Generalized Map and Reduce Lab

Learning Goals

Identify duplication that can be avoided with blocks
State two ways of constructing blocks
Execute a block from within a method
State the purpose of the yield keyword
Pass data between methods and blocks

Introduction

As you saw in the previous lesson, there's a lot of repeated code between our various map-based and reduce-based methods. This should trigger an "allergic" reaction in you. You learned to "DRY out" your code. In this lesson, we'll learn to DRY out our home-grown enumerable methods using a powerful language feature of Ruby: the block.

Identify Duplication That Can Be Avoided With Blocks

Let's look at this solution code from the previous lesson:

def map_to_negativize(source_array)
  new = []
  i = 0
  while i < source_array.length do
    new.push( source_array[i] * -1 ) # <== Unique work
    i += 1
  end
  return new
end

def map_to_no_change(source_array)
  new = []
  i = 0
  while i < source_array.length do
    new.push( source_array[i] ) # <== Unique work
    i += 1
  end
  return new
end

def map_to_double(source_array)
  new = []
  i = 0
  while i < source_array.length do
    new.push( source_array[i] * 2 ) # <== Unique work
    i += 1
  end
  return new
end

def map_to_square(source_array)
  new = []
  i = 0
  while i < source_array.length do
    new.push( source_array[i] * source_array[i] ) # <== Unique work
    i += 1
  end
  return new
end

As you can see, the only thing that varies between these methods is the line which we've labeled with Unique work. We want to get rid of that duplicated "noise" around those lines.

When you learned to create methods, you learned that the way to abstract the method is to pass in the stuff that varies as an argument in the call of the method. We'd like to do the same thing here. But in Ruby, we know we can pass Strings, Arrays, Hashes, and Floats. With blocks, Ruby lets us pass in "work to do later," although it looks a little bit different than passing an argument to a method.

State Two Ways of Constructing Blocks

We write blocks in Ruby using two syntaxes:

Curly brace {}
do...end

We generally use curly brace when the work inside is only one expression. Since expressions in Ruby always return a value, the return value of the block will be the value of that single expression.

We use do...end when the work takes multiple lines. Just like methods, whatever is on the last line of a do...end block will be the return value of the block.

Code Writing Tip

It's wise to start out with a do...end blocks. If you need to add debugging data like p "this should be #{i}", need to assign some local variables, or maybe use a less-fancy bit of code to do the work, you're already in the "multi-line-friendly" style.

Once you're sure you've gotten your implementation correct, you can use the shorter, and more elegant {} format.

If you refuse this advice, if you need to include multiple expressions inside of a {}, you should separate the expressions with ;.

On the other hand, it's hard to see how an expression like -1 * x could go wrong, so maybe in some cases you can start with {}. Using do...end is our recommendation while you're starting out.

Pass a Block to a Method

We pass a block to a method by including either a {} or a do...end block after our call to the method.

method_using_block { puts "hi" }

# Is the same as...
method_using_block do
  puts "hi"
end

Execute a Code Passed as a Block Inside a Method

When we pass arguments into a method, we have parameters that allow us to refer to them:

def make_sandwich(element1, element2)
end

make_sandwich("Peanut Butter", "Yakisoba")

Inside of the make_sandwich method, we could get the Strings passed into make_sandwich by working with the local variables (or "parameters") element1 or element2.

When blocks are passed in, they are not stored in a parameter name. They are, instead, implicitly passed. We "run" the code in the block by using the Ruby keyword yield.

ADVANCED: You actually can capture blocks. We hope you'll stick with Ruby long enough to learn this super-cool concept.

State the Purpose of the `yield` Keyword

The yield keyword executes the block passed into the method. When it is executed, it can be passed arguments. You could pass it local variables that were defined within the method that is, in turn, executing the block.

Let's add a block to our call to make_sandwich and add a call to yield inside of make_sandwich.

def make_sandwich(element1, element2)
  base = "A #{element1} and #{element2}"
  puts base
  yield
  base
end

make_sandwich("chicken", "a sense of malaise") do |innards|
  puts "making some tasty stuff..."
end #=> "A chicken and a sense of malaise"

And outputs:

A chicken and a sense of malaise
making some tasty stuff...

