GEOG 485:
GIS Programming and Automation

1.5.1 Working with variables

PrintPrint

It’s time to get some practice with some beginning programming concepts that will help you write some simple scripts in Python by the end of Lesson 1. We’ll start by looking at variables.

Remember your first introductory algebra class where you learned that a letter could represent any number, like in the statement x + 3? This may have been your first exposure to variables. (Sorry if the memory is traumatic!) In computer science, variables represent values or objects you want the computer to store in its memory for use later in the program.

Variables are frequently used to represent not only numbers, but also text and “Boolean” values (‘true’ or ‘false’). A variable might be used to store input from the program’s user, to store values returned from another program, to represent constant values, and so on.

Variables make your code readable and flexible. If you hard-code your values, meaning that you always use the literal value, your code is useful only in one particular scenario. You could manually change the values in your code to fit a different scenario, but this is tedious and exposes you to greater risk of making a mistake (suppose you forget to change a value). Variables, on the other hand, allow your code to be useful in many scenarios and are easy to parameterize, meaning you can let users change the values to whatever they need.

To see some variables in action, open PythonWin and type this in the Interactive Window:

>>> x = 2

You’ve just created, or declared, a variable, x, and set its value to 2. In some strongly-typed programming languages, such as Java, you would be required to tell the program that you were creating a numerical variable, but Python assumes this when it sees the 2.

When you hit Enter, nothing happens, but the program now has this variable in memory. To prove this, type:

>>> x + 3

You see the answer of this mathematical expression, 5, appear immediately in the Interactive Window, proving that your variable was remembered and used.

You can also use the print command to write the results of operations. We’ll use this a lot when practicing and testing code.

>>>print x + 3
5

Variables can also represent words, or strings, as they are referred to by programmers. Try typing this in the Interactive Window:

>>>myTeam = "Nittany Lions"
>>>print myTeam
Nittany Lions

In this example, the quotation marks tell Python that you are declaring a string variable. Python is a powerful language for working with strings. A very simple example of string manipulation is to add, or concatenate, two strings, like this:

>>> string1 = "We are "
>>> string2 = "Penn State!"
>>> print string1 + string2
We are Penn State!

You can include a number in a string variable by putting it in quotes, but you must thereafter treat it like a string; you cannot treat it like a number. For example, this results in an error:

>>>myValue = "3"
>>>print myValue + 2

In these examples you’ve seen the use of the = sign to assign the value of the variable. You can always reassign the variable. For example:

>>> x = 5
>>> x = x - 2
>>> print x
3

When naming your variables, the following tips will help you avoid errors.

  • Variable names are case sensitive. myVariable is a different variable than MyVariable.
  • Variable names cannot contain spaces.
  • Variable names cannot begin with a number.
  • A recommended practice for Python variables is to name the variable beginning with a lower-case letter, then begin each subsequent word with a capital letter. This is sometimes known as camel casing. For example: myVariable, mySecondVariable, roadsTable, bufferField1, etc.
  • Variables cannot be any of the special Python reserved words such as "import" or "print."

Make variable names meaningful so that others can easily read your code. This will also help you read your code and avoid making mistakes.

You’ll get plenty of experience working with variables throughout this course and will learn more in future lessons.

Readings

Read Zandbergen chapter 4.5 (Variables and naming).

  • Chapter 4 covers the basics of Python syntax, loops, strings and other things which we will look at in more detail in Lesson 2 but feel free to read ahead a little now if you have time or come back to it at the end of Lesson 1 to prepare for Lesson 2.