Decisions in Python
If / else
A two-way branch
• Many times you need to test for a condition
and do actions BOTH if it is True or if it is False
• This is where the if/else statement becomes
useful
Syntax of if/else
• First part is an if statement with condition and
colon
• Then the statements which will be done if the
condition is True, indented as usual
• Then the keyword else and a colon, lined up
under the if statement
• Then the statements which will be done if the
condition is False, indented as the other one is
Example
• The two branches cannot be empty, they can
be as small as one statement but they have to
have something in each
• if x > 0:
print(“ok”)
else:
print(“sorry”)
Semantics of if/else
• The condition at the if line is always executed,
as with a plain if statement
• If the condition is True, the statements
indented under the if line are executed
• If the condition is False, the statements
indented under the else line are executed
• Exactly one of the two branches will be
executed on any particular run of the program
Alignment of else with if
• You do not have to have an else with every if
statement.
• BUT if you have an else: you must have an if to
go with it. You cannot start a statement with
else!
• The indentation determines which if that an
else goes with
Do these do the same thing?
if x > y:
if y > 25:
print(“a”)
else:
print(“b”)
if x > y:
if y > 25:
print(“a”)
else:
print(“b”)
“Factoring out”
• If a statement appears in both branches, at the same
point in execution, then ‘factor it out’ and put it just
one time either before or after the if/else statement
• if y + z > x:
print(“ok”)
t=1
else:
print(“ok”)
t=2
• The print(“ok”) statement can be done before the if
statement starts
Shortcut for Booleans
• If you are writing an if statement to test a
Boolean variable, you can write the condition as a
‘short cut’
• if bool_var == True: can be written as just
if bool_var:
• Remember that the if statement needs a Boolean
value at that point and the bool_var is assumed
to contain one, so comparing it to True is not
necessary
• If you needed the opposite test, you can write
“if not bool_var:”