2.1. Assigning a Name¶
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.
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.
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.
To see it in context, run the program below. The first line sets a value for a, but does not
produce any output we can see. The print command on the second line will show print out a value
so we can see it. In this case, it prints 4 since that is the value stored in a.
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.
Changing the value of a variable in memory.¶
Try modifying the code above. Add a new line of code, after a = 4 and before print(a),
that sets a to 7.2. If you do so correctly, the program will print 7.2 when it runs.
Warning
Although we use the = symbol to do assignment, you should not think of it as “equals”.
Most programmers will pronounce a = 4 as “a gets 4” or “a is assigned the value 4”.
Assignment always takes the value of the right side of the = and stores it into
the variable named on the left hand side. It is actually an error to switch the order and
try to write 4 = a - that says “4 is assigned the value a” which does not make
sense… we can’t change the value of 4.