Download Python 7: Program flow control

OCR GCSE Computing
Python programming
7: Program flow control
OCR Computing GCSE
© Hodder Education 2013
Slide 1
Python 7: Program flow control
All programs can be made from 3 constructs:
• Sequence
– One command after another
• Selection
– Decision making
• Iteration
– repetition
OCR Computing GCSE
© Hodder Education 2013
Slide 2
Python 7: Program flow control
Sequence
• Do this, then this, then this …
#int function demo
name=(input('What is your name? '))
income=int((input('What is your monthly income in pounds? ')))
annual_income=income*12
print(name, ' you earn ',annual_income, ' pounds per year.')
OCR Computing GCSE
© Hodder Education 2013
Slide 3
Python 7: Program flow control
Selection
Decision making
How much money do you have?
• More than £4: you can have fish and chips
• More than £3: you can have fish
• More than £2: you can have chips
• Otherwise: nothing!
OCR Computing GCSE
© Hodder Education 2013
Slide 4
Python 7: Program flow control
Selection
Use if… elif… else
Remember the colon and the indentation:
OCR Computing GCSE
© Hodder Education 2013
Slide 5
Python 7: Program flow control
Selection
if… elif… else
What happens after an ‘if’ condition
must be indented
End the list of consequences by unindenting.
OCR Computing GCSE
© Hodder Education 2013
Slide 6
Python 7: Program flow control
Iteration
This means repetition.
A section of program code repeats.
This is called a loop.
Python has various ways of achieving this.
‘While’ and ‘for’ are the most common ways.
OCR Computing GCSE
© Hodder Education 2013
Slide 7
Python 7: Program flow control
Iteration with ‘while’
While a condition is true (think boolean operators)
Do something
The condition is tested on entry to the loop.
answer=''
while answer!='yes':
answer=input('do you like computing? ')
print('That is the correct answer')
OCR Computing GCSE
© Hodder Education 2013
Slide 8
Python 7: Program flow control
• You can control a ‘while’ loop with a counter:
• Notice that the variable ‘count’ is incremented at the
end of each iteration.
OCR Computing GCSE
© Hodder Education 2013
Slide 9
Program flow control
The ‘for’ loop
This is widely used in Python. It lets you iterate a
loop based on a sequence.
A sequence can be lots of things:
– a a string, a list, a tuple
Here we use a sentence (a string). The ‘for’ loop
iterates through the letters in the sentence.
OCR Computing GCSE
© Hodder Education 2013
Slide 10
Python 7: Program flow control
The ‘for’ loop
As usual with Python, notice the colon and the
indentation used to indicate a program block.
OCR Computing GCSE
© Hodder Education 2013
Slide 11
Python 7: Program flow control
The ‘for’ loop
We usually want to do some processing inside a
loop. Here, we count the letters ‘a’:
OCR Computing GCSE
© Hodder Education 2013
Slide 12
Python 7: Program flow control
A for loop can be controlled with the range()
function:
i is a variable used here to keep track of the
iterations
We get an unexpected result:
Python starts counting at zero
and stops after 9.
OCR Computing GCSE
© Hodder Education 2013
Slide 13
Python 7: Program flow control
We can use whatever steps we want to iterate
through the loop.
We give the range function the start, the finish and
the step.
OCR Computing GCSE
© Hodder Education 2013
Slide 14
Python 7: Program flow control
We can use negative steps to count backwards.
OCR Computing GCSE
© Hodder Education 2013
Slide 15