Control Flow Techniques in Ruby

17 Feb 2014

Ruby has a variety of control flow techniques that let you control the execution of the program. Many of them (e.g. if, while, etc..) are similar to their counterparts in other programming languages. This article focuses on syntax and idiosyncrasies of Ruby control flow techniques.

Ruby provides if..else, unless..else and case constructs for conditional execution. if..else is same as other programming languages such as Python, Java, C. unless is opposite of if, its body gets executed if condition is false. If you are new to Ruby, you might find unless awkward to use, but as you code more and more in Ruby, you will intuitively start using unless. You can use if !(condition) or if not condition to achieve the same result as unless.

>>   a = true
>>   b = false
?>   if a
>>     puts "Inside IF a"
>>   elsif b
>>     puts "Inside ELSE IF b"
>>   else
?>     puts "Inside ELSE"
>>   end
Inside IF a
?>   unless b
>>     puts "Inside UNLESS"
>>   else
?>     puts "Inside ELSE"
>>   end
>> # This works too
>> puts "IF a is true" if a
IF a is true
>> puts "IF b is false" unless b
IF b is false

It is possible to assign value to a variable inside conditional test. In such cases, the return value of the assignment is considered while evaluating condition. Also, if a local variable is initialized in the body of a conditional it comes into existence even if conditional body containing the variable is not executed.

>> a = 1
>> if (b = a)  # b gets a's value, which is interpreted as true.
>>   puts "True that!"
>> else
?>   t1 = 2
>> end
True that!
?> p b
>> p t1 # t1 is nil, NOT undefined
>> p t2
NameError: undefined local variable or method `t2' for main:Object

In the above example, variable t1 does not throw NameError. This is because when Ruby parses the code, it sees t1 = 2 and allocates memory for t1. So t1 exists as result of parsing process but it is not assigned any value.

case..when is similar to C or Java’s switch case which one caveat. when does not use == to test equality, rather it uses === operator, which is syntactic sugar for method call .===. For most of the common Ruby classes such as String, === is same as ==.

case test_var
when 1
  puts "test_var is 1"
when 2
  puts "test_var is 2"
  puts "test_var is something else"

Ruby provides loop(for unconditional looping), while and untill (for conditional looping) and for (for looping through a list of value). Ruby has break and next keywords which allow the program to exit looping block or skip remaining part of the iteration respectively.

## Ruby Looping constructs for printing 1 to 10
a = 1
loop do
  puts a
  a += 1
  break if a > 10

a = 1
while a <= 10
  puts a
  a += 1

# until is same as while but with reverse logic
a = 1
until a > 10
  puts a
  a += 1

# Similar to do..while in C, Java.
a = 1
  puts a
  a += 1
end while a <= 10

# for is similar to its counterpart in Python
lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for a in lst
  puts a

An exception in Ruby is a special kind of object of class Exception or its descendants. Ruby provides rescue clause to handle exceptions and ensure clause to make sure that certain code gets executed whether an exception is thrown or not. Ruby’s begin..rescue..ensure is similar to Java’s try..catch..finally block. You can raise exception programatically using raise keyword. While rescuing from an exception, you can catch the thrown exception object using special operator =>. Here is a contrived example that tries to explain the constructs.

>> begin
?>   raise RuntimeError, "Problem occurred!"
>> rescue RuntimeError => e
>>   puts "Handled:"
>>   p e
>>   puts '-----------'
>>   # re-raising Exception to pass it up the call stack
?>   raise
>> ensure
?>   puts "I will get executed for sure"
>> end
#<RuntimeError: Problem occurred!>
I will get executed for sure
RuntimeError: Problem occurred!
        from (irb):2

In the above example, raise RuntimeError is syntactic sugar for raise You raise exception object, not class.

If Ruby’s built-in exceptions do not suit your needs, you can define you own exception class by subclassing Exception or one of its descendants.

That’s it for now. In the next article we will take a look at more powerful control flow constructs, code block.