Python Data Structures

This post explains the data structures used in Python. It is essential to understand the data structures in a programming language. In python, there are many data structures available. They are as follows :
  1. strings
  2. lists
  3. tuples
  4. dictionaries
  5. sets
Python Data Structures
1. Strings
Python String is a sequence of characters.
How to create a string in Python
You can create Python string using a single or double quote.
mystring = "Hello Python3.6"
print(mystring)
Output:
Hello Python3.6
Can I use multiple single or double quotes to define string?
Answer is Yes. See examples below -
Multiple Single Quotes
mystring = '''Hello Python3.6'''
print(mystring)
Output:
Hello Python3.6
Multiple Double Quotes
mystring = """Hello Python3.6"""
print(mystring)
Output:
Hello Python3.6
How to include quotes within a string?
mystring = r'Hello"Python"'
print(mystring)
Output:
Hello"Python"
How to extract Nth letter or word?
You can use the syntax below to get first letter.
mystring = 'Hi How are you?'
mystring[0]
Output
'H'
mystring[0] refers to first letter as indexing in python starts from 0. Similarly, mystring[1] refers to second letter. To pull last letter, you can use -1 as index.
mystring[-1]
To get first word
mystring.split(' ')[0]
Output : Hi
How it works -

1. mystring.split(' ') tells Python to use space as a delimiter.

Output : ['Hi', 'How', 'are', 'you?']

2. mystring.split(' ')[0] tells Python to pick first word of a string.

2. List
Unlike String, List can contain different types of objects such as integer, float, string etc.
  1. x = [142, 124, 234, 345, 465]
  2. y = [‘A’, ‘C’, ‘E’, ‘M’]
  3. z = [‘AA’, 44, 5.1, ‘KK’]
Get List Item
We can extract list item using Indexes. Index starts from 0 and end with (number of elements-1). Syntax : list[start : stop : step]
  1. start : refers to starting position.
  2. stop : refers to end position.
  3. step : refers to increment value.
k = [124, 225, 305, 246, 259]
k[0]
k[1]
k[-1]
k[0]
124

k[1]
225

k[-1]
259

Explanation :
k[0] picks first element from list. Negative sign tells Python to search list item from right to left. k[-1] selects the last element from list.
To select multiple elements from a list, you can use the following method :
k[:3] returns [124, 225, 305]
k[0:3] also returns [124, 225, 305]
k[::-1] reverses the whole list and returns [259, 246, 305, 225, 124]
Sort list
sorted(list) function arranges list in ascending order. sorted(list, reverse=True) function sorts list in descending order.
sorted(k) returns [124, 225, 246, 259, 305]
sorted(k, reverse=True) returns [305, 259, 246, 225, 124]
Add 5 to each element of a list
In the program below, len() function is used to count the number of elements in a list. In this case, it returns 5. With the help of range() function, range(5) returns 0,1,2,3,4.
x = [1, 2, 3, 4, 5]
for i in range(len(x)):
    x[i] = x[i] + 5
print(x)
[6, 7, 8, 9, 10]
It can also be written like this -
for i in range(len(x)):
   x[i] += 5
print(x)
Combine / Join two lists
The '+' operator is concatenating two lists.
X = [1, 2, 3]
Y = [4, 5, 6]
Z = X + Y
print(Z)
[1, 2, 3, 4, 5, 6]
Sum of values of two list
X = [1, 2, 3]
Y = [4, 5, 6]
import numpy as np
Z = np.add(X, Y)
print(Z)
print(Z)
[5 7 9]
Similarly, you can use np.multiply(X, Y) to multiply values of two list.
Repeat List N times
The '*' operator is repeating list N times.
X = [1, 2, 3]
Z = X * 3
print(Z)
[1, 2, 3, 1, 2, 3, 1, 2, 3]
Note : The above two methods also work for string list.
Modify / Replace a list item
Suppose you need to replace third value to a different value.
X = [1, 2, 3]
X[2]=5
print(X)
print(X)
[1, 2, 5]
Add / Remove a list item
We can add a list item by using append method.
X = ['AA', 'BB', 'CC']
X.append('DD')
print(X)
Result : ['AA', 'BB', 'CC', 'DD'] Similarly, we can remove a list item by using remove method.
X = ['AA', 'BB', 'CC']
X.remove('BB')
print(X)
Result : ['AA', 'CC']
3. Tuple
Like list, tuple can also contain mixed data. But tuple cannot be mutable or changed once created whereas list can be mutable or modified.

