3.2 - Dictionary

Ha Khanh Nguyen (hknguyen)

• 1. What is a dictionary?
• 2. Elements of a Dictionary
  • 2.1 Access, insert or set elements
  • 2.2 Delete elements
  • 2.3 Iterators of dict's keys and values
  • 2.4 Merge dictionaries
• 3. Creating Dictionary From Sequences
  • 3.1 The zip() function
  • 3.2 Unzip!
• Exercise

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1. What is a dictionary?

• Dictionary, dict, is a flexibly sized collection of key-value pairs, where key and value are Python objects.
• It is also known as hash map or associative array.

empty_dict = {}
empty_list = []
empty_tuple = ()
empty_string = ''

d1  = {'a' : 'some value', 'b' : [1, 2, 3, 4]}
d1

{'a': 'some value', 'b': [1, 2, 3, 4]}

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2. Elements of a Dictionary

2.1 Access, insert or set elements

• You can access, insert, or set elements using the same syntax as for accessing elements of a list or tuple:

# access an element
d1['a']

'some value'

# insert an element
d1[7] = 'an integer'
d1

{'a': 'some value', 'b': [1, 2, 3, 4], 7: 'an integer'}

# set a different value for an exisiting key
d1['a'] = 1
d1

{'a': 1, 'b': [1, 2, 3, 4], 7: 'an integer'}

• To check if a dict contains a key, we can use the in keyword:

'a' in d1

True

2.2 Delete elements

• Use the del keyword to delete a key-value pair from a dict:

del d1['b']
d1

{'a': 1, 7: 'an integer'}

2.3 Iterators of dict's keys and values

d1.keys()

dict_keys(['a', 7])

d1.values()

dict_values([1, 'an integer'])

2.4 Merge dictionaries

• You can merge one dict into another using the update() method:

d1.update({'b': 'here again', 'c': [1, 2, 3], 9: (1, 2)})
d1

{'a': 1, 7: 'an integer', 'b': 'here again', 'c': [1, 2, 3], 9: (1, 2)}

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3. Creating Dictionary From Sequences

• Let's say you are given two lists a and b and you want to create a dictionary where the key are elements of a and the values are elements of b.

key_list = ['a', 'b', 'c']
value_list = [1, 2, 3]

mapping = {}

for key, value in zip(key_list, value_list):
    mapping[key] = value

mapping

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

• Now, what happens if the two lists have different length?!
  • Well, let's try that out in Jupyter Notebook!

3.1 The zip() function

• Note the zip() function that we used in the above code segment, this is a very useful function (especially with for loop).
• zip() “pairs” up the elements of a number of lists, tuples, or other sequences to create a list of tuples.

seq1 = ['foo', 'bar', 'baz']
seq2 = ['one', 'two', 'three']

zipped = zip(seq1, seq2)
zipped
list(zipped)

[('foo', 'one'), ('bar', 'two'), ('baz', 'three')]

• zip() can take an arbitrary number of sequences, and the number of elements it produces is determined by the shortest sequence:

seq3 = [False, True]
list(zip(seq1, seq2, seq3))

[('foo', 'one', False), ('bar', 'two', True)]

3.2 Unzip!

Given a "zipped" sequence, the zip() function can also be applied in a clevery way to "unzip" the sequence.

pitchers = [('Nolan', 'Ryan'), ('Roger', 'Clemens'), ('Schilling', 'Curt')]
first_names, last_names = zip(*pitchers)

first_names

('Nolan', 'Roger', 'Schilling')

last_names

('Ryan', 'Clemens', 'Curt')

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Exercise

Let s be a string that contains a simple mathematical expression, e.g.,

s = '1.5 + 2.1'

s = '10.0-1.6'

s = '3.1*5.8'

s = '4.7 /7.2'

The expression will only have 2 operands and the operator will be one of the following: +, -, * and /.

Write a program that interpret the expression, then evaluate it and store the result in the result variable.

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This lecture note is modified from Chapter 3 of Wes McKinney's Python for Data Analysis 2nd Ed.