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Making decisions

The last missing piece in our basic set of commands is the ability to make decisions in a program. This is done in Python using the if command.

Warming up

Add your favourite movie to the following program and execute it:

movie = input('Please enter your favourite movie: ')
if movie == 'Star Trek':
    print('Live long and prosper')
elif movie == 'Star Wars':
    print('These are not the droids you are looking for')
elif movie == 'Dirty Dancing':
    print('I fetched the melons')
else:
    print("Sorry, I don't know the movie %s." % (movie) )

Exercises

Exercise 1

Which of these if statements are syntactically correct?

  • if a and b:
  • if len(s) == 23:
  • if a but not b < 3:
  • if a ** 2 >= 49:
  • if a != 3
  • if (a and b) or (c and d):

Exercise 2

Write a program that lets the user enter a number on the keyboard. Find the number in the list that is closest to the number entered and write it to the screen.

data = [1, 2, 5, 10, 20, 100, 200, 500, 1000]
query = int(input())

The Challenge: Filter a list

You have a list of the top 20 girls names from 2000:

['Emily', 'Hannah', 'Madison', 'Ashley', 'Sarah', 
'Alexis', 'Samantha', 'Jessica', 'Elizabeth', 'Taylor', 
'Lauren', 'Alyssa', 'Kayla', 'Abigail', 'Brianna', 
'Olivia', 'Emma', 'Megan', 'Grace', 'Victoria']

Write a program that prints all names that start with an A.

Extra challenge

  • count the number of names starting with A and print that number as well.