Assigning a Name¶

Learning Objectives:

Understand the concept of a variable.
Assign a value to a variable.
Use assignment in calculations.
Understand the ways that students get assignments wrong.
Reuse variables across different assignment statements.

A computer can associate a name with a value. It does this by creating a variable, which is space in computer memory that can represent a value. An example of a variable is a score in a computer game. The score usually starts at 0 and increases as you play the game. The score can change or vary during the game, which is why we call it a variable. You also associate a name with a value when you enter a new contact name and phone number in your cell phone. When you tell your phone to call “Alexa” it will look up the phone number associated with that name and call it.

Figure 1: A pong game in Scratch with a score shown in the upper left.

Think of a variable as a box that has a label on it and you can store a value in the box. The value can be anything that can be represented on a computer and stored in a computer’s memory. A computer’s memory is only made up of numbers (really, just patterns of voltages, but we can think about them as numbers). Everything that a computer can remember in its memory is translated into these numbers – but don’t worry about how this works right now.

Figure 2: Creating a variable and setting its value in memory.

In programming languages, setting a variable’s value is also called assignment. A statement like a = 4 means that the symbol a refers to space (in the computer’s memory) that is assigned the value 4. When we use the symbol a in a program the computer will substitute the value 4. If we later change the value stored at a, say by doing a = 7.2 then we say that the variable a now has the value 7.2 meaning that the value in the box (memory) associated with the name a is changed to 7.2.

Figure 3: Changing the value of a variable in memory

Click on the right arrow below to play the following video.

Legal Variable Names¶

There are restrictions on what you can use as a variable name.

It must start with a letter (uppercase like A or lowercase like a) or an underscore _
It can also contain digits, like 1 or 9, just not as the first character
It can’t be a Python keyword such as and, def, elif, else, for, if, import, in, not, or, return, or while. These have special meaning in Python and are part of the language.
Case does matter. A variable named result is not the same as one named Result.

Since you can’t have spaces in a variable name you can either join words together by uppercasing the first letter of each new word like heightInInches or use underscores between words height_in_inches. Uppercasing the first letter of each new word is called camel-case or mixed-case.

Note

Different programming language communities have different conventions or styles in addition to the legal rules defined by the language. Camel-case is commonly used for naming variables in the Java programming language, for example, while in Python underscores between words is preferred. Python even has its own Style Guide for Python Code.

not

_a1
You can use an underscore as the first character in a variable name
my_name
You can use an underscore between words in a variable name.
amountOfStuff
You can use both uppercase and lowercase letters in a variable name.
BMP
You can use only uppercase letters in a variable name.
1A
You can't use a digit as the first letter in a variable name.

not

_my_name
This is legal, but you don't usually start a variable name with an underscore.
my name
You can't have a space in a variable name.
myname
This may be hard to read, but it is legal.
myName
Since you can't have spaces in names, one way to make variable names easier to read is to use camel case (uppercase the first letter of each new word).
my_name
Since you can't have spaces in names, one way to make variable names easier to read is to use an underscore between two words.

Next Section - Expressions