COS 100: Introduction to Programming

Fall 2019

Lab 14: Bonus lab

Due to time constraint in this course, we've skipped over a lot of things. This optional bonus lab contains a few ideas that are very useful.

Lab 14A: Control flow in loops

• In the body of a for or a while loop, you can use continue to skip the rest of the loop and start the next iteration or use break to get out of the loop.
• Run this code to understand the effect of break and continue:

  i = 0
  while True:
      if i == 10:
          break
      i = i + 1
      print(i)
      if i % 2 != 0:
          continue
      print("even:", i)
  

• Suppose gradebook.txt has the following format:

  # max score
  Max Perfect,6,10,20,20
  # what follows are students
  Rory Gilmore,5,9,13,7
  Paris Geller,6,10,20,19
  --
  Paris, as always, did well.
  Rory drifted off at the end.
  

  where a line starting with # is a comment to be ignored and a line that is just -- marks the end of student records (what's below should be ignored).
• Modify your load_file from the gradebook homework project to ignore comments (lines starting with #) and stop processing when (if) a line -- is reached.
• See Zelle 8.5 for more details.

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Lab 14B: Exceptions

• When an error occurs, the program raises an exception.
• A try block allows you to catch an exception (of a certain type) and do something in the except block.

  try:
      num = int(input("Integer please: "))
  except ValueError:
      print("That was not an integer!")
  

• Modify your silo homework so that if the user provided bad input, you will repeatedly prompt the user until a good input is given.
• Modify the animation from the graphics lab to exit gracefully (close the window, say bye to the user, and don't display error messages) when the user types Ctrl-C to quit.
• See Zelle 7.4 for more details.

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Lab 14C: String formatting

• Strings have a format() method.
• Run the code below and deduce its usage:

  name = input('Name, please: ')
  number = int(input('Number, please: '))
  print('Hi {0}. You like {1}?'.format(name, number))
  

• What's the curly braces { and } for? What happens if you swap the 0 and the 1? What if you omit the 0 and the 1?
• You can make pretty columns with some special features of formatted strings.

  people = ['Alice', 'Bob', 'Chrysanthemum', 'Donald']
  hourlyWages = [15.456, 6.8, 254.25, 20]
  
  for k in range(len(people)):
      print('{0:15}{1:f}'.format(people[k], hourlyWages[k]))
  

• Look at the official documentation to see if you can figure out a way to force the hourly wages to be right-justified with exactly two digits beyond the decimal point.
• See Zelle 5.8.2 for more details.

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Lab 14D: Dictionaries

• A dictionary can store key-value pairs.

  wages = {'Alice': 15.456, 'Bob': 6.8, 'Donald': 20}
  wages['Chrysanthemum'] = 254.25
  print(wages)
  print(wages['Alice'])
  for name in wages:
      print(name, wages[name])
  del wages['Bob']
  print('Alice' in wages)
  print('Bob' in wages)
  print(20 in wages)
  for (name, wage) in wages.items():
      print(name, wage)
  

• See Zelle 11.7 for more details.

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Done with all that? Remember that you can play with Kattis. You can also ask me for an extra secret bonus lab.