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Ruby Programming
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If and Unless Expressions

An if expression in Ruby is pretty similar to ``if'' statements in other languages.

if aSong.artist == "Gillespie" then
  handle = "Dizzy"
elsif aSong.artist == "Parker" then
  handle = "Bird"
else
  handle = "unknown"
end

If you lay out your if statements on multiple lines, you can leave off the then keyword.

if aSong.artist == "Gillespie"
  handle = "Dizzy"
elsif aSong.artist == "Parker"
  handle = "Bird"
else
  handle = "unknown"
end

However, if you lay your code out more tightly, the then keyword is necessary to separate the boolean expression from the following statements.

if aSong.artist == "Gillespie" then  handle = "Dizzy"
elsif aSong.artist == "Parker" then  handle = "Bird"
else  handle = "unknown"
end

You can have zero or more elsif clauses and an optional else clause.

As we've said before, if is an expression, not a statement---it returns a value. You don't have to use the value of an if expression, but it can come in handy.

handle = if aSong.artist == "Gillespie" then
           "Dizzy"
         elsif aSong.artist == "Parker" then
           "Bird"
         else
           "unknown"
         end

Ruby also has a negated form of the if statement:

unless aSong.duration > 180 then
  cost = .25
else
  cost = .35
end

Finally, for the C fans out there, Ruby also supports the C-style conditional expression:

cost = aSong.duration > 180 ? .35 : .25

The conditional expression returns the value of either the expression before or the expression after the colon, depending on whether the boolean expression before the question mark evaluates to true or false. In this case, if the song duration is greater than 3 minutes, the expression returns .35. For shorter songs, it returns .25. Whatever the result, it is then assigned to cost.
Ruby Programming
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