Correspondence summary of array operation of ruby and python

Overview

A summary of typical operations of Ruby Array and Python list.

This is an article for Rubyist (Pythonista) who has just started learning Python (Ruby).

Reference: Click here for C ++ version Ruby-C ++ Array operation support summary

Prerequisites

• Python 3.5.1
• Ruby 2.2.4

Reference information

• Ruby
  • http://docs.ruby-lang.org/ja/2.2.0/class/Enumerable.html
• Python
  • http://docs.python.jp/3.5/tutorial/introduction.html#lists
  • http://docs.python.jp/3.5/tutorial/datastructures.html#more-on-lists
  • http://kesin.hatenablog.com/entry/2013/05/12/004541
  • http://www.walbrix.com/jp/blog/2014-01-findfirst-in-scala-and-python.html

Method

size

• ruby

[1,2,3].size   #=> 3

• python

len( [1,2,3] )   #=> 3

each

Loop around the element

• ruby

[1,2,3,4,5].each do |x|
  puts 2*x    #=> 2,4,6,8,10
end

• python

for x in [1,2,3,4,5]:
    print(2*x)

each_with_index

Turn the loop with index

• ruby

[5,4,3,2,1].each_with_index do |x,idx|
  print "#{idx}:#{x}, "
end
# => 0:5, 1:4, 2:3, 3:2, 4:1,

• python

for idx,x in enumerate( [5,4,3,2,1] ):
    print("{0}:{1}, ".format(idx,x), end="")

Joining arrays

• ruby --concat is a destructive operation,+is a non-destructive operation

[1,2,3] + [4,5]     # [1,2,3,4,5]

• python --ʻExtendis a destructive operation. Non-destructive operation with+`

[1,2,3] + [4,5]

push (<<)

Add element at the end

• ruby

a = [1,2]
a << 3
# a => [1,2,3]

• python

a = [1,2]
a.append(3)
# a => [1,2,3]

map

• ruby

[1,2,3].map {|x| 2*x }  #=> [2,4,6]

• python --You can use the map function, but the list comprehension seems to be modern.

[ 2*x for x in [1,2,3] ]                 # => [2,4,6]
list( map( lambda x: 2*x, [1,2,3] ) )    # => [2,4,6]

find

• ruby --Nil is returned if the element is not found

[7,8,9,10,11].find {|x| x%5 == 0 }  #=> 10

• python --I can't find a very good notation

[ x for x in [7,8,9,10,11] if x % 5 == 0 ][0]   #=> 10

--Addition: http://stackoverflow.com/questions/9868653/find-first-list-item-that-matches-criteria --The second argument is the default value

next((x for x in [7,8,9,10,11] if x % 5 == 0), None)

find_all

• ruby

[1,2,3,4,5].find_all {|x| x>3 }  #=> [4,5]

• python --Use filter or list comprehension

[ x for x in [1,2,3,4,5] if x > 3]     #=> [4,5]
list( filter( lambda x: x>3, [1,2,3,4,5] ) )    #=> [4,5]

sort

• ruby

[4,1,3,2,5].sort  #=> [1,2,3,4,5]

• python

sorted( [4,1,3,2,5] )   #=> [1,2,3,4,5]

sort_by

• ruby

[4,1,2,3,5].sort_by {|x| (3-x).abs }  #=> [3,4,2,1,5]

• python

sorted( [4,1,3,2,5], key= lambda x: abs(3-x) )

index

• ruby --Nil is returned if there is no corresponding element

[5,1,4,2,3].index(2)  #=> 3

• python --If there is no corresponding element, an exception will occur

[5,1,4,2,3].index(2)  #=> 3

all?

• ruby

[2,4,6,8].all? {|x| x % 2 == 0 }  #=> true
[2,4,6,9].all? {|x| x % 2 == 0 }  #=> false

• python

all( x%2==0 for x in [2,4,6,8] )   #=> True
all( x%2==0 for x in [2,4,6,9] )   #=> False

sum

• ruby --Use inject

[1,3,5].inject(:+)   #=> 9

• python --Use the built-in function sum

sum( [1,3,5] )

inject (reduce)

• ruby

[1,2,3,4,5].inject(1) {|acc,x| acc*x }   #=> 120
[1,2,3,4,5].inject("") {|acc,x| acc + (x%2==0?'e':'o') }   #=> oeoeo

• python --Use the reduce function. Built-in functions up to Python2. From Python3 you need to import functools.

import functools
functools.reduce( lambda acc,x: acc*x, [1,2,3,4,5], 1 )
functools.reduce( lambda acc,x: acc+("e" if x%2==0 else "o") , [1,2,3,4,5], "" )

uniq

• ruby

[5,1,1,2,3,5,5].uniq  #=> [5,1,2,3]

• python --Convert to list after putting in set. The order is not saved. --If you want to save the order, it seems that you have to run the loop. - http://www.lifewithpython.com/2013/11/python-remove-duplicates-from-lists.html

list( set( [5,1,1,2,3,5,5] ) )

li_uniq = []
for x in [5,1,1,2,3,5,5]:
  if x not in li_uniq:
    li_uniq.append(x)
li_uniq    #=> [5,1,2,3]

uniq {block}

• ruby --Delete duplicate block evaluation results

[4,5,1,2,3,4,5].uniq {|x| x % 3 }   #=> [4,5,3]

• python

li_uniq = []
set_uniq = set()
for x in [4,5,1,2,3,4,5]:
  if x % 3 not in set_uniq:
    li_uniq.append(x)
    set_uniq.add(x%3)
li_uniq                    #=> [4,5,3]

join

• ruby

[1,5,3,2].join(":")  #=> "1:5:3:2"

• python --It's very hard to see that String has a join method. Arrays must be explicitly converted to string type.

":".join(map(str, [1,5,3,2]))

reverse

• ruby

[1,2,3].reverse  #=> [3,2,1]
a = [1,2,3]
a.reverse!
a                #=> [3,2,1]

• python --List's reverse method is a destructive operation. --For non-destructive operations, use the built-in function reversed.

list( reversed([1,2,3]) )

a = [1,2,3]
a.reverse()
a                #=> [3,2,1]

group_by

• ruby

["cat","bat","bear","camel","alpaca"].group_by {|x| x[0]}
# {"c"=>["cat", "camel"], "b"=>["bat", "bear"], "a"=>["alpaca"]}

• python --Turn the loop to add elements to the dictionary

d = {}
for x in ["cat","bat","bear","camel","alpaca"]:
    key = x[0]
    if key not in d:
        d[key] = []
    d[key].append(x)
d        # => {'c': ['cat', 'camel'], 'a': ['alpaca'], 'b': ['bat', 'bear']}

shuffle

• ruby

[1,2,3,4,5,6,7].shuffle   #=> [2, 6, 4, 7, 5, 3, 1]

• python

import random
a = [1,2,3,4,5,6,7]
random.shuffle(a)
a                       #=> [4, 5, 6, 2, 7, 1, 3]