Another difference is a tuple is created inside parentheses ( ). Whereas, list is created inside square brackets [ ]

Examples
mytuple = (123,223,323)
City = ('Delhi','Mumbai','Bangalore')
Perform for loop on Tuple
for i in City:
    print(i)
Delhi
Mumbai
Bangalore
Tuple cannot be altered
Run the following command and check error
X = (1, 2, 3)
X[2]=5
TypeError:
'tuple' object does not support item assignment
4. Dictionary
It works like an address book wherein you can find an address of a person by searching the name. In this example. name of a person is considered as key and address as value. It is important to note that the key must be unique while values may not be. Keys should not be duplicate because if it is a duplicate, you cannot find exact values associated with key. Keys can be of any data type such as strings, numbers, or tuples.
Create a dictionary
It is defined in curly braces {}. Each key is followed by a colon (:) and then values.
teams = {'Dave' : ['teamA','teamAA', 'teamAB'],
         'Tim'  : ['teamB','teamBB','teamBC'],
         'Babita' : ['teamC','teamCB','teamCC']
        }
Extract Keys and Values of Dictionary
teams.keys() returns dict_keys(['Dave', 'Tim', 'Babita'])
teams.values() returns dict_values([['teamA', 'teamAA', 'teamAB'], ['teamB', 'teamBB', 'teamBC'], ['teamC', 'teamCB', 'teamCC']])
Find Values of a particular key
teams['Dave']
Output
['teamA', 'teamAA', 'teamAB']
Delete an item
In the code below, we are removing 'Babita' from teams dict.
del teams['Babita']
Output
{'Dave': ['teamA', 'teamAA', 'teamAB'], 
'Tim': ['teamB', 'teamBB', 'teamBC']}
Add an item
Here we are adding one more key named 'Deep' and value against it is 'team D'.
teams['Deep'] = 'team D'
Output
{'Dave': ['teamA', 'teamAA', 'teamAB'],
 'Deep': 'team D',
 'Tim': ['teamB', 'teamBB', 'teamBC']}

You can also create dictionary like the way it is shown below

d={}
d['a'] = 1
d['b'] = 2
print(d)
{'a': 1, 'b': 2}
How to create a dictionary from lists
Suppose you have keys and values stored in two separate lists. You can use and zip them to create a dictionary.
keys = ['a', 'b', 'c']
values = [1, 2, 3]
d1 = dict(zip(keys, values))
5. Sets
Sets are unordered collections of simple objects. They are mainly used to check whether an object is present in the set and compute mathematical operations such as intersection, union, difference etc.
X = set(['A', 'B', 'C'])
Q. Does 'A' exist in set X?
'A' in X
Result : True
Q. Does 'D' exist in set X?
'D' in X
Result : False
Q. How to add 'D' in set X?
X.add('D')
Q. How to remove 'C' from set X?
X.remove('C')
Q. How to create a copy of set X?
Y = X.copy()
Q. Which items are common in both sets X and Y?
Y & X
Practical Examples : Python Data Structures
The examples below would help you to understand what kind of operations on data structures are commonly used in real-world.
1. How to find intersection and union of two lists
x = [1, 2, 3, 4]
y = [2, 3, 6, 5]

list(set(x) & set(y))
list(set(x) | set(y))
& symbol refers to 'and' condition which means common between two lists. | symbol refers to 'or' condition.
2. Check whether an item exist in list
x = [1, 2, 3, 4]
3 in x

True
3. Check whether multiple items exist in list
all returns True only when all the items exist. any returns when any of the item exist.
all(i in x for i in [1,6])
False

any(i in x for i in [1,6])
True
About Author:

Deepanshu founded ListenData with a simple objective - Make analytics easy to understand and follow. He has over 8 years of experience in data science. During his tenure, he has worked with global clients in various domains like Banking, Insurance, Telecom and Human Resource.

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