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Topics

Sequences
Introduction to Lists
List Slicing
Finding Items in Lists with the in Operator
List Methods and Useful Built-in Functions

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Topics (cont’d.)

Copying Lists
Processing Lists
Two-Dimensional Lists
Tuples

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Sequences

Sequence: an object that contains multiple items of data

The items are stored in sequence one after another

Python provides different types of sequences, including lists and tuples

The difference between these is that a list is mutable and a tuple is immutable

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Introduction to Lists

List: an object that contains multiple data items

Element: An item in a list
Format: list = [item1, item2, etc.]
Can hold items of different types

print function can be used to display an entire list
list() function can convert certain types of objects to lists

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Introduction to Lists (cont’d.)

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The Repetition Operator and Iterating over a List

Repetition operator: makes multiple copies of a list and joins them together

The * symbol is a repetition operator when applied to a sequence and an integer

Sequence is left operand, number is right

General format: list * n

You can iterate over a list using a for loop

Format: for x in list:

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Indexing

Index: a number specifying the position of an element in a list

Enables access to individual element in list
Index of first element in the list is 0, second element is 1, and n’th element is n-1
Negative indexes identify positions relative to the end of the list

The index -1 identifies the last element, -2 identifies the next to last element, etc.

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The len function

An IndexError exception is raised if an invalid index is used
len function: returns the length of a sequence such as a list

Example: size = len(my_list)
Returns the number of elements in the list, so the index of last element is len(list)-1
Can be used to prevent an IndexError exception when iterating over a list with a loop

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Lists Are Mutable

Mutable sequence: the items in the sequence can be changed

Lists are mutable, and so their elements can be changed

An expression such as

list[1] = new_value can be used to assign a new value to a list element

Must use a valid index to prevent raising of an IndexError exception

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Concatenating Lists

Concatenate: join two things together
The + operator can be used to concatenate two lists

Cannot concatenate a list with another data type, such as a number

The += augmented assignment operator can also be used to concatenate lists

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List Slicing

Slice: a span of items that are taken from a sequence

List slicing format: list[start : end]
Span is a list containing copies of elements from start up to, but not including, end

If start not specified, 0 is used for start index
If end not specified, len(list) is used for end index

Slicing expressions can include a step value and negative indexes relative to end of list

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Finding Items in Lists with the in Operator

You can use the in operator to determine whether an item is contained in a list

General format: item in list
Returns True if the item is in the list, or False if it is not in the list

Similarly you can use the not in operator to determine whether an item is not in a list

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List Methods and Useful Built-in Functions

append(item): used to add items to a list – item is appended to the end of the existing list
index(item): used to determine where an item is located in a list

Returns the index of the first element in the list containing item
Raises ValueError exception if item not in the list

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List Methods and Useful Built-in Functions (cont’d.)

insert(index, item): used to insert item at position index in the list
sort(): used to sort the elements of the list in ascending order
remove(item): removes the first occurrence of item in the list
reverse(): reverses the order of the elements in the list

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List Methods and Useful Built-in Functions (cont’d.)

del statement: removes an element from a specific index in a list

General format: del list[i]

min and max functions: built-in functions that returns the item that has the lowest or highest value in a sequence

The sequence is passed as an argument

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Copying Lists

To make a copy of a list you must copy each element of the list

Two methods to do this:

Creating a new empty list and using a for loop to add a copy of each element from the original list to the new list
Creating a new empty list and concatenating the old list to the new empty list

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Copying Lists (cont’d.)

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Processing Lists

List elements can be used in calculations
To calculate total of numeric values in a list use loop with accumulator variable
To average numeric values in a list:

Calculate total of the values
Divide total of the values by len(list)

List can be passed as an argument to a function

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Processing Lists (cont’d.)

A function can return a reference to a list
To save the contents of a list to a file:

Use the file object’s writelines method

Does not automatically write \n at then end of each item

Use a for loop to write each element and \n

To read data from a file use the file object’s readlines method

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Two-Dimensional Lists

Two-dimensional list: a list that contains other lists as its elements

Also known as nested list
Common to think of two-dimensional lists as having rows and columns
Useful for working with multiple sets of data

To process data in a two-dimensional list need to use two indexes
Typically use nested loops to process

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Two-Dimensional Lists (cont’d.)

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