Skip to content

Variables, expressions and statements: variables

Learning objectives

For teachers

Teaching goals are:

  • Learners have worked with variables
  • Learners understand the purpose of variables
  • Learners have obtained the data type of a variable
  • Learners have heard about what a data type is
  • Learners have practiced convert an equation and a text question to Python

Lesson plan:

Exercises:

  • 5 mins: prior knowledge
  • 5 mins: presentation
  • 15 mins: challenge
  • 5 mins: feedback

Prior questions:

  • What is a data type?
  • Can you name a Python data type?
  • What is an object?
  • What is a variable?
  • What are rules for naming a variables?
  • What are good practices for the name of a variable?
  • What is an operator?
  • Can you name an operator?

Overview

In any non-trivial calculation, we want to let our computer remember things, such as the content of a data file, or the parameters/settings we use in our program. Variables allows one to make a computer store ('remember') information. Here we use our first simple variables.

Also we practice converting an equation and a mathematical problem to Python code. We will discover the helpful modulo operator.

flowchart TD
    python[[Python]]

    %% Give a white background to all nodes, instead of a transparent one
    classDef node fill:#fff,color:#000,stroke:#000

    subgraph sub_programming_language[Programming language]
      interpreted_language[Interpreted language]
      programming_language[Programming language]
      scripting_language[Scripting language]
      interpreter[Interpreter]
      scripts[Scripts]
      text_files[Text files]
    end
    style sub_programming_language fill:#ccf,color:#000,stroke:#fcc

    subgraph sub_programming_terms[Programming terms]
      user_input[User input]
      %% file_io[File I/O]
      %% command_line_arguments[Command-line arguments]
      variables[Variables]
      operators[Operators]
      %% functions[Functions]
      python_packages[Python packages]
      %% graphics[Graphics]
    end 
    style sub_programming_terms fill:#cfc,color:#000,stroke:#fcc

    subgraph sub_uppmax[UPPMAX]
      uppmax[UPPMAX]
      uppmax_modules[UPPMAX modules]
      uppmax_clusters[UPPMAX clusters]
      text_editors[Text editors]
      x_forwarding[X-forwarding]
      remote_desktop[Remote desktop]
    end
    style sub_uppmax fill:#fcc,color:#000,stroke:#fcc

    python --> |is a| programming_language
    python --> |is a| interpreted_language
    python --> |is a| scripting_language

    programming_language --> |uses| text_files
    interpreted_language --> |has a| interpreter
    scripting_language --> |is|interpreted_language
    scripting_language --> |runs| scripts
    scripts --> |are| text_files
    %% scripts --> |can use| command_line_arguments

    python --> |has| python_packages
    python --> |has| variables
    %%python --> |has| functions
    python --> |has| operators
    %%functions --> |use| variables
    operators --> |work on| variables
    %%python_packages --> |allow| graphics
    python_packages --> |allow to use| user_input
    %% python_packages --> |allow to do| file_io
    %% python_packages --> |allow to use| command_line_arguments

    uppmax --> |has| uppmax_clusters
    uppmax_clusters --> |have| uppmax_modules
    uppmax_clusters --> |have| text_editors
    uppmax_clusters --> |allow| x_forwarding
    uppmax_clusters --> |has| remote_desktop
    uppmax_modules --> |allow the use of| python_packages
    uppmax_modules --> |allow the use of| interpreter

    text_editors --> |work on|text_files
    %%x_forwarding --> |allows|graphics
    %%remote_desktop --> |allows|graphics


    variables ~~~ uppmax

Exercises

See the exercise procedure here.

Exercise 1: working with variables

Learning objectives

  • get the type of an object
  • create a variable
  • use a variable

Read the following sections of How to Think Like a Computer Scientist: Learning with Python 3:

  • 2.1. Values and data types
  • 2.2. Variables
  • 2.3. Variable names and keywords

Then in section 2.14, do exercises 1 to and including 4.

2.14.1

Take the sentence: 'All work and no play makes Jack a dull boy'. Store each word in a separate variable, then print out the sentence on one line using print.

Answer 
a = 'All'
b = 'work'
c = 'and'
d = 'no'
e = 'play'
f = 'makes'
g = 'Jack'
h = 'a'
i = 'dull'
j = 'boy'
print(a, b, c, d, e, f, g, h, i, j)

2.14.2

Add parenthesis to the expression 6 * 1 - 2 to change its value from 4 to -6.

Answer 
6 * (1 - 2)

2.14.3

Place a comment before a line of code that previously worked, and record what happens when you rerun the program.

Answer

I use this code as a starting point:

a = 'All'
b = 'work'
c = 'and'
d = 'no'
e = 'play'
f = 'makes'
g = 'Jack'
h = 'a'
i = 'dull'
j = 'boy'
print(a, b, c, d, e, f, g, h, i, j)

Commenting out the first line:

    print(a, b, c, d, e, f, g, h, i, j)
          ^
NameError: name 'a' is not defined

Commenting out the last line results in no sentence being printed.

2.14.4

Start the Python interpreter and enter bruce + 4 at the prompt. This will give you an error:

NameError: name 'bruce' is not defined

Assign a value to bruce so that bruce + 4 evaluates to 10.

Answer

Here is a solution:

bruce = 6
print(bruce + 4)