Tuples & Lists

Agenda

• List
• Tuple
• String method
• More on string
• (List) comprehension

List

Lists are one of four major built-in data types to store collections of data

• The others are tuples, dictionaries, and sets
• Used to store “things” in a single variable

List

Items in a list do not need to be of the same type

List

Can initialize an empty list with just two squared brackets

List

Lists are ordered

• So you can access an item at a specific index
• This is called indexing/slicing
  • Indexing is used to access a single element at a specific position
  • Slicing is used to access a range of elements

List

Lists are ordered

• Sets are not ordered/indexed (this can be good!)

List

Lists are ordered

• The index can be negative too

List

Lists are ordered

• An item can occur more than once (unlike sets)

List

Lists are mutable

• Lists are mutable objects (so are sets and dictionaries)
• That means they can be modified once they are created
• For example

List

Many objects are immutable though

• Tuple

• String

List

Length of a list

• len() returns the number of items in (or length of) a list

List

List methods

• In Python, methods are functions that belong to an object
  • A function is a reusable block of code that performs a specific task
  • So, methods are a subset of functions
• They only work with that object
• They are used with dot notation: object.method()

List

List methods

• There are many useful methods that come with list objects
• We will be able to look at some of them

List

append()

• Often used inside for loops

List

insert()

• Adds an element at the specified position
• No reassignment

List

reverse()

• Reverses the order of the list
• Again, no reassignment

List

sort()

• Sorts the list (no re-assignment too)

List

sort()

• Elements are not always comparable to each other, but see the following

List

sorted() function

• sorted() is a function but not a method

• Returns a sorted version of the list, but original stays intact

List

index()

• Returns the index of the first element matching the specified value

List

extend()

• How about this? Why does it work or not?

List

+

• To add lists together without modifying original lists, just +

List

Slicing

• Lists can be sliced with the following syntax (similar to range()):

• start at start (default 0)

• stop one step before stop (default is length of list)

• step specifies how many indices to jump

List

Slicing

• Get everything

List

Slicing

• What will this return?

List

Slicing

• What will this return?

List

Slicing

• What will this return?

List

Slicing

• Look how easy it is to get first n elements, and then the rest

List

Slicing

• A few more examples

List

Slicing

• A few more examples

Tuple

Tuples can be ordered and be indexed

Tuple

Used to conveniently swap variable values

Tuple

Used to return more than one value from a function

• It conveniently packages many values of different type into one object

Tuple

Tuples have just two methods: count() and index()

String method

Python has really rich string methods

• See a full list here
• Let’s explore some of them

String method

startswith(), endswith(), in

• Return a Boolean value
• Use in operator to see if a string ‘just contains’ a substring

String method

capitalize(), title(), upper(), lower()

• capitalize() capitalizes the first character
• title() capitalizes the first character of every word
• upper() capitalizes everything
• lower() converts string to all lowercase

String method

capitalize(), title(), upper(), lower()

List comprehension

Comprehensions

• list, set, and dictionary comprehensions

• It is possible to create a list, set, or dictionary using:

  • for / while loops
  • if / else statements in the loop

• Comprehensions provide simple syntax to achieve it in a single line

• Better readability too

List comprehension

Let’s look at speed: for loop vs. comprehension

List comprehension

Simple example: create a copy of a list

• Let’s do this with a for loop:

List comprehension

Create a copy of the list with list comprehension

List comprehension

The more common scenario is “do something” to every element of a list

• With for loop

List comprehension

The more common scenario is “do something” to every element of a list

• With list comprehension
• “Square the number, for each number in numbers”

List comprehension

Do something to element only if a condition is true

• New list with only even numbers squared
• With for loop

List comprehension

With list comprehension

• “Square the number, for each number in numbers, if the number is even”