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Python while loops
Girls’ Programming Network
School of Information Technologies
University of Sydney
Mini-lecture 6
Loops
while loops
Summary
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Outline
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Loops
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while loops
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Summary
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
3
Our programs usually don’t run for very long
• Because we can only execute a few lines and then the
program finishes
• We could make it longer by duplicating the code
• But it would always be a fixed number of lines
• So we need some way of doing things over and over again
• The idea of repetition is called looping or iteration
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
4
Different languages, different loops
• All procedural languages provide control structures for:
• making decisions (if statements)
• repeating things over and over
(while, for, do-while, repeat-until loops)
• Almost all procedural languages have if statements
• But there’s lots of variation across languages about loops
• Python has two types of loops: while loops and for loops
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
5
A Python while loop in action
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>>> i = 0
>>> while i < 3:
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print i, 'is less than 3'
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i += 1
...
0 is less than 3
1 is less than 3
2 is less than 3
>>>
• A while loop starts with the keyword while
Remember, all Python control structures start with a keyword
• Next is the conditional expression i < 3 and then a colon
Watch out for missing the colon!
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
5
A Python while loop in action
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>>> i = 0
>>> while i < 3:
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print i, 'is less than 3'
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i += 1
...
0 is less than 3
1 is less than 3
2 is less than 3
>>>
• A conditional expression is either evaluates to True or False
• Here the comparison operator < tests if i is less than 3
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
6
A while loop simply a repeating if statement
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>>> i = 0
>>> while i < 3:
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print i, 'is less than 3'
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i += 1
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0 is less than 3
1 is less than 3
2 is less than 3
>>>
• If the condition is True, run the loop body again
• The body is indented just like the body in an if statement
• The i += 1 statement is the same as i = i + 1
• It adds one to the value in i each time it is run
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
7
The body must change the condition!
• If not, we’ll never ever stop looping
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>>> i = 0
>>> while i < 3:
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print i, 'is less than 3'
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0 is less than 3
0 is less than 3
...
• We’ve removed the i += 1, so the condition doesn’t change
• This is called an infinite loop
• You can interrupt an infinite loop by pressing Ctrl-C
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
8
General pattern for making while loops
• Your while loops will (usually) do the following:
1 Setup the loop variable(s) (the initialisation)
2 Test whether to continue running loop (the condition)
3 Run the body if the condition is true (the body)
4 Update the loop variable (the update)
• You should check each time to make sure they’re all there!
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
9
We’ve seen looping over a list of numbers
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>>> i = 1
>>> while i < 12:
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print i, 'times 5 =', i*5
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i += 1
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1 times 5 = 5
2 times 5 = 10
...
>>>
• i is the loop variable and is initialised to 1
• i < 12 is the condition
• print i, 'times = 5', i*5 is the body
• i += 1 is the update step
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
10
We can also loop over user input
• use raw_input inside the loop as the update:
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>>> number = int(raw_input('Enter a number? '))
>>> while number != 73:
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print 'Wrong! try again.'
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number = int(raw_input('Enter a number? '))
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>>>
• The raw_input lines are both initialiser and update
Girls’ Programming Network
while loops
Mini-lecture 6
Loops
while loops
Summary
11
You should now be able to:
• Explain why programming languages need loops
• Write Python while loops
• Interrupt an infinite loop in Python
• Read user input using a loop
Girls’ Programming Network
while loops
Mini-lecture 6