# 3.1.7. Conditional Sentences¶

## 3.1.7.1. Semantics of Conditional Sentences¶

• Conditional Sentences, like functions affect the flow of control

• Conditional Sentences affect the flow of control by values

## 3.1.7.2. Syntax of Conditional Sentences¶

• Conditional Sentences are compound sentences consisting of one or more clauses headed by the keywords:

• if

• elif

• else

• Each if or elif clause is followed by a Logical Expression and a colon :

• If the Logical Expression for a clause is True the body of the clause is executed

• The body must be indented by 4 spaces (or a tab)

## 3.1.7.3. Examples of Conditional Sentences¶

def greet(friend, money):        # Function has two predicates
if friend and (money > 20):
print "Hi!"
money -= 20
elif friend:
print "Hello!"
else:
print "Ha ha!"
money += 10
return money

money = 15

money = greet(True, money)
print "Money:", money
print ""

money = greet(False, money)
print "Money:", money
print ""

money = greet(True, money)
print "Money:", money
print ""

Hello!
Money: 15

Ha ha!
Money: 25

Hi!
Money: 5

# Conditionals Examples

# Return True if year is a leap year, False otherwise

def is_leap_year(year):          # Testing a predicate - convention to use is_function_name
if(year % 400) == 0:
return True
elif (year % 100) == 0:
return False
elif (year % 4) == 0:
return True
else:
return False

year = 2010
leap_year = is_leap_year(year)

if leap_year:
print year, "is a leap year"
else:
print year, "is not a leap year"

2010 is not a leap year