4. Dictionaries, User Input and While Loops - LAC

Lecture notes

• Read Python Crash Course Chapter 6 and 7 (the last time we do two chapters)
• A dictionary is a collection of key-value pairs which is indexed and changeable. Keys must be unique. (from Python 3.7 it is ordered)
  vocab = dict()
vocab = {"kore":"これ", "sore":"それ", "are":"あれ" } (define with values)
  vocab['kore'] = 'これ'
• Set is a collection which is unordered (and thus unindexed) and changeable. No duplicate members.
  vocab = set()
vocab.add('word') #not append
vocab = {"kore", "sore", "are"} (define with values)
• while loops continue until the condition is not true
  be careful not to loop forever
• the input() function in Python reads a line of text input from the user and returns it as a string.
  name = input("Enter your name: ")
• Cheat Sheet: if/while, Cheat Sheet: if/while BW, Cheat Sheet dict., Cheat Sheet dict. BW

Before class (code, output)

• Make a dictionary of some new terms you have learnt, and their translations in Czech. Assume (contrary to real life) that each term only has one translation.

      en2cs = {
      "list": "seznam",
      "set": "množina",
      "boolean": "booleovská hodnota",
      "tuple": "ntice",
      "dictionary": "slovník",
      "string": "řetězec" }
  

  Loop through the dictionary and print them nicely formatted, something like:
  list = seznam
• Now make a reversed dictionary cs2en (hint: loop through and set the values as keys ), then loop through the new dictionary and print it nicely formatted.
• Make a dictionary called food. Use the names of three dishes as in your dictionary. For each food, make a dictionary of details, like ingredients, cooking time, comments. Then loop through the foods and the details, printing everything nicely.
• Write a program that asks the user how many languages they speak. If they say 1, tell them to study more, if they say 2 or 3, say 'good' if they speak more than 3, say 'well done'.

Practical work (code, output)

• Write a loop that prompts the user to enter a series of foods until they enter a 'quit' value. As they enter each one, print a message saying you’ll cook that for the party. The program should stop when they write 'quit'.
  Do this in three different ways:
  • Use a conditional test in the while loop
  • Use a variable active
  • Use a break statement
• This is the first paragraph of The Adventure of the Dancing Men (Conan Doyle), split into tokens.

  tokens = [
      "Holmes", "had", "been", "seated", "for", "some", "hours", "in", "silence",
      "with", "his", "long", ",", "thin", "back", "curved", "over", "a", "chemical",
      "vessel", "in", "which", "he", "was", "brewing", "a", "particularly",
      "malodorous", "product", ".", "His", "head", "was", "sunk", "upon", "his",
      "breast", ",", "and", "he", "looked", "from", "my", "point", "of", "view",
      "like", "a", "strange", ",", "lank", "bird", ",", "with", "dull", "gray",
      "plumage", "and", "a", "black", "top-knot", "." ]
      

  We will learn how to do this later!
  • Use set() to determine how many different tokens there are
  • Should you count the words as they are, or should you make them lower case? How different are the results?
  • Loop through the tokens and use a dictionary to count the frequency.
    If you have not seen a word before, set its frequency to 1
    If you have seen it before, add one to the frequency.
    Hint: you can use in to see if a key is in a dictionary
  • Loop through and show the frequency for each word that has a frequency greater than one, in two different ways.
    • Use items()
    • Just loop through the keys

  Further work (code, output)

  This is a continuation of the work in class.

  • Loop through and show the frequency for each word as a histogram
    Print n '*'s if the frequency is n
    **** (a)
    * (chemical)
  • Loop through the tokens and use a dictionary to count the frequency.
    use get to make your code shorter.
  • Make a new list that has none of these words in it:
    ['.', ',', ';', 'a', 'an', 'of', 'the', 'he', 'his', 'him', 'me', 'my', 'I', 'you', 'your', 'we', 'our', 'us']
    Make sure to remove words regardless of their case.

Summary

You learned:

• how to define a dictionary and work with the information stored in it
• how to access and modify individual elements in a dictionary
• how to loop through a dictionary's key-value pairs, keys, and values
• how to nest multiple dictionaries in a list, lists in a dictionary, and dictionaries inside a dictionary
• how to use the input() function to allow users to provide their own information in programs
• to work with both text and numerical input
• how to use while loops to keep programs running as long as the users want them to
• several ways to control the flow of a while loop:
  • setting an active flag
  • using the break statement
  • using the continue statement
• how to use a while loop to move items from one list to another
• how to remove all instances of a value from a list
• how while loops can be used with dictionaries

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LAC: Language and the Computer Francis Bond.