
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
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.
Popular Pragmatic Bookshelf topics

Hi Travis! Thank you for the cool book! :slight_smile:
I made a list of issues and thought I could post them chapter by chapter. I’m rev...
New

Hi,
build fails on:
bracket-lib = “~0.8.1”
when running on Mac Mini M1 Rust version 1.5.0:
Compiling winit v0.22.2
error[E0308]: mi...
New

This is as much a suggestion as a question, as a note for others.
Locally the SGP30 wasn’t available, so I ordered a SGP40. On page 53, ...
New

I think I might have found a problem involving SwitchCompat, thumbTint, and trackTint.
As entered, the SwitchCompat changes color to hol...
New

I’m under the impression that when the reader gets to page 136 (“View Data with the Database Inspector”), the code SHOULD be able to buil...
New

Hi, I’ve got a question about the implementation of PubSub when using a Phoenix.Socket.Transport behaviour rather than channels.
Before ...
New

Hi, I’m working on the Chapter 8 of the book.
After I add add the point_offset, I’m still able to see acne:
In the image above, I re...
New

@mfazio23
I’ve applied the changes from Chapter 5 of the book and everything builds correctly and runs. But, when I try to start a game,...
New

Hello faithful readers! If you have tried to follow along in the book, you are asked to start up the dev environment via dx/build and ar...
New

@mfazio23
Android Studio will not accept anything I do when trying to use the Transformations class, as described on pp. 140-141. Googl...
New
Other popular topics

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

Rust is an exciting new programming language combining the power of C with memory safety, fearless concurrency, and productivity boosters...
New

Just done a fresh install of macOS Big Sur and on installing Erlang I am getting:
asdf install erlang 23.1.2
Configure failed.
checking ...
New

This looks like a stunning keycap set :orange_heart:
A LEGENDARY KEYBOARD LIVES ON
When you bought an Apple Macintosh computer in the e...
New

Think Again 50% Off Sale »
The theme of this sale is new perspectives on familiar topics.
Enter coupon code ThinkAgain2021 at checkout t...
New

Biggest jackpot ever apparently! :upside_down_face:
I don’t (usually) gamble/play the lottery, but working on a program to predict the...
New

We’ve talked about his book briefly here but it is quickly becoming obsolete - so he’s decided to create a series of 7 podcasts, the firs...
New

Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New

The File System Access API with Origin Private File System.
WebKit supports new API that makes it possible for web apps to create, open,...
New

This is a very quick guide, you just need to:
Download LM Studio: https://lmstudio.ai/
Click on search
Type DeepSeek, then select the o...
New
Latest in A Common-Sense Guide to Data Structures and Algorithms, Second Edition
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /ruby
- /wasm
- /erlang
- /phoenix
- /keyboards
- /rails
- /python
- /js
- /security
- /go
- /swift
- /vim
- /clojure
- /emacs
- /haskell
- /java
- /onivim
- /typescript
- /svelte
- /c-plus-plus
- /crystal
- /kotlin
- /tailwind
- /react
- /gleam
- /ocaml
- /flutter
- /elm
- /vscode
- /ash
- /html
- /opensuse
- /centos
- /php
- /deepseek
- /zig
- /scala
- /textmate
- /lisp
- /sublime-text
- /nixos
- /debian
- /react-native
- /agda
- /kubuntu
- /arch-linux
- /django
- /revery
- /ubuntu
- /manjaro
- /spring
- /nodejs
- /diversity
- /lua
- /deno
- /julia
- /c
- /slackware