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
Python Testing With Pytest - Chapter 2, warnings for “unregistered custom marks”
While running the smoke tests in Chapter 2, I get these...
New
Title: Web Development with Clojure, Third Edition, pg 116
Hi - I just started chapter 5 and I am stuck on page 116 while trying to star...
New
your book suggests to use Image.toByteData() to convert image to bytes, however I get the following error: "the getter ‘toByteData’ isn’t...
New
Title: Hands-On Rust (Chapter 11: prefab)
Just played a couple of amulet-less games. With a bit of debugging, I believe that your can_p...
New
Page 28: It implements io.ReaderAt on the store type.
Sorry if it’s a dumb question but was the io.ReaderAt supposed to be io.ReadAt?
...
New
Title: Web Development with Clojure, Third Edition, vB17.0 (p9)
The create table guestbook syntax suggested doesn’t seem to be accepted ...
New
I think I might have found a problem involving SwitchCompat, thumbTint, and trackTint.
As entered, the SwitchCompat changes color to hol...
New
When I run the coverage example to report on missing lines, I get:
pytest --cov=cards --report=term-missing ch7
ERROR: usage: pytest [op...
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
Modern front-end development for Rails, second edition - Struggling to get the first chapter to work
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New
Other popular topics
Stop developing web apps with yesterday’s tools. Today, developers are increasingly adopting Clojure as a web-development platform. See f...
New
Which, if any, games do you play? On what platform?
I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
New
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
Curious to know which languages and frameworks you’re all thinking about learning next :upside_down_face:
Perhaps if there’s enough peop...
New
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
Continuing the discussion from Thinking about learning Crystal, let’s discuss - I was wondering which languages don’t GC - maybe we can c...
New
Create efficient, elegant software tests in pytest, Python's most powerful testing framework.
Brian Okken @brianokken
Edited by Kat...
New
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc.
However, I don’t...
New
If you’re getting errors like this:
psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
New
This is cool!
DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON
We just witnessed something incredible: the largest open-s...
New
Latest in A Common-Sense Guide to Data Structures and Algorithms, Second Edition
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /python
- /js
- /rails
- /security
- /go
- /swift
- /vim
- /clojure
- /java
- /emacs
- /haskell
- /typescript
- /svelte
- /onivim
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /elm
- /vscode
- /flutter
- /ash
- /html
- /deepseek
- /opensuse
- /zig
- /centos
- /php
- /scala
- /react-native
- /lisp
- /sublime-text
- /textmate
- /nixos
- /debian
- /agda
- /deno
- /django
- /kubuntu
- /arch-linux
- /nodejs
- /spring
- /ubuntu
- /revery
- /manjaro
- /lua
- /diversity
- /julia
- /markdown
- /laravel









