Lists in Python

Introduction to Collections

• Collections are essential for tracking related values.
• Examples of collections include:
  • Grades for a class
  • Names of employees
• Prior methods involved using sentinel values and loops with no long-term storage options.
• Collections enable the storage of many related values under a single variable name.

Indexing a Collection

• A collection is a data type that permits the storing of multiple values.
• Indexing is essential for accessing individual elements within a collection.
  • Numeric indexing starts from 0:
  • Index 0: first item
  • Index 1: second item
  • Index 2: third item, etc.
• Avoids the need for multiple variable names (e.g., grade1, grade2).
• Instead of naming each value, individual elements can be accessed via the collection name and index:
  • Example: grades[0], grades[1], grades[2], etc.

Introduction to Lists

• Lists are ordered sequences that store multiple values within a single collection.
• Creation of lists is achieved by enclosing values in square brackets, separating them with commas:
  • Example:
  • $[1, 2, 3, 4, 5]$
  • $["Hello", "This", "is", "a", "list", "of", "strings"]$
  • $[]$ (an empty list)
• Lists can contain any data type and can include mixed data types:
  • Example: $["Hello", 1, 2, 3.7, ["a list", "inside a list"]]$
• Declaring a variable of list type:
  • Example: $Mylist = [37, 42, 64, 12, 37]$

Accessing Values in a List

• To access or modify a value within a list, use the syntax:
  • $listName[index]$
• Indexing conventions:
  • Starts at 0 and ends at (number of items - 1).
• Access examples:
  • $Mylist = [42, 37, 64, 12, 37]$
  • $Last ext{ }value = Mylist[4] <br /> ightarrow 37$
  • $First ext{ }value = Mylist[0] <br /> ightarrow 42$
• Using variables for indexing:
  • Example:
  • $i = 2$
  • $val = Mylist[i] <br /> ightarrow 64$
  • $val = Mylist[i+1] <br /> ightarrow 12$
• Modifying values in a list:
  • Example:
  • $Mylist[3] = 17 <br /> ightarrow Mylist = [42, 37, 64, 17, 37]$

Handling Out of Bounds Values

• When accessing an element in a list, ensure the index is within valid bounds.
  • Example List: $Mylist = [42, 37, 64, 12, 37]$
• Valid indices range from 0 to 4 in this scenario (5 items total).
• Attempting to access an out of bounds index:
  • Example:
  • $Mylist[6]$ → ERROR: index out of bounds
• Utilize the built-in $len()$ function to determine list size:
  • Example:
  • $print(Mylist[len(Mylist) - 1])$

Printing a List

• The print function can display entire lists in square brackets.
  • Example:
  • $Mylist = [42, 37, 64, 12, 37]$
  • $print(Mylist)$ → Output: $[42, 37, 64, 12, 37]$
• Printing a single value using indexing:
  • Example:
  • $print(Mylist[0])$

Iterating Through a List

• Accessing list values using a for loop allows for read-only access:
  • Example:
  • $for ext{ }name ext{ }in ext{ }MyList:$
    • $print(name)$
• Utilizing a while loop provides control over the loop index:
  • Example:
  • $idx = 0$
  • while ext{ }idx < len(MyList):
    • $print(MyList[idx])$
    • $idx = idx + 1$
• Example List: $MyList = ["Neil", "Garrett", "Stacy", "Vicki", "Magnitude"]$

List Operators

• Available operators for lists include:
  • Concatenation:
  • The + operator combines two lists:
    • Example: $list1 + list2$
  • Repetition:
  • The * operator repeats a list a defined number of times:
    • Example: $list * n$
• Both operators return a new list.

Boolean List Operators

• in: Checks if a list contains a specific value, returning true or false:
  • Example:
  • $if ext{ }"Vicki" ext{ in } MyList:$
    • $print("Found Vicki")$
• not in: Checks if a list does NOT contain a specific value, returning true or false:
  • Example:
  • $if ext{ }"Leonard" ext{ not in } MyList:$
    • $print("Leonard likes this post!")$

List Indexing Operator

• The [] operator retrieves individual values in a list by specifying the index:
  • Can also create slices (subsections of lists).
• Slicing:
  • $myList[start:stop]$
  • Start: First index included
  • Stop: Up to, but NOT including, this index
• Example of slicing:
  • $MyList[1:3]$ → $["Garrett", "Stacy"]$
  • $MyList[:2]$ → $["Neil", "Garrett"]$
  • $MyList[3:]$ → $["Vicki", "Magnitude"]$

In-Class Activity

• Create a program that initializes a list:
  • $[1, 5, 6, 7, 8, 12, 14, 15, 18, 20]$
• Iterate over the list, squaring each element, then slice the list and print elements 2-7:
  • Expected output: $[36, 49, 64, 144, 196, 225]$

Removing Items from a List

• Using the del operator to erase an item by index:
  • Example:
  • $T = [4, 6, 9]$
  • $del T[1] <br /> ightarrow T = [4, 9]$
• Can also remove a slice:
  • Example:
  • $del MyList[2:5]$ (This will remove the slice from index 2 to 5.)

