CSC/ECE 517 Fall 2012/ch1 1w58 am

Ruby Blocks, Iterators, Functional Idioms

This wiki-page serves as a knowledge source for understanding Ruby Blocks, Iterators, Functional Idioms.

Introduction

Ruby has simplified the way programmers use loops and iterators. Ruby helps programmers to use DRY principle effectively by using blocks and defining iterators over collections.<ref>Alan Skorkin. "A wealth of ruby loops and iterators" http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators</ref> This helps in minimizing the development work that programmers often find in any other O-O style programming language. In ruby, these iterators, blocks and functional idioms are explained as:

Iterators are Collection defined methods that helps to iterate over all the elements present in a Collection.<ref>"Ruby Iterators" http://www.tutorialspoint.com/ruby/ruby_iterators.htm</ref> Ruby Collections are basically objects that store a group of data members of various types. Examples of Collection are arrays, hashes etc. A block consists of chunks of codes with a name assigned to it.<ref>"Ruby blocks" http://www.tutorialspoint.com/ruby/ruby_blocks.htm</ref> For example,

  my_block { puts "Hello World" }

Functional idioms are ... (please feel free to add about functional idioms here and become the author of it :) )

Iterators

Iterators in ruby can be written in following ways:

The times iterator:

The times<ref>Alan Skorkin. "A wealth of ruby loops and iterators" http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators</ref> iterators works similar to the for loop used in programming languages. As the name suggests it allows to loop over a chunk of code n number of times. For example:

   5.times { puts "Hello Viewers!" }

This code produces the following output:

   Hello Viewers!
   Hello Viewers!
   Hello Viewers!
   Hello Viewers!
   Hello Viewers!

Any chunk of code enclosed within the curly braces will execute 5 times in the above example.

The upto iterator:

The upto<ref>Alan Skorkin. "A wealth of ruby loops and iterators" http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators</ref> iterator is also similar to the for loop. However, the difference between times and upto iterator is that, upto allows the programmer to specify the start index m and the end index n of the loop. So, if we define an upto iterator as m.upto(n) { # do some work }, then the code in curly braces will be executed n - m + 1 number of times. For example:

   m = 45
   n = 48
   m.utpo(n) {puts "Hello Viewers! This is really cool, isn't it?"}

This code produces the following output:

   Hello Viewers! This is really cool, isn't it?
   Hello Viewers! This is really cool, isn't it?
   Hello Viewers! This is really cool, isn't it?
   Hello Viewers! This is really cool, isn't it?

Here the loop will be executed 48 - 45 + 1 = 4 number of times. If we had used times than 45.times would have printed the puts statement 45 number of times. Hence it allows us to make loop execution dynamic as compared to the times iterator which is relatively static.

The step iterator:

The above iterators times and upto performs the loop execution in increments of 1. Step<ref>Alan Skorkin. "A wealth of ruby loops and iterators" http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators</ref> iterator allows us to vary the increments of loop execution. It gives the programmers the flexibility to skip a few iterations of the loop if it is required as per the programmers logic. For example, consider the code:

   guess_my_string = "HfezlvlbomWyowrqlad"
   0.step(guess_my_string.length, 2) {|i| print guess_my_string[i]}

This code produces the following output -

   HelloWorld

In the above code, the variable i collects the iteration number and the print statement prints the character of the String guess_my_string at position i. Hence we get the above output.

The each iterator:

All collections in Ruby have an each<ref>Alan Skorkin. "A wealth of ruby loops and iterators" http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators</ref><ref>"Ruby Iterators" http://www.tutorialspoint.com/ruby/ruby_iterators.htm</ref> method defined, that will allow programmers to go over all of the data members in collection for which the each method is called and do some operation with them. For example, consider the code:

   example = [82, 117, 98, 121, 32, 105, 115, 32, 103, 114, 101, 97, 116, 33]
   example.each {|i| print i.chr }

The above code prints the following output -

   Ruby is great!

In the above code the each iterator takes every element of the array example and prints its character equivalent.

The each iterator can also be used for hash traversal. Ruby defines the following methods for hash traversal:

1. each_key
   This method returns a collection of all the keys present in the hash on which the each_key method was called.
2. each_value
   This method returns a collection of all the values present in the hash on which the each_value method was called.
3. each_pair
   This method returns a collection of all the key, value pairs present in the hash on which the each_pair method was called.

