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Lists

References:

Lists

Make a list when you want to store multiple items as a single variable.

In this exercise, we'll use lists to keep track of students' grades.

StudentExam 1Exam 2
Brooke 85 95
Coen 90 100
Blas 80 100
George 50 96

Notes and Code
Outcome

I. Creating Lists

We'll make three lists. One for the student names, and one each for the two exams. 


                students = ['Brooke', 'Coen', 'Blas', 'George']
                exam1 = [85, 90, 80, 50]
                exam2 = [ 95, 100, 100, 96]

Observe:

If we'd like to print out the whole list we can just write: 


                  print(students)
which gives, 

                  ['Brooke', 'Coen', 'Blas', 'George']

However, if we want to print out just one of the items in the list, we have to reference it by its index. Indexing starts with 0 so to get the first student's name we'd use: 


                  print(students[0])
which gives, 

                  Brooke

Assignment
Write a program that prints out Blas' name and his grade for exam 1.
Answer ▼
Assignment
Make a list with the days of the week and use the list to have it print out: 

                    Monday and Friday
Answer ▼
Assignment
Make a list with the first 10 multiples of 2 (2, 4, 6...), and have it print out the last number.
Answer ▼

II. Referencing Items in a List

So the program: 

                students = ['Brooke', 'Coen', 'Blas', 'George']
                print('students[0]:', students[0])
                print('students[1]:', students[1])
                print('students[-1]:', students[-1])
                print('students[-2]:', students[-2])
                print('students[1:3]:', students[1:3])
                print('students[:2]:', students[:2])
                print('students[1:]:', students[1:])

Outputs:


                students[0]: Brooke
                students[1]: Coen
                students[-1]: George
                students[-2]: Blas
                students[1:3]: ['Coen', 'Blas']
                students[:2]: ['Brooke', 'Coen']
                students[1:]: ['Coen', 'Blas', 'George']
Assignment
Print the name of the second to last student using both forward and backward indexing :
Result ▼
Code ▼
Assignment
Print the Exam 1 grades of the last two students using slicing:
Result ▼
Code ▼

III. Looping (Iterating) Through Lists

You can quickly go through each item in a list using a for ... in ... loop. 


                students = ['Brooke', 'Coen', 'Blas', 'George']

                for s in students:
                    print( s, 'is a student')

In this case, the variable s takes the value of each item in the list.

Outputs:


                Brooke is a student
                Coen is a student
                Blas is a student
                George is a student

It is sometimes necessary to get the index as well as the item while you go through the list, so you can reference parallel items in other lists.

Let's loop through the three lists and print out the total grade of each student (Exam 1 + Exam 2). We use the enumerate function to get the index and the item simultaneously. 


                students = ['Brooke', 'Coen', 'Blas', 'George']
                exam1 = [85, 90, 80, 50]
                exam2 = [ 95, 100, 100, 96]

                for i, s in enumerate(students):
                    total = exam1[i] + exam2[i]
                    print( i, s, total)

i becomes the index, and s is the value from the enumerated list.

Outputs:


                0 Brooke 180
                1 Coen 190
                2 Blas 180
                3 George 146
Assignment
Find, and print out, the student name and their average score from the two exams:
Result ▼
Code ▼
Assignment
For each of the students, print their name, grades on both exams, their total score, and their average score.:
Result ▼
Code ▼
Fancy Column Code ▼

IV. Creating Lists with Loops

Use for loops to create numerical lists.

To create a list with the first 20 multiples of 3, we can first create an empty list (nums = [] ) and then append items using a loop. 


                  nums = []
                  for i in range(20):
                      n = i * 3
                      nums.append(n)
                  print(nums)

Outputs:


                  [0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57]

You will have noticed that the output is not quite right, since it starts at zero instead of three. You can fix this either by changing the range function to range(1, 21) or by changing the calculation of n to n = i * 3 + 3

Assignment
Make a list of the first 10 multiples of 5.
Result ▼
Code ▼
Assignment
Create two lists, one of the first 10 multiples of 4, and the second of the first 10 multiples of 7. Then use these two lists to create a new list of the products of the corresponding numbers from only the first five numbers of the two lists:
Result ▼
Code ▼