Summary of Hash (Dictionary) operation support for Ruby and Python

Overview

A summary of typical operations of Array in Ruby and Hash (Dictionary) in Python.

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

Prerequisites

• Python 3.5.2
• Ruby 2.3.1

Reference information

• Ruby
  • https://docs.ruby-lang.org/ja/latest/class/Hash.html
• Python
  • http://docs.python.jp/3/tutorial/datastructures.html#dictionaries
  • http://www.pythonweb.jp/tutorial/dictionary/

Method

Initialization

• ruby

h = {"a" => 1, "b" => 2, "c" => 3}
h2 = {a: 1, b: 2, c: 3}   #When symbol is key

• python

d = {'a': 1, 'b': 2, 'c': 3}

Extracting elements

• ruby

h['c']    #=> 3
h['d']    #=> nil

d['c']    #=> 3
d['d']    #=>Exception KeyError occurs

size

• ruby

h.size   #=> 3

• python

len( d )   #=> 3

each

Loop around the element

• ruby

h.each do |key,val|
  puts key,val
end

• python

for k,v in d.items():
  print(k,v)

keys, values

Get a list of key and value

• ruby

h.keys    #=> ["a", "b", "c"]
h.values  #=> [1, 2, 3]

• python --In Python3, dict_keys and dict_values objects are returned.
  • http://www.atmarkit.co.jp/ait/articles/0902/06/news139.html --Since it returns a reference to the dictionary, changing the dictionary itself will change the keys themselves. --Use list () if you want a list.

keys = d.keys()       #=> dict_keys(['b', 'a', 'c'])
values = d.values()   #=> dict_values([2, 1, 3])

d['d'] = 4
keys                     #=> dict_keys(['b', 'd', 'a', 'c'])
values                   #=> dict_values([2, 4, 1, 3])

list(keys)               #=> ['b', 'd', 'a', 'c']

Generated from an array

• ruby --Easy to use ʻArray # to_h`

a = [1,2,3,4]
a.map {|x| [x,x+3] }.to_h      #=> {1=>4, 2=>5, 3=>6, 4=>7}

• python --List comprehension

a = [1,2,3,4]
{ i:i+3 for i in a }     #=> {1: 4, 2: 5, 3: 6, 4: 7}

key, value map

• ruby --Use map if you want to convert key and value. The map itself returns an Array of Arrays, so convert it to Hash with Hash []

h = {"a" => 1, "b" => 2, "c" => 3}
h.map {|k,v| [k.upcase, -v] }            #=> [["A", -1], ["B", -2], ["C", -3]]
h.map {|k,v| [k.upcase, -v] }.to_h       #=> {"A"=>-1, "B"=>-2, "C"=>-3}

• python

d = {'a': 1, 'b': 2, 'c': 3}
{ k.upper():-v for k,v in d.items() }    #=> {'A': -1, 'B': -2, 'C': -3}

has_key?

• ruby

h = {"a" => 1, "b" => 2, "c" => 3}
h.has_key?("a")               #=> true
h.has_key?("d")               #=> false

• python --It seems that you could use the method d.has_key ('a') up to python2.

d = {'a': 1, 'b': 2, 'c': 3}
'a' in d                      #=> True
'd' in d                      #=> False

merge

• ruby --Use Hash # merge. Non-destructive method. The destructive method is merge! --If there is the same key, the value of the argument takes precedence by default. Can be specified by passing a block - https://docs.ruby-lang.org/ja/latest/method/Hash/i/merge.html

h1 = {"a" => 1, "b" => 2, "c" => 3}
h2 = {"d" => 4, "e" => 5, "f" => 6}
h1.merge(h2)
# => {"a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5, "f"=>6}

• python --There is a method called ʻupdate`, but it's a destructive operation ...

d1 = {"a": 1, "b": 2, "c": 3}
d2 = {"d": 4, "e": 5, "f": 6}
d1.update(d2)
d1
#=> {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

delete

• ruby

h = {"a" => 1, "b" => 2, "c" => 3}
h.delete('a')                      #=> 1
h                                  #=> {"b"=>2, "c"=>3}

• python

d = {"a": 1, "b": 2, "c": 3}
d.pop('a')                         #=> 1
d                                  #=> {'b': 2, 'c': 3}

Setting the default value

Case of using an associative array when taking a histogram

• ruby

chars = ["a", "b", "b", "c", "c", "c"]
h = Hash.new(0)
chars.each {|c| h[c] += 1 }            #Counting the number of characters
h                                      #=> {"a"=>1, "b"=>2, "c"=>3}

• python --Refer to http://www.lifewithpython.com/2015/05/python-dict-default-value.html --There is also a way to use default_dict

chars = ["a", "b", "b", "c", "c", "c"]
d = {}
for c in chars:
  d[c] = d.get(c,0) + 1             #Get if there is a value, return 0 if not
d                                   #=> {'a': 1, 'b': 2, 'c': 3}

from collections import defaultdict
chars = ["a", "b", "b", "c", "c", "c"]
d = defaultdict( lambda: 0 )
for c in chars:
  d[c] += 1

d
# => defaultdict(<function __main__.<lambda>>, {'a': 1, 'b': 2, 'c': 3})

When creating an index of an array

• ruby --Caution Hash.new ([]) is a bug
  • https://docs.ruby-lang.org/ja/latest/method/Hash/s/new.html

words = ["a1", "b1", "b2", "c1", "c2", "c3"]
h = Hash.new {|hash,key| hash[key] = [] }
words.each {|w| h[ w[0] ] << w }
h
# => {"a"=>["a1"], "b"=>["b1", "b2"], "c"=>["c1", "c2", "c3"]}

• python

words = ["a1", "b1", "b2", "c1", "c2", "c3"]
d = defaultdict( list )
for w in words:
  key = w[0]
  d[key].append(w)

d
#=> defaultdict(list, {'a': ['a1'], 'b': ['b1', 'b2'], 'c': ['c1', 'c2', 'c3']})