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Python if statements
Girls’ Programming Network
School of Information Technologies
University of Sydney
Mini-lecture 5
Control Structures
if statements
Summary
2
Outline
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Control Structures
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if statements
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Summary
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
3
End-to-end programs aren’t very useful
ˆ So far our Python programs have been pretty boring
ˆ So far the interpreter has only run a single statement at a time
ˆ Every time we run the program the statements are executed in
the same order
ˆ We need the computer to do more work for us!
ˆ For our programs to be more useful they need to:
ˆ make simple decisions on our behalf to execute certain
statements
ˆ repeat statements over and over again
ˆ Computers are great at doing this until you pull out the plug
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
4
Control structures change program execution
ˆ Control structures control one or more statements by
changing the order (or whether) statements are executed
ˆ That is, they change the flow of control within a program
ˆ The decision control structure is called an if statement
ˆ The repetition control structure is called a loop statement
ˆ All imperative programming languages (like Python) provide
these two
ˆ Most languages provide very similar looking if statements
ˆ But there are many varieties of loop statement
ˆ Today we will cover Python’s if statement
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
5
A Python if statement in action
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>>> x = 3
>>> if x == 3:
...
print 'x is equal to 3'
...
x is equal to 3
>>>
ˆ First we create a variable x set to the value 3
ˆ An if statement starts with the keyword if
All Python control structures start with a keyword
ˆ Next is the conditional expression x == 3 and then a colon
ˆ A conditional expression either evaluates to True or False
ˆ Here the comparison operator == tests if x is equal to 3
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
6
Python uses indentation to indicate blocks
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>>> x = 3
>>> if x == 3:
...
print 'x is equal to 3'
...
x is equal to 3
>>>
ˆ The conditional expression is True since we set x to 3
ˆ This means the body of the if statement is executed
ˆ The body is a block of code controlled by the if statement
ˆ Python uses indentation to identify blocks
ˆ Here the body is a single print statement
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
7
The if body is not executed on False
ˆ Let’s now set the variable x to 4 and try again:
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>>> x = 4
>>> if x == 3:
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print 'x is equal to 3'
...
>>>
ˆ Notice this time the print statement does not get run
ˆ That’s because the conditional x == 3 is now False
ˆ So the if statement body is skipped
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
8
A block can contain one or more statements
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>>> x = 10
>>> if x > 3:
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print 'x is bigger than 3'
...
if x > 6:
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print 'x is also bigger than 6'
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x is bigger than 3
x is also bigger than 6
ˆ Here one if statement is controlled by another if statement
ˆ We can tell this because the second if statement is indented
ˆ So the first if statement body consists of two statements:
a print statement and a nested if statement
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
9
The else clause is executed on False
ˆ if statements can have an else clause:
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>>> x = 4
>>> if x == 3:
...
print 'x is equal to 3'
... else:
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print 'x is not equal to 3'
x is not equal to 3
>>>
ˆ Notice again, the else keyword is followed by a colon
All Python control structures end with a colon
ˆ We could write this using two if statements, with the second
having x != 3 as the conditional expression
ˆ else is more efficient and avoids redundancy in the logic
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
10
Deeply nested ifs quickly becomes messy
ˆ If we want to consider more alternatives we must nest ifs:
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>>> if x < 3:
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print "x is less than three"
... else:
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if x == 3:
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print "x is equal to three"
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else:
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print "x is greater than three"
...
>>>
ˆ This can get ugly once there are many alternatives
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
11
elif clauses avoid nested ifs
ˆ We can write the previous example more elegantly as:
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>>> if x < 3:
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print "x is less than three"
... elif x == 3:
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print "x is equal to three"
... else:
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print "x is greater than three"
...
>>>
ˆ Each conditional expression is evaluated until one is True
ˆ The corresponding block is then executed
ˆ If none of the if or elif conditionals are True
the else block is run
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
12
and and or
ˆ There are two additional keywords that are quite useful, and
and or.
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>>> if name == "David" or name == "Paul":
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print "That's my brother's name!"
ˆ How would you write this without using or?
ˆ You can test conditional expressions in the interpreter:
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>>> True and True
True
>>> True or False
True
>>> False and True
False
Girls’ Programming Network
if statements
Mini-lecture 5
Control Structures
if statements
Summary
13
You should now be able to:
ˆ Explain what a control structure is
ˆ Write Python if/elif/else statements
Girls’ Programming Network
if statements
Mini-lecture 5