Learning Goals

By the end of the lesson I will be able to:

• Define the term variable
• Identify and use the standard data types associated with Python code
• Write the code necessary to store data into a variable in Python

A variable declaration is, in general, composed of three parts:

1. type
2. label
3. value/assignment

Types

Every variable has a type. The type determines the types of actions you can take using the data. There are four most common types of data in computing:

1. integer - a number with no fractional decimal component. This means there is no . in the number. Examples: 0, 42, -1056
2. real/floating point number - a number that may or may not contain a fractional decimal component. Examples: 1.01, 0.33333333333333333, 3.01x10^-3.
3. strings - a sequence of characters grouped together. A character is any one letter, number, or punctuation. Strings are surrounded by quotation marks (usually double, but in some cases single quotes work). Examples: "Hello World!" "My name is Mr. Rivard" "Pi is equal to 3.14159"
4. boolean - a data type that has only two values: True or False. These data types are used to make decisions.

Label

The label is the name we give to a variable. Variables should always be named carefully and appropriate to the data that is being stored inside it. If we are storing someone's name into a variable, an appropriate label would be "name." Examples of good variable names:

• interestRate = 0.02
• name = "Sean"
• age = currentYear - birthYear

In most programming languages, including Python, the label used for a variable can only have alphanumeric (a to z, A to Z and 0 to 9) characters and underscores (_) in them. In addition, the label MUST start with a letter.

Declaring Variables

The process of choosing a location and giving it a label is called declaring a variable. We can set values into those locations immediately. This is helpful if you know something has an initial value. For example, if you are writing a program that counts to 100, you know that you want to start at a particular value, either 0 or 1. So, when we declare the variable, we can assign it a value immediately, then use it or change it later.

IMPORTANT: A variable must be given a value BEFORE it is used. If I told you to get the contents of bucket 4 in the example above before putting anything in there, what would happen? The same is true for variables in computers. If you attempt to use a variable that is not declared or does not have a value stored in it, you will see an error message.

In Python, variables are declared at the same time they are first given a value. We will explore that next.

Value/Assignment

The value of a variable is the data that is stored in the container.

Assignment is the process of putting data into a variable.

Examples:

Analogy:

So now we have 2 buckets, each storing some data that we may need later.

In programming we do something similar: Choose a location (bucket), give it a label, then fill it with information. To fill it, we write our label first, use a single equal sign (=), followed by the value we wish to store.

Python:

Here are some examples of storing a value into a variable in Python:

i = 0

s = "Hello World!"

f = 0.1

b = True

i = 1

s = "I am changing my value!"

f = i

i = s

In our first four lines, we are declaring four variables:

• i, an integer
• s a string
• f a floating point number
• b a boolean

We are also storing values into those variables immediately, thus i contains 0, s contains "Hello World!", f contains 0.1 and b contains True. Notice that we did not tell Python about the type of each variable. Python determines the type automatically based on the value assigned.

Note that "Hello World!" is in quotations. This is because a string must be surrounded by quotes to be valid. In fact, 0 and "0" are two different values; 0 is the integer representation of the number 0 (00000000_{base 2}), whereas "0" is a string represented by the 0 character (00110000_{base 2}).

The next two lines are variable assignments of variables that have been previously declared. Think of this like changing what we are storing in the bucket: The old value is removed in order to store the new value.

The last two lines show us that we can store values from one variable to another. On the line "f = i" f is no longer a floating point number! Now it is an integer!

The last line has a similar effect: the value stored in i is now a string!

Naming Conventions:

Most programming languages try to use a common way of creating labels so that we can easily identify a variable. Consistency of variable names is a key component in making your code readable.

In Python, convention dictates that the underscores be used between words and we write everything in lower case. For example:

• A variable containing the interest rate becomes interest_rate
• Day of the Week becomes day_of_the_week

Additional Reading and Work

An Informal Introduction to Python - Basic Python mathematical and string operations. Read through 3, 3.1, 3.1.1 and 3.1.2 and try it out in your development environment.

Also read this article on string slicing.

1. Create a Python file which does the following:

• Demonstrate simple mathematical operations:
  • Addition
  • Subtraction
  • Multiplication
  • Division
  • Floor Division
  • Modulus Division (Remainder)
  • Exponents
• Store the value of a mathematical operation into a variable and print the variable to the console.

2. Create a Python file which does the following:

• Create a variable which stores the word "hamburger"
• Use the stored word to do the following:
  • Print the first character of the variable.
  • Print the last character of the variable.
  • Print the word backwards.
  • Print the sub-word "ham" and "burger" on two separate lines.
  • Print every other letter in the word
  • Print every other letter in the word, backwards

3. Identify one reason you may want to use Modulus division.