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.
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
.
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.