Consider the example which explains the behavior of all the above methods defined for hash.

   breakfast_menu = {"Martini" => 4, "apple pie" => 3, "pan cakes" => 5 }
   
   puts "Today's break fast menu -"
   breakfast_menu.each_key { |i| puts i }
   
   puts "\nTheir respective prices -"
   breakfast_menu.each_value { |i| puts "$#{i}" }
   
   puts "\nThe breakfast menu with their respective prices -"
   breakfast_menu.each_pair { |i, j| puts "#{i}: $#{j}" }

The above code produces the following output:

   Today's break fast menu -
   Martini
   apple pie
   pan cakes
   
   Their respective prices -
   $4
   $3
   $5
   
   The breakfast menu with their respective prices -
   Martini: $4
   apple pie: $3
   pan cakes: $5

In the above code, breakfast_menu defines a hash. It contains key, value pair. When each_key method is called on breakfast_menu, it prints all the keys present in the hash breakfast_menu. Similarly for each_value. For each_pair it prints all the key value pairs present in the hash. Hence the output.

The collect iterator

This simply returns all the elements of the collection. The collect method works for arrays as well as hashes. For example:

   a = ["ruby", "rails", "ruby on rails"];
   b = Array.new
   b = a.collect{ |i| i.capitalize}
   puts b

This produces the following output:

   Ruby
   Rails
   Ruby on rails

In the above code, a.collect iterates over all the elements of a and returns a collection of all elements in a and assigns it to b

The map iterator

The map method creates a temporary array and stores in it whatever value is returned from each iteration of the block of code on which the map method is called. Then that temporary array is returned as a collection. For example consider the ruby code

   fruits = [ "apple", "banana", "cherry" ]
   puts fruits.map { |i| i.reverse }

The above code produces the following output:

   elppa
   ananab
   yrrehc

In the above code the map method iterates over all the elements in fruits and reverse them. These reverse strings are stored in a temporary array which map method uses and returns that array when all the elements in fruits are iterated.

In ruby, there are many methods that iterate over a range of values. These iterators are so written that they take a code block as part of their calling syntax and then yield control to that code block. This code block is executed as many times as specified by the iterator. In all of the above examples, the code that is written in curly braces is the block that the iterators yield their control to.

Blocks

A Block consists of a chunk of codes with a name assigned to it. Alternately, a block is a code which is passed to the iterator. These blocks can either be defined by enclosing them in curly braces or by using the do...end syntax as shown below

   2.times { puts "This is simple a linear block" }

   2.times do
       puts "This is a block using do..end syntax"
       puts "You can add multiple statements inside the do...end syntax"
   end

Also, In the curly braces syntax multiple lines of code can be added but they need to be separated with a semicolon.

Block can also take parameters. For example consider the code shown below:

   5.times { |i| puts i}

In the above code i is passed as a parameter to the block. Thus any thing between the pipe symbols can be considered as parameters. Multiple parameters can also be passed to a block but they need to be separated with commas. For example,

  example = {"one" => 1, "two" => 2, "three" => 3}
  example.each_pair { |key, value| puts "#{key} : #{value}"}

In the all the above block examples, an interesting thing to note is that these blocks are not executed until a yield statement is encountered. A block is always invoked from a function having the same name as that of the defined block. For, examples if there is a block with the name myblock, then there should be a function with the name myblock which invokes this block. For example, consider the code shown below:

   def block_testing
       puts "Inside function block_testing."
       yield
       puts "Back to block_testing function."
   end
   
   block_testing { puts "Yielding, to print within block." }

The code produces the following output:

   Inside function block_testing.
   Yielding, to print within block.
   Back to block_testing function.

In the above code, when the block_testing method executes, it prints the first statement, then yield is encountered and the control is passed to the block_testing block which prints the second statement. After the block finishes execution, the control is once again passed to the block_testing function which prints the third statement.

Defining your own iterator

Ruby allows programmers to define their own iterator other than the iterators listed above. For example if a programmer wishes to define an iterator which prints an array in reverse order, then he can do so as shown below:

   class Array
       def reverse_iterate
           if block_given?
               current_index = self.size-1
               while current_index >= 0
                   yield self[current_index]
                   current_index -= 1
               end
           else
               print self.reverse
           end
       end
   end

In order to use the above iterator, we just need to call this iterator on an array as shown below.

   puts "Fall break begins in 10 secs ..."
   puts [0,1,2,3,4,5,6,7,8,9, 10].reverse_iterate {|i| puts i }
   puts ".. yay!! "

The above code produces the following output:

   Fall break begins in 10 secs ...
   10
   9
   8
   7
   6
   5
   4
   3
   2
   1
   0
   
   .. yay!!

In the above code, the reverse_iterator iterates the array in reverse order and prints the array elements as shown in the output.

Topical References

<references/>