learn-co-students/programming-univbasics-nds-nested-arrays-iteration-lab-part-1-nyc01-seng-ft-100520

Nested Data Structures: Printing Data Lab

Learning Goals

• Display the cells in an Array of Arrays

Introduction

In this lab, we're going to practice what we reviewed in the previous lesson - displaying content from an Array of Arrays.

We're going to give you a bit of "reference code." Reference code is code that's a little generic. It probably doesn't solve any real problem, but which is kept intentionally very simple so that you can see how it might be adapted to your particular need. Some people call these "reference implementations."

Display the Cells in an Array of Arrays

It's not often we say this, but this next bit of code is worth memorizing. If you want to learn to speak any human language, it's a good idea to learn the grammar, grow your vocabulary and talk with native speakers as much as possible.

But

That takes years, and, in a pinch, you should really know how to ask for food, water, emergency services, and a bathroom. Even if you don't understand what's happening grammatically, having a few "stock phrases" in times of need is helpful. The following code is a good "stock phrase."

Assuming the following AoA:

spice_rack = [
  ["Posh", "Scary", "Sporty"],
  ["Paprika", "Fajita Mix", "Coriander"],
  ["Parsley", "Sage", "Rosemary"]
]

We display it like:

spice_rack = [
  ["Posh", "Scary", "Sporty"],
  ["Paprika", "Fajita Mix", "Coriander"],
  ["Parsley", "Sage", "Rosemary"]
]

row_index = 0
while row_index < spice_rack.count do
  element_index = 0
  while element_index < spice_rack[row_index].count do
    p spice_rack[row_index][element_index]
    element_index += 1
  end
  row_index += 1
end #=> nil

This prints out:

Posh
Scary
Sporty
Paprika
Fajita Mix
Coriander
Parsley
Sage
Rosemary

This "formula" can be expanded and changed. Maybe you want something more descriptive than row_index or element_index index. Or, perhaps, you need to adjust behavior slightly. That's fine! But if you're not sure how to start, this is always a good start and will get you un-stuck!

Instructions

For this lab, your task is to write the implementation for one method, find_even_values. Write your code in lib/iteration_with_loops.rb using the guidance in the comments there and below.

find_even_values

This method is already defined but does not contain any code. It should accept an array of arrays as a parameter. Your task is to iterate over the AoA to access the elements of each nested array and print out every even number present. So, given the following:

array_of_arrays = [
  [1,2,3],
  [4,5,6],
  [7,8,9]
]

find_even_values(array_of_arrays)

The expected output would be:

2
4
6
8

Use learn to check your solution and learn submit to submit it. Try to solve this on your own first, but if you get stuck, a walkthrough is provided below.

...

...

Solving this Lab

Our solution is going to be similar to the first example provided in this lesson, but before we get there, let's look at solving this for a single array of arrays using specific, concrete code. If we had the following array of arrays:

array_of_arrays = [
  [1,2,3],
  [4,5,6],
  [7,8,9]
]

And we only wanted to print out even values, a specific solution could be:

p array_of_arrays[0][1]
 # => 2
p array_of_arrays[1][0]
 # => 4
p array_of_arrays[1][2]
 # => 6
p array_of_arrays[2][1]
 # => 8

We can see which values are even, and go straight to printing only those. What if we had this exact structure (an array of three arrays, each with three element), but we couldn't see the values contained?

We would have to check every element using a conditional statement. Integers in Ruby have a built-in methods to tell if something is even or odd - .even? and .odd?. We can use this to determine whether or not a value should be printed:

if array_of_arrays[0][0].even?
 p array_of_arrays[0][0]
end
if array_of_arrays[0][1].even?
 p array_of_arrays[0][1]
end
if array_of_arrays[0][2].even?
 p array_of_arrays[0][2]
end
if array_of_arrays[1][0].even?
 p array_of_arrays[1][0]
end
if array_of_arrays[1][1].even?
 p array_of_arrays[1][1]
end
# etc...

Now, regardless of the elements contained, we would be able to print out only the even values.

This code still relies on the assumption that the array of arrays is structured precisely how we expect. When writing a looping solution, the implementation for deciding whether or not to print a value is going to be like we see above. Loops, however, abstract away the specific indices being used.

To implement a looping solution, first, we'll start with a generic double loop:

def find_even_values(src)
  row_index = 0
  while row_index < src.count do
    element_index = 0
    while element_index < src[row_index].count do
      # inside this loop, src[row_index][element_index] will access the current element
      element_index += 1
    end
    row_index += 1
  end
end

We know thata the src parameter is an array of arrays, so we can use src.count to get the length of the outer array and compare it to row_index when setting up the first while loop. Inside this loop, we create a second variable, element_index. Every time the outer loop runs, element_index is set to 0. When setting up the inner while loop, we use src[row_index].count. This gives us the length of the current nested array we're working with.

Inside this second while loop, we can access nested values with src[row_index][element_index]. To output only even values, here is where we will put a conditional statement:

def find_even_values(src)
  row_index = 0
  while row_index < src.count do
    element_index = 0
    while element_index < src[row_index].count do
      if src[row_index][element_index].even?
        p src[row_index][element_index]
      end
      element_index += 1
    end
    row_index += 1
  end
end

Conclusion

Using loops with an array of arrays, we can access every value regardless of how many nested arrays are present and how long each nested array is. We will see all sorts of data structures, arrays of arrays of arrays, hashes of arrays, arrays of hashes of arrays, etc... to work with each, we will need a slightly customized loop implementation. An array of array of arrays, for instance, would require three while loops. Once we've worked out the particular implementation, though, we gain a powerful tool in working with data!