" then 2 else 3 end # Display result. puts id Output 2
When. This can have multiple values in it. Here we see a when statement that matches if the variable equals 100 or 200. This syntax form is a good choice when a range is unnecessary.
Ruby program that uses when, multiple values apartment = 200 case apartment when 100, 200 # Has two possible matching values. puts "100 or 200" when 0 puts "Zero" end Output 100 or 200
Regexp. A case can use Regexp in the when statements. Here we use a regular expression to test first letters. Any regular expression can be used. And we can even capture groups.

Here: The case block detects that the word "Alexandria" starts with the letter "A." The Regexp is described.

Ruby program that uses Regexp, case value = "alexandria" # Use case with regular expressions. case value when /^a/ puts "Starts with letter A" when /^b/ puts "Starts with B" end Output Starts with letter A Pattern description ^a String starts with a lowercase letter A. ^b String starts with a "b".
Regexp, captures. This example combines many constructs in Ruby. We use an iterator (each) over a string array. We then use a case-statement with regular expressions.

Tip: After a Regexp matches a value in a case, its groups are stored in a special variable $1. We get the captured value there.

Tip 2: We can also directly display the value of $1 in a puts call with the code #$1. This helps us develop a simple text parser.

Ruby program that uses case, Regexp, captures # An array of strings. values = ["value 100", "string box"] # Loop over strings. values.each do |value| # Use regular expression in case-statement. # ... Use capturing. case value when /value (\d+)/ # Access the capture from $1. argument = $1 # Use format string to display argument. puts "Value argument = %s" % argument when /string (.+)/ # Directly display captured value. puts "String argument = #$1" end end Output Value argument = 100 String argument = box
Performance. In this test, case has no performance advantage. It is many times slower than an equivalent if-statement. This depends on the data being tested.

However: To boost performance, it is sometimes helpful to replace cases with if-statements.

Ruby program that times case-statement count = 100000 n1 = Time.now.usec x = 0 v = 5 # Assign a variable with a case-statement. count.times do x = case v when 0..1 then 1 when 2..3 then 2 when 4..6 then 3 else 0 end end puts x n2 = Time.now.usec # Assign a variable with an if-statement. count.times do if v >= 0 && v <= 1 x = 1 elsif v >= 2 && v <= 3 x = 2 elsif v >= 4 && v <= 6 x = 3 else x = 0 end end puts x n3 = Time.now.usec # Times. puts ((n2 - n1) / 1000) puts ((n3 - n2) / 1000) Output 3 3 92 ms, case-statement 15 ms, if-elsif-else statements
A summary. Case is a selection statement. It is similar, conceptually, to switch statements and to select-case statements in other languages.
Case has advantages. Its syntax is clear and easy to read. It also has limitations—it may evaluate slower. We used a case-statement in a method.
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