Conditional Statements
The simplest conditional statement is the if statement.
if 5 + 7 == 12
puts "This code will be executed!"
end
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If the test expression following the if keyword is true, then the Ruby
interpreter will execute the subordinate block of code. An if statement always
begins with the if keyword and ends with the end keyword.
Like English, Ruby features an if-else statement:
if 2.odd?
puts "Two is odd. Who knew?"
else
puts "Two is even. All is well!"
end
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The Ruby interpreter executes the block subordinate to else (puts "Two is
even. All is well!") if 2.odd? is false. If the test expression following
if were true, then the block subordinate to else would not be executed.
Unlike English, Ruby also has an if-elsif-else construction. Like if, elsif
precedes a test expression. One can stack an arbitrary number of elsif
statements, but there can be only one if and one else, though else is
optional. if introduces the control structure, and else acts as a kind of
fail-safe. The block subordinate to else is executed only if none of the prior
if or elsif conditions are true. If no conditions are true and the
conditional statement lacks an else block, then the entire statement evaluates
to nil.
if 2 == "dog"
puts "Not executed."
elsif 2 == "cat" && 0 < 2
puts "Not executed."
elsif "cat" == "dog" || 2 == "cat"
puts "Not executed."
else
puts "2, cat, and dog are not equal!"
end
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Only one subordinate block is ever executed in an if, if-else, or if-elsif-else
statement. If one of the test expressions is true, then the result of its
subordinate block is the result of the entire conditional statement. The
interpreter skips over all subsequent code in the conditional statement and
resumes execution after the end.
Ruby permits nested conditional statements, though overly nested code can be difficult to read. Note that subordinate blocks may be any number of lines. As in method definition, the result of the last line in a block is the result of the entire block.
if 2.odd?
"2 is apparently odd"
elsif 2.even?
puts "2 is even: we're in the outer elsif branch"
if 2 > 100
"2 is a large even number!"
elsif 2 > 30
"2 is a medium even number"
else
puts "2 is a small even number"
puts "Then it must be true that the result of 2 + 30 is less than 60 and is even!"
puts "I'll prove it!"
"2 + 30 < 60 && (2 + 30).even? is " + (2 + 30 < 60 && (2 + 30).even?).to_s
# The conditional statement evaluates to "2 + 30 < 60 && (2 + 30).even? is true"
end
else
"2 is apparently neither even nor odd."
end