Methods and Functions in Lists

• Two categories exist: functions and methods.
• Functions:
  • A named section of code callable with data input.
  • Example:
  • $mylength = len(mylist)$
• Methods:
  • Special type of function initiated on a variable using the dot operator.
  • Example:
  • $mylist.append(17)$

List Methods

• pop(): Removes an item by index and returns its value.
  • Example:
  • $T = [4, 6, 9]$
  • $x = T.pop(1) <br /> ightarrow T = [4, 9], x = 6$
• remove(value): Removes an item by value (not by index).
  • Example:
  • $T = [4, 6, 9]$
  • $T.remove(6) <br /> ightarrow T = [4, 9]$

Common List Methods

• append(item): Adds an item to the end of the list.
  • Example:
  • $list1 = [8, 43, 120, 4, 33]$
  • $list1.append(45) <br /> ightarrow [8, 43, 120, 4, 33, 45]$
• index(item): Returns index of the first element equal to item; raises ValueError exception if not found.
  • Example:
  • $list1.index(43) <br /> ightarrow 1$
• insert(index, item): Inserts item at a specified index, shifts subsequent elements to the right.
  • Example:
  • $list1.insert(2, 10) <br /> ightarrow [8, 43, 10, 120, 4, 33]$

Additional Methods

• sort(): Sorts list in ascending order.
  • Example:
  • $list1 = [8, 43, 120, 4, 33]$
  • $list1.sort() <br /> ightarrow [4, 8, 33, 43, 120]$
• reverse(): Reverses the order of the list.
  • Example:
  • $list1.reverse() <br /> ightarrow [33, 4, 120, 43, 8]$

List Functions

• len(mylist): Returns the length of the list (number of items).
• max(mylist): Returns the maximum value within the list.
• min(mylist): Returns the minimum value within the list.
• sum(mylist): Returns the sum of the values in the list.
  • Notably, these functions accept a list or a slice of a list as input.

In-Class Activity

• In your main function:
  • Create a list with students' quiz grades:
  • $[92, 89, 76, 68, 72, 90, 87, 97, 85]$
  • Pass this list to a function that calculates the average, which should then be displayed to the user.
  • Do NOT use the built-in $sum()$ function.

Lists and Strings

• Strings in Python share similarity with lists.
• A string can be viewed as a list of characters:
  • Example:
  • $"Spam" <br /> ightarrow [‘S’, ‘p’, ‘a’, ‘m’]$
• A string can be transformed into a list:
  • Example:
  • $A = list("Hello")$ → $A = [‘H’, ‘e’, ‘l’, ‘l’, ‘o’]$

Using Split and Join

• split(): Converts a string into a list based on a specified delimiter.
  • Default delimiter is a space.
  • Example:
  • $S = "Won’t change how mustard tastes"$
    • $myList = S.split() <br /> ightarrow ["Won’t", "change", "how", "mustard", "tastes"]$
• Commonly used to process CSV (comma-separated values) files.
• join(): Reverses the process, combining a list into a string using a specified delimiter.
  • Example:
  • $t = ["Why", "Go", "Greendale?", "Just", "Because!"]$
  • $delim = " "$
    • $s = delim.join(t) <br /> ightarrow "Why go Greendale? Just Because!"$

In-class Activity

• Write a function that takes in three parameters:
  • index1, index2, and a list.
  • Swap the values located at index1 and index2.

Additional In-Class Activity

• Write a function that accepts a list and returns two values:
  • The minimum value in the list.
  • The index of the minimum value.

Challenge Activity

• Develop a selection sort algorithm to sort values in an array:
  • Implement logic to continuously find the minimum value and swap it with the value at index 0, and subsequently with the others in subsequent iterations until the list is sorted.
• Repeats this process for the second and third lowest values, positioning them in the appropriate respective indices.