LESSON 03 – LISTS

 

 

LESSON NOTE/GUIDE

PROGRAM 1 – CREATING A LIST

·       Create a list that contains numbers.  Output a few of the numbers in the list.


THE CODE

 

#Creating a list of numbers.

numbers = [4,6,9,2,0]

 

#We can print any element

print(numbers[0])   #will output 4

print(numbers[3])   #will output 2

#We can also print the whole list

print(numbers)      #will output [4,6,9,2,0]

 


EXPLANATION

 

It is often necessary to work with large amounts of data.  Creating a variable name for each value would be very difficult.  Instead, we create a single name and store all the data under that name.  This is called a data structure and there are many different types of these.  In Python, the most commonly used data structure is the list.

 

The first statement above creates a list called numbers.  We know it’s a list because of the square brackets used around the values.  This list contains five values that we tend to refer to as elements.

 

Each element in a list is associated with an index number.  For example, the first element which has value 4, has the index number 0.  The next element, with value 6, has index 1.  And so on…

 

In the second statement, we see how we can access the value of an element.  We simply use the list name followed by the index number inside square brackets.  In this case, we access index 0 and print it out to screen.

 

In the third statement, we output element with index 3 to screen.

 

In the fourth statement, we output the entire list.

 

It is important to note that index numbers in Python start at zero.

 

 

PROGRAM 2 – CREATING A LIST WITH A RANGE

  • Create a list using the range function.

 


THE CODE

 

numbers = list(range(1, 10))

print(numbers)

 

EXPLANATION

 

In the first statement, we create a sequence from 1 to 9 (10 is not included).  We then convert that to a list.

 

The output will be:

 

[1, 2, 3, 4, 5, 6, 7, 8, 9]

 

So using range, we can conveniently create lists of consecutive numbers.

 

 

 

PROGRAM 3 – GETTING VALUES FROM A LIST

  • Create a list that contains strings.   Use those strings in a few statements.

 


THE CODE

 

veggies = ["carrots", "onions", "peas"]

print("I love", veggies[2])

print ("I hate", veggies[1])

 

 

EXPLANATION

 

We can create lists that contain strings as values.

 

The first statement creates a list called veggies.  It contains three elements that are all strings (as opposed to the numbers in the first program).

 

The second statement outputs “I love peas” and the third statement outputs “I hate onions”.

 

 

PROGRAM 4 – SETTING VALUES IN A LIST

  • Create a list that contains 5 numbers.  Then change the values of a few of the elements in the list.

 

THE CODE

 

my_list = [4, 9, 12, -4, 7]

print(my_list)

 

my_list[0] = 2

my_list[3] = my_list[1]*2

 

print(my_list)

 

 

EXPLANATION

 

 

The program will output:

 

[4, 9, 12, -4, 7]

[2, 9, 12, 18, 7]

 

Here is a line by line explanation:

 

The first line creates a list called mylist with five numbers in it.  The second line outputs it to screen.

 

The third line sets the value of element 0 to 2.  So the value 4 is now gone and replaced by a 2.

 

The fourth line will set the value of element 3.  It will be set to the value of element 1 times 2.  Since element 1 has a value of 9, and since 9 times 2 is 18, then element 3 gets the value 18.

 

The last line simply outputs the list with the new values for elements 0 and 3 showing.

 

 

PROGRAM 5 – LENGTH OF A LIST

  • Create a list that contains every student in the class’ favourite number.
  • Use the length function to see how many numbers are in the list.  In this case, this will give you the number of students in the class.

 


THE CODE

 

fnlist = [4,5,8,7,2,4,25,6,7,4,1,99,9,1]

    

count = len(fnlist)

    print("There are", count, "numbers in the list.")

 

EXPLANATION

 

The first line of code simply creates a list called fnlist which is short for favourite numbers list that contains the favourite number of each student in the class.

 

The next line makes use of the len function to get the length of the list.  This is the number of elements in the list.  This value gets stored in the variable called count.

 

The final line simply outputs the value of count along with a message.

 

 

PROGRAM 6 – GETTING A RANDOM VALUE FROM A LIST

  • Create a list that contains possible treasures (for an RPG game).
  • Use randomness to randomly select one of the items.

