Raspberry Pi A, User Manual, Page: 198 / 314

P A R T I I I
P R O G R A M M I N G W I T H T H E R A S P B E R R Y P I
182
In its most basic form, a calculator takes two numbers as input and performs a mathematical
operation on them. To make your calculator work, first take the two numbers from the user
with the following lines:
firstNumber = int(raw_input(“Type the first number: “))
secondNumber = int(raw_input(“Type the second number: “))
These lines not only use the
raw_input
instruction to ask for two numbers, but they also use
int
. Short for integer , the
int
instruction tells Python to treat input as a number rather than a
string. Obviously, this is important for a calculator program since it won’t be calculating words.
With the two numbers stored as variables, the program can perform its calculations. Type
the following lines, which add, subtract and multiply the two numbers and send the output
to the user:
print firstNumber, “added to”, secondNumber, “equals”,
↵
first secondNumber
print firstNumber, “minus”, secondNumber, “equals”,
↵
firstNumber - secondNumber
print firstNumber, “multiplied by”, secondNumber, “equals”,
↵
firstNumber * secondNumber
Notice that while the addition and subtraction operations use the expected plus and minus
symbols, multiplication uses the
*
symbol. Also notice that there are no formatting spaces
between the quotation marks. This is because Python automatically adds spaces where required
Why ==?
Previously, you’ve been using a single equals symbol to set the value of variables. The
while
loop, however, uses two. Using two equals symbols next to each other performs an
evaluation , which compares a variable’s value to whatever follows. A single equals symbol
instead sets the variable to the value that follows.
There are other evaluations as well as the double-equals, which is only true if the variable
matches the given value exactly:
>
means greater than,
<
means less than,
>=
means
greater than or equal to,
<=
means less than or equal to, and
!=
means not equal to.
Using these evaluation symbols, you can control the flow of a program according to the
rules of Boolean logic. For more information on Boolean logic, see Chapter 11, “An
Introduction to Scratch”.