Hardware Reference
In-Depth Information
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 low of a program according to the
rules of Boolean logic. For more information on Boolean logic, see Chapter 10, “An
Introduction to Scratch”.
he irst line creates a new variable called
goAgain
and sets it to
1
. he second line begins the
loop, and tells Python that
while
the
goAgain
variable is equal to
1
, it should continue to
loop through the following code. As the next few lines are written, they will need to be
indented
by inserting four spaces at the start of each line. hese spaces tell Python which lines are part of
the loop and which lines are outside the loop. If you're using IDLE, the spaces will be inserted
automatically; if you're using a text editor, remember to insert the spaces manually.
In its most basic form, a calculator takes two numbers as input and performs a mathematical
operation on them. To make your calculator work, irst 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: “))
hese 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 fol-
lowing lines, which add, subtract and multiply the two numbers and send the output to the user:
print firstNumber, “added to”, secondNumber, “equals”, Æ
firstNumber + secondNumber
print firstNumber, “minus”, secondNumber, “equals”, Æ
firstNumber - secondNumber
