Using Loops and Arrays to Find Data

Learning Goals

• Work with arrays and loops to extract specific values
• Use loops to compare values in an array

Introduction

We've stepped away from the idea of programming as conversation in these last few lessons. But now that you understand what an Array is and how it mirrors real life. You might realize that you've been working with Arrays all the time!

• Do any of the children have a runny nose?
• Do all of the children have a runny nose?
• "Is there a doctor in the house?"
• Do all of the dogs have their rabies vaccination?
• Who is the fastest runner?
• What is the lowest temperature Maui will see during my vacation?

Given an Array with the appropriate data, you now can loop through it and return the critical bit of information.

We're going to practice working with arrays and loops in a bit more depth. We will be using them to implement a typical pattern: looping through an array to find the position of a value.

Instructions

For this lab's tests, assume all arrays are sets of positive integers. The method and parameter names are already provided for you in lib/find_in_array.rb. Your task is to write the implementation.

Finding Where an Element is Located on an Array

This method, find_element_index(array, value_to_find), takes in two parameters, an array of integers and a value to find. This method should return the index of the value that was passed in. If the value is not found, this method should return nil.

To implement this method, you will need loop over the provided array and compare each value in the array to the value provided. Remember though, that this is not the value that needs to be returned.

A walkthrough for this lab is provided below, but first, try your best to solve this on your own.

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Solving this Lab

In previous labs, we used loops to use or modify elements in an array. This time, we need to loop through an array and compare each element to a provided value. We'll first need to set up the basic loop:

def find_element_index(array, value_to_find)
  count = 0
  while count < array.length do
    # comparison code will go here
    count += 1
  end
end

The above code will let us loop through the provided array. We can access each element with array[count] starting with array[0]. We now need to write code that will compare each array element to value_to_find. For now, we can simply puts the result of this comparison:

def find_element_index(array, value_to_find)
  count = 0
  while count < array.length do
    puts array[count] == value_to_find
    count += 1
  end
end

Since array[count] == value_to_find returns true or false, if we were to run this in IRB, we'd get a list:

2.6.1 :010 > array = [6, 5, 4, 3, 2, 1]
 => [6, 5, 4, 3, 2, 1]
2.6.1 :011 > find_element_index(array, 2)
false
false
false
false
true
false
 => nil

Above, the number 2 is the value to find. We can see in the array that it is in the fifth position and find_element_index method confirms by the outputting true during the fifth loop.

We haven't solved this lab yet, though. The task is to return the index of an element in the array that matches the value to find. In the example above, 2 is in the fifth position in the array, but we start array indexes at 0, so its index is equal to 4. How might we capture this value and return it?

Fortunately for us, we are already capturing the a value equivalent to the index we need: count! The count variable starts at 0 and increments by one on every loop.

One possible solution - we could create a second variable (let's call it found_value_index) and assign it when we find the value we're looking for. Then, after the looping completes, we could return this variable.

def find_element_index(array, value_to_find)
  count = 0
  found_value_index = nil
  while count < array.length do
    if array[count] == value_to_find
      found_value_index = count
    end
    count += 1
  end
  found_value_index
end

In this example, we start by assigning found_value_index to nil. The second test in this lab expects find_element_index to return nil if no match is found. The way we've set this method up, found_value_index will continue to equal nil until a match is found. During every loop, if array[count] is equal to value_to_find, found_value_index is set to the current count. At the end, found_value_index is returned.

This is a valid solution to this lab. There are, however, other solutions that behave slightly differently. In the above solution, for instance, the code will always loop through the entire array. Another thing to notice - if the provided array has repeating values that match the value to find, this solution will return the last matching index.

A slightly modified solution changes this:

def find_element_index(array, value_to_find)
  count = 0
  while count < array.length do
    if array[count] == value_to_find
      return count
    end
    count += 1
  end
end

This solution drops the extra variable. Instead, if a match is found between an array element and the value to find, we immediately return count, breaking out of the loop. This solution will always return the first index of a matching element in the provided array. It also will stop working as soon as it finds a match, rather than continuing to loop.

If there is no match found, the loop exits and the method ends. The while loop statement doesn't return anything, so find_element_index returns nil when no match is found.

Conclusion

When working with arrays, it is often necessary that we find specific array elements. This is such a common task that Ruby provides a built-in method that will complete this task on any array: .index.

2.6.1 :001 > array = [5, 4, 3, 2, 1]
 => [5, 4, 3, 2, 1]
2.6.1 :002 > array.index(5)
 => 0
2.6.1 :003 > array.index(1)
 => 4

Technically, this lab could be solved with the following:

def find_element_index(array, value_to_find)
  array.index(value_to_find)
end

Though in a real coding situation, this method would be redundant - we would skip writing a custom method and use array.index directly.

You might ask - why do we learn the loop way if something exists that does this work for us? The main reason is that implementing our own version allows us to understand and appreciate how something works. There is no magic. By writing the code, you will be able to better visualize what happens when a method like .index is called.

Building our own versions is also useful for times when you need to do something that doesn't quite work with the existing tools.

Resources

• Arrays