Introduction to Lists and List Operations

2. Topics • Sequences • Lists • Copying Lists • Processing Lists • Two-Dimensional Lists

3. Sequences • An object that contains multiple items of data • The items are stored in sequence one after another • You can examine and manipulate items stored in it • Python provides different types of sequences, including lists and tuples • Both can hold various types of data • List is mutable - program can change the contents • Tuple is immutable

4. Introduction to Lists • An object that contains multiple data items • Each item is an element • Items can be added and removed • Can pull data in various ways • Has more capabilities than arrays in other languages • Format: list = [item1, item2, etc.] • Example: even_numbers = [2, 4, 6, 8, 10] • Can hold items of different types • info = [‘Alicia’, 27, 1550.87]

5. Introduction to Lists

6. Displaying Lists • print function can be used to display an entire list Example: numbers = [5, 10, 15, 20] print(numbers) Output: [5, 10, 15, 20]

7. list()Function • list() function can convert certain types of objects to lists like the range function does Example 1: numbers = list(range(5)) print(numbers) Output: [0, 1, 2, 3, 4] Example 2: numbers = list(range(1, 10, 2)) print (numbers) Output: [1, 3, 5, 7, 9]

8. 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 • General format: list * n • Variable is left operand, how many times to replicate on right • Example: numbers = [0] * 5 • Produces: [0, 0, 0, 0, 0] • Numbers = [1, 2, 3] * 3 • Produces: [1, 2, 3, 1, 2, 3, 1, 2, 3]

9. Iterating Over a List • Lists and ranges used in for loops have similar structures • You can traverse items over a list using a for loop • Format: for x in list: • Example: numbers = [99, 100, 101, 102] for n in numbers: print(n) Output: 99 100 101 102

10. Indexing • Index: a number specifying the position of an element in a list • Allows 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.

11. Indexing Examples my_list = [10, 20, 30, 40] print (my_list[0], my_list[1], my_list[2], my_list[3]]) Output: 10 20 30 40 my_list = [10, 20, 30, 40] for i in range(4): print(my_list[i]) Output: 10 20 30 40

12. Negative Indexing my_list = [10, 20, 30, 40] print (my_list[-1], my_list[-2], my_list[-3], my_list[-4]]) Output: 40 30 20 10

13. 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 • Can be used to prevent an IndexError exception when iterating over a list with a loop

14. 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

15. Concatenating Lists Example List1 = [1, 2, 3, 4] List2 = [5, 6, 7, 8] List = list1 + list2 Output: [1, 2, 3, 4, 5, 6, 7, 8]

16. List Slicing • Slice: a range of items 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 • Ex: numbers[:3] • If end not specified, len() of list is used for end index • numbers[2:] • Slicing expressions can include a step value and negative indexes relative to end of list

17. List Slicing Example days = [‘Sunday’, ‘Monday’, ‘Tuesday’,\ ‘Wednesday’, ‘Thursday’, ‘Friday’, ‘Saturday’] mid_days = days[2:5] print (mid_days) Output: [‘Tuesday’, ‘Wednesday’, ‘Thursday’ ]

18. Finding Items in Lists with the in Operator • The in operator is used 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

19. In List Example # This program demonstrates the in operator used with a list. # Create a list of product numbers. prod_nums = ['V475', 'F987', 'Q143', 'R688'] # Get a product number to search for. search_value = input('Enter a product number: ') # Determine whether the product number is in the list. if search_value in prod_nums: print(search_value, 'was found in the list.') else: print(search__value, 'was not found in the list.') Output: Enter a product number: V475 V745 was found in the list. Output: Enter a product number: v475 v745 was not found in the list.

20. Exceptions • Error that occurs while a program is running • Usually causes program to abruptly halt • Ex: user enters string when expecting integer • Traceback: error message that gives information regarding line numbers that caused the exception • Indicates the type of exception and brief description of the error that caused exception to be raised

21. Exceptions • Many exceptions can be prevented by careful coding • Example: input validation • Some exceptions cannot be avoided by careful coding • Examples • Trying to convert non-numeric string to an integer

22. Exception Handler • Code that responds when exceptions are raised • Prevents program from crashing • Python uses try/except statement • General format: try: Statements except exceptionName: Statements • try suite: statements that can potentially raise an exception • exceptblock: event handler

23. Exceptions • If statement in try suite raises exception: • Use statements in except clause • Handler immediately following except clause executes • Continue program after try/except statement • Other exceptions: • Program halts with traceback error message • If no exception is raised, handlers are skipped

24. Exceptions Example # This program calculates gross pay. try: # Get the number of hours worked. hours = int(input('How many hours did you work? ')) pay_rate = float(input('Enter your hourly pay rate: ')) gross_pay = hours * pay_rate # Display the gross pay. print('Gross pay: $', format(gross_pay, ',.2f'), sep='') except ValueError: print('ERROR: Hours worked and hourly pay rate must') print('be valid integers.')

25. 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

26. List Append Example # This program demonstrates how the append # method can be used to add items to a list. # First, create an empty list. name_list = [ ] # Create an empty list again = 'Y‘ # Create a variable to control the loop. while again.upper() == 'Y': # Add some names to the list. name = input('Enter a name: ') # Get a name from the user. name_list.append(name) # Append the name to the list. print('Do you want to add another name?') again = input('y = yes, anything else = no: ') print() print('Here are the names you entered.') for name in name_list: print(name)

27. Index List Example # This program demonstrates how to get the index of an item # in a list and # then replace that item with a new item. food = ['Pizza', 'Burgers', 'Chips'] print('Here are the items in the food list print(food) item = input('Which item should I change? ') try: item_index = food.index(item) # Get index in list new_item = input('Enter the new value: ') food[item_index] = new_item # Replace old item print('Here is the revised list:') print(food) except ValueError: print('That item was not found in the list.')

28. List Methods and Useful Built-in Functions • 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

30. List Methods and Useful Built-in Functions • 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 return the item that has the lowest or highest value in a sequence • The sequence is passed as an argument

31. Copying Lists • If you set one list equal to another list they will point to same place in memory • When you make change to one list the other will also change • 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

32. Copying Lists

33. Two-Dimensional Lists • 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

34. Two-Dimensional Lists

35. Two-Dimensional Lists

36. Two-Dimensional List Example Students = [[‘Joe’, ‘Kim’], [‘Sam’, ‘Sue’], [‘Kelly’, ’Chris’]] Output: [[‘Joe’, ‘Kim’], [‘Sam’, ‘Sue’], [‘Kelly’, ’Chris’]] print(student[0]) Output: [‘Joe’, ‘Kim’]