Ruby Control Flow

1. Ruby Control Flow and other languages…

2. Standard Control Flow • Selection Statements • Iterative Statements • Unconditional Branching • Not covered • Guarded Commands

3. Conditionals if expr code end # newline after expr # false or nil, don’t execute, else do # no () around expr # end required even if only one statement in code if expr then code end if expr code else code end if expr1 code elsif expr2 code2 … else code end

4. Conditional return value • Return value is last expression executed OR nil x=5 # topic: return values name = if x==1 then "Cyndi" else "Nancy" end puts name

5. Expression Modifier Perl also has this syntax if expr then code end equivalent to: code if expr Best practice: use when condition is trivial or normally true

6. Other conditionals unless expr code end code unless expr tax = case income when 0..7550 income * 0.1 when 7550..30650 income * 0.15 when 3065..50000 income * 0.25 else income * 0.9 end also other forms of case, not covered Compare to switch. Consider language readabilty.

7. Iteration while expr do code end code while expr until expr do code end code until expr Pascal had repeat… until.

8. More iteration for var in collection do code end # do is optional, can use newline hash.each do |key, value| puts “#{key} => #{value}” end

9. Iterators • Integer.times, Enumerable.each, map and Integer.upto • make use of yield (next slide) • Examples 2.times { puts “OK” } array.each {|x| puts x } [5,10,15].map { |x| x*x*x } factorial = 1 2.upto(20) { |x| factorial *= x}

10. yield yield temporarily returns control from iterator to calling method QUICK EXERCISE: Trace the code on the next two slides. Turn in for class participation.

11. yield You are in the method You are in the block You are again back to the method You are in the block from: http://www.tutorialspoint.com/ruby/ruby_blocks.htm • yield temporarily returns control from iterator to calling method def test puts "You are in the method" yield puts "You are again back to the method" yield end test {puts "You are in the block"} • method must be invoked with a block (otherwise, what would you yield to?)

12. another example, same site You are in the block 5 You are in the method test You are in the block 100 Java = pull (caller controls iteration), Ruby = push (iterator controls iteration) def test yield 5 puts "You are in the method test" yield 100 end test {|i| puts "You are in the block #{i}"}

13. Unconditional Branching Transfers execution control to a specified place in the program Represented one of the most heated debates in 1960’s and 1970’s Well-known mechanism: goto statement Major concern: Readability Some languages do not support goto statement (e.g., Module-2, Java, Python, Ruby) C# offers goto statement (can be used in switch statements) Loop exit statements are restricted and somewhat camouflaged goto’s

14. Topic Summary • Language Concepts • selection/conditionals • return value • iteration • unconditional branching • Ruby • if/elsif • conditional return value • expression modifier • unless • while/until • for • each • yield