Make sure you understand what's going on here or work with this simple code in IRB.

Pass Data Between Methods and Blocks

In the same way that you provide parameters in methods in order to "catch" things passed in, we define _block-parameters_ by placing their name(s) between a pair of "pipe" characters (|), separated by commas (,). Then, within the block, we can use the passed-in data in whatever way we deem fit.

def make_sandwich(element1, element2)
  base = "A #{element1} and #{element2}"
  yield(base)
end

make_sandwich("gator", "gumbo") do |innards|
  "#{innards} on rye"
end #=> "A gator and gumbo on rye"

Since this is a simple transformation, we'll convert to a {}-based block. We'll also rewrite the internals of make_sandwich to be a bit cleaner. Make sure you can step through this code and understand what's going on with return values, implicit return. This is would be typical of a professional codebase.

We'll also add the word "sandwich" to our default implementation.

def make_sandwich(element1, element2)
  yield("A #{element1} and #{element2} sandwich")
end

make_sandwich("grits", "abject terror") { |innards| "#{innards} on rye" }
#=> "A grits and abject terror sandwich on rye"

But now we are prepared to see the power of blocks.

Demonstrate How Blocks Allow for Flexible Method Calls

Because we've "exposed" how to customize the sandwich in the block, it's possible for us to satisfy many types of calls to this method flexibly. We can provide the final bit of "work" (look back to the first lesson, a little bit of "work" is common to all Enumerables!).

def make_sandwich(element1, element2)
  yield("A #{element1} and #{element2} sandwich")
end

# Wheat, sure!
make_sandwich("Creamy peanut butter", "glittering sense of accomplishment") { |b| "#{b} on wheat" }

# Bueno!
make_sandwich("Creamy peanut butter", "glittering sense of accomplishment") { |b| "#{b} on tortilla" }

# Lewis Carroll's Kitchen
make_sandwich("Creamy peanut butter", "glittering sense of accomplishment") { |b| "#{b.reverse} on #{"bread".reverse}" }

# Try some more yourself!

This produces:

"A Creamy peanut butter and glittering sense of accomplishment sandwich on wheat"
"A Creamy peanut butter and glittering sense of accomplishment sandwich on tortilla"
"hciwdnas tnemhsilpmocca fo esnes gnirettilg dna rettub tunaep ymaerC A on daerb"

KEEP GOING: Did you notice that some of the template sentence doesn't quite work? Consider updating our code with conditionals. Add more logic in the method or in the block. Play with how you can move logic from the block to the method and back. Make this piece your own!

By now it should be clear that a block allows us to separate small bits of varying work from methods that, for the most part, remain the same. This should be the key to making a generalized, abstract map and reduce.

Lab

In this lab, you should write a generalized map and reduce method. Both of these methods will take a block and require that you pass information between the method and the block. Write your code in the lib/my_code.rb file.

Note: This is a challenging lab. Do your best to solve using the information below. If you do find yourself stuck, a walkthrough video is provided at the end of this Readme.

`map`

Your implementation should expect a source array and a block. All the tests will pass an Array and a block.

Remember, map returns a new Array.

`reduce`

Your implementation should expect a source array and optionally (recall optional parameters in methods!) a starting value. All the tests will pass an Array and sometimes, a starting point.

Remember, reduce returns a value.

You might be interested to compare the code in the previous lesson to the code in this lesson: they're the exact same expectations but since we know how to use blocks we need only call map versus map_to_negativize; map versus map_to_no_change, etc...

map_to_square([1, 2, 3, -9]) #=> [1, 4, 9, 81]

became generalized as:

map([1, 2, 3, -9]){|n| n * n} #=> [1, 4, 9, 81]

Video Walkthrough

Conclusion

Congratulations you've linked your conceptual grasp of Enumeration with the understanding of the code needed in Enumerables. Now would be a great time to go back to Ruby's Enumerables documentation and try to follow along with the code examples to see how you can use Enumerable methods to make your coding easier.

mattkuras/ruby-enumerables-generalized-map-and-reduce-lab-online-web-prework