The Case Statement and Relationship Operator

Ruby supports a case statement, in which the value of something is matched against a set of options

case value
when 1, 2, 5
do_this
when 3
do_that
else
do_the_other
end

In this example the first when will catch three different values. Note that unlike the C family of languages, there is no break statement used. Options cannot fall though to the next one.

You do not need to give a parameter to the case statement, as seen here.

case
when @t == 7
p 't is 7'
when @s == :this
p 's is :this'
else
p 'none of the above'
end

In this form, the case is like an if/elsif chain.

if @t == 7
p 't is 7'
elsif @s == :this
p 's is :this'
else
p 'none of the above'
end

So why use case? Well, case returns a value, so instead we could do this:

s = case
when @t == 7
't is 7'
when @s == :this
's is :this'
else
'none of the above'
end

The Relationship Operator

The case statement uses the relationship operator, === (aka triple equals or trequals operator) when comparing the value to each when. The relationship operator is really a method, and in Object the relationship operator is defined to do the same as the equals operator. However, the important point here is that it can be overridden as required. Patterns override it to check for a match, and the Range class overrides it to check if the value is within the range. That allows you to do things like this:

mark = 56

grade = case mark
when 1..25
'Fail'
when 26..50
'C'
when 51..75
'B'
when 76..100
'A'
else
'Out of range!'
end

Here grade is set to 'B' because (51..75) === 56 evaulates to true. Note that this is calling the === method on 51..75. Write it the other way around, 56 === (51..75), and the === method of 56 is invoked, and the expression evaluates to false.

See more here:
http://www.pmamediagroup.com/2009/07/dont-call-it-case-equality/


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