 


THE CODE

 

import random

 

treasureItems = ["50 gold", "a sword", "boots", "a skull"]

rn = random.randint(0,3)

print("You open the chest and find", treasureItems[rn])

 

EXPLANATION

 

The first line of code simply imports the random library.

 

The second line of code creates a list called treasureItems that contains 4 elements.  Each element is a possible treasure find in a game.

 

The third line of code generates a random integer between 0 and 3 inclusively.  Those numbers are specifically chosen because the possible index values for the list treasureItems range from 0 to 3. 

 

The last line outputs a message that ends with a randomly selected item from the list.

 

PROBLEM WITH THE ABOVE CODE

 

The problem with the above program is that if the programmer changes the amount of items in the list, then the range for the random number also needs to be updated – more specifically, the upper value of the range needs to be updated.  While this may seem easy in our short program, in a real application, those two lines of code may be thousands of lines apart from one another.

 

THE SOLUTION TO THE PROBLEM

 

The solution is to tell Python to calculate the upper value of the range for the random number.  This upper value is always the length of the list minus one.

 

So, the upper value should be:

 

     len(listname) - 1

 

 

IMPROVED CODE

 

Below, the code in red is the code that has been changed from the original solution.

 

import random

 

treasureItems = ["50 gold", "a sword", "boots", "a skull"]

rn = random.randint(0, len(treasureItems) - 1)

print("You open the chest and find", treasureItems[rn])

 

 

PROGRAM 7 – ADDING TO A LIST

  • Create a list that contains the four coolest people in class.  J
  • Add one more person at the end of the list using append.
  • Add one more person at a specific location using insert.

 


THE CODE

 

coolPeople = ["Maddy", "Alyx", "Nicky", "Matti"]

 

#we can add at the end by using append

coolPeople.append("Scott")

print(coolPeople)

 

#we can add anywhere using insert

coolPeople.insert(0, "Jay")

print(coolPeople)

 

EXPLANATION

 

The first line creates a list called coolPeople with four names in it.

 

The second line of code adds the name “Scott” to the list by using the append method.

 

The third line of code outputs the list to screen:

 

["Maddy", "Alyx", "Nicky", "Matti", "Scott"]

 

The fourth line of code inserts “Jay” at element zero.  It shuffles everything afterwards.

 

The fifth line of code outputs the list to screen:

 

["Jay", "Maddy", "Alyx", "Nicky", "Matti", "Scott"]

 

 

PROGRAM 8 – REMOVING FROM A LIST

  • Create a list that contains some of the coolest people in class.  J
  • Remove the least cool person from that list (haha) by using remove.
  • Remove a person by using the del function

 


THE CODE

 

coolPeople = ["Noah", "Emma", "Nicky", "Matti"]

 

#we can remove from the list using remove and the value

coolPeople.remove("Matti")

print(coolPeople)

 

#we can remove an element using del and the index number

del(coolPeople[1])

print(coolPeople)

 

EXPLANATION

 

The first line creates a list called coolPeople with four names in it.

 

The second line removes the element with “Matti” in it.

 

The third line outputs the list to screen:

 

["Noah", "Emma", "Nicky"]

 

The fourth line deletes the element at index 1 (which contains “Alyx”).

 

The fifth line outputs the list to screen:

 

["Noah", "Nicky"]

 

 

PROGRAM 9 – STARTING WITH AN EMPTY LIST

  • Create an empty list and add items into it one at a time.

 


THE CODE

 

swlist = []

newname = "Leia"

swlist.append(newname)

newname = "Luke"

swlist.append(newname)

newname = "Han"

swlist.append(newname)

newname = "Ahsoka"

swlist.append(newname)

print(swlist)

 

 

EXPLANATION

 

The first line creates an empty list named swlist.

 

The second line creates a variable with the value “Leia” stored in it.  The third line then adds the value of the variable to the end of the list using the append method.

 

Lines 4 and 5 append the word “Luke” to the end of the list.

 

Lines 6 and 7 append “Han” to the end of the list.

 

Lines 8 and 9 append “Ahsoka” to the end of the list.

 

Line 10 prints the list out to screen.

 

 

FUTURE PROGRAM 10 – CHECKING IF A LIST CONTAINS A VALUE

 

data = [2, 6, 4, 8, 0]

if 2 in data:

   print (“yep”)