paolotormon

paolotormon

A Common-Sense Guide to Data Structures and Algorithms, Second Edition: Quickselect In Python (page 221)

Hi, I am trying to convert the ruby code of Quickselect into python and I noticed that I have to add return statements in the if else conditions like so

def partition(left_p, right_p, arr=[]):

    pivot_index = right_p

    pivot = arr[pivot_index]

    right_p -= 1

    while True:

        while arr[left_p] < pivot:

            left_p += 1

        while arr[right_p] > pivot:

            right_p -= 1

        if left_p >= right_p:

            break

        else:

            arr[left_p], arr[right_p] = arr[right_p], arr[left_p]

            left_p += 1

    arr[left_p], arr[pivot_index] = arr[pivot_index], arr[left_p]

    return left_p

def quickselect(kth_lowest_value, left_index, right_index, arr=[]):

    print(arr)

    if right_index - left_index <= 0:

        return arr[left_index]

    pivot_index = partition(left_index, right_index, arr)

    if kth_lowest_value < pivot_index:

        return quickselect(kth_lowest_value, left_index, pivot_index-1, arr)

    elif kth_lowest_value > pivot_index:

        return quickselect(kth_lowest_value, pivot_index+1, right_index, arr)

    else:

        print(f"item = {arr[pivot_index]}")

        return arr[pivot_index]

array = [200, 97, 100, 101, 211, 107, 63, 123, 11, 34]

index = quickselect(6, 0, len(array)-1, array)

print(index)

In the book version written in Ruby, there “return” is only in “return @array[pivot_index]”, so I think we either remove the return or also put returns on the statements after the other conditionals. Unedited code below:

    attr_reader :array
    def initialize(array)
        @array = array
    end
    def quickselect!(kth_lowest_value, left_index, right_index)
        # If we reach a base case - that is, that the subarray has one cell,
        # we know we've found the value we're looking for:
        if right_index - left_index <= 0
            return @array[left_index]
        end
        # Partition the array and grab the index of the pivot:
        pivot_index = partition!(left_index, right_index)
        # If what we're looking for is to the left of the pivot:
        if kth_lowest_value < pivot_index
            # Recursively perform quickselect on the subarray to
            # the left of the pivot:
            return quickselect!(kth_lowest_value, left_index, pivot_index - 1)
            # If what we're looking for is to the right of the pivot:
        elsif kth_lowest_value > pivot_index
            # Recursively perform quickselect on the subarray to
            # the right of the pivot:
            return quickselect!(kth_lowest_value, pivot_index + 1, right_index)
        else # if kth_lowest_value == pivot_index
            # if after the partition, the pivot position is in the same spot
            # as the kth lowest value, we've found the value we're looking for
            return @array[pivot_index]
        end
    end
    
    def partition!(left_pointer, right_pointer)
        # We always choose the right-most element as the pivot.
        # We keep the index of the pivot for later use:
        pivot_index = right_pointer
        # We grab the pivot value itself:
        pivot = @array[pivot_index]
        # We start the right pointer immediately to the left of the pivot
        right_pointer -= 1
        while true
            # Move the left pointer to the right as long as it
            # points to value that is less than the pivot:
            while @array[left_pointer] < pivot do
                left_pointer += 1
            end
            # Move the right pointer to the left as long as it
            # points to a value that is greater than the pivot:
            while @array[right_pointer] > pivot do
                right_pointer -= 1
            end
            # We've now reached the point where we've stopped
            # moving both the left and right pointers.
            # We check whether the left pointer has reached
            # or gone beyond the right pointer. If it has,
            # we break out of the loop so we can swap the pivot later
            # on in our code:
            if left_pointer >= right_pointer
                break
            # If the left pointer is still to the left of the right
            # pointer, we swap the values of the left and right pointers:
            else
                @array[left_pointer], @array[right_pointer] = @array[right_pointer], @array[left_pointer]
            # We move the left pointer over to the right, gearing up
            # for the next round of left and right pointer movements:
                left_pointer += 1
            end
        end
        # As the final step of the partition, we swap the value
        # of the left pointer with the pivot:
        @array[left_pointer], @array[pivot_index] = @array[pivot_index], @array[left_pointer]
        # We return the left_pointer for the sake of the quicksort method
        # which will appear later in this chapter:
        return left_pointer
    end
end



array = [0, 50, 20, 10, 60, 30]
sortable_array = SortableArray.new(array)
p sortable_array.quickselect!(5, 0, array.length - 1)

First Post!

jaywengrow

jaywengrow

Author of A Common-Sense Guide to Data Structures and Algorithms

Good point, thank you! This will be modified in a future version of the book.

Where Next?

Popular Pragmatic Bookshelf topics Top

jeffmcompsci
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list) Typo:...
New
mikecargal
Title: Hands-On Rust (Chap 8 (Adding a Heads Up Display) It looks like ​.with_simple_console_no_bg​(SCREEN_WIDTH*2, SCREEN_HEIGHT*2...
New
mikecargal
Title: Hands-on Rust: question about get_component (page 295) (feel free to respond. “You dug you’re own hole… good luck”) I have somet...
New
HarryDeveloper
Hi @venkats, It has been mentioned in the description of ‘Supervisory Job’ title that 2 things as mentioned below result in the same eff...
New
alanq
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
New
gilesdotcodes
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions: In Gemfile, change gem 'rails' t...
New
jgchristopher
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ” Excerpt From: Br...
New
s2k
Hi all, currently I wonder how the Tailwind colours work (or don’t work). For example, in app/views/layouts/application.html.erb I have...
New
Keton
When running the program in chapter 8, “Implementing Combat”, the printout Health before attack was never printed so I assumed something ...
New
dachristenson
I just bought this book to learn about Android development, and I’m already running into a major issue in Ch. 1, p. 20: “Update activity...
New

Other popular topics Top

AstonJ
You might be thinking we should just ask who’s not using VSCode :joy: however there are some new additions in the space that might give V...
New
AstonJ
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
New
AstonJ
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
Rainer
Not sure if following fits exactly this thread, or if we should have a hobby thread… For many years I’m designing and building model air...
New
mafinar
Crystal recently reached version 1. I had been following it for awhile but never got to really learn it. Most languages I picked up out o...
New
AstonJ
If you get Can't find emacs in your PATH when trying to install Doom Emacs on your Mac you… just… need to install Emacs first! :lol: bre...
New
PragmaticBookshelf
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
New
PragmaticBookshelf
Author Spotlight: Sophie DeBenedetto @SophieDeBenedetto The days of the traditional request-response web application are long gone, b...
New
sir.laksmana_wenk
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc. However, I don’t...
New
AnfaengerAlex
Hello, I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New

Sub Categories: