Dictionaries

Reading

Chapter 12.

Key ideas

From chapter 12

• A dictionary is an unordered collection of key-value pairs.
• Creating an empty dictionary: homeruns = {}
• Adding a key-value pair: homeruns["Babe Ruth"] = 713 (note: he hit 714)
• Adding another pair: homeruns["Mickey Mantle"] = 536
• Deleting an item: del homeruns["Mickey Mantle"]
• Modifying an item: homeruns["Babe Ruth"] = 714
• Determining the number of items in a dictionary: len(homeruns)

Keys

• To retrieve all keys in a dictionary named inventory: inventory.keys()
• To retrieve all keys as a list: list(inventory.keys())
• To retrieve keys one at a time: for key in inventory:
• To check if a key named “kiwi” exists: if "kiwi" in inventory

Values

• To retrieve all values: inventory.values()
• To retrieve a specific value, producing a runtime error if the key is not present: inventory[key-name]
• To retrieve a specific value, producing NO runtime error if the key is not present: inventory.get(key-name) or inventory.get(key-name, value-to-return-if-key-not-present)

Key-Value Pairs

• To retrieve all key-value pairs: inventory.items()

Aliasing vs. copying

• An example of an alias: dictionary1 = dictionary2
• The expression dictionary2 is dictionary1 returns True
• An example of a copy: dictionary2 = dictionary1.copy()
• The expression dictionary2 is dictionary1 returns False
• A dictionary handles sparse data well. For example, consider keeping track of how many times a word occurs in a novel such as The Catcher in the Rye. Only words that appear one or more times need to be stored.

Active learning

Activity 1

On paper, answer the following questions:

1. What is printed by the following statements?

   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   keylist = list(mydict.keys())
   keylist.sort()
   print(keylist[3])
   

2. What is printed by the following statements?

   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   answer = mydict.get("cat") // mydict.get("dog")
   print(answer)
   

3. What is printed by the following statements?

   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   print("dog" in mydict)
   

4. What is printed by the following statements?

   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   print(23 in mydict)
   

5. What is printed by the following statements?

   total = 0
   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   for akey in mydict:
   if len(akey) > 3:
      total = total + mydict[akey]
   print(total)
   

6. What is printed by the following?

   mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
   yourdict = mydict
   yourdict["elephant"] = 999
   print(mydict["elephant"])
   

Activity 2

Download raven.py and raven.txt. Edit raven.py to find the counts of every letter in the text using a dictionary.

Activity 3

Download raven_words.py and

1. Modify raven_words.py to produce a file word_counts.txt that contains the counts of each word in the file raven.txt, in alphabetical order. The first few lines of word_counts.txt should look something like this:

   a 15
   above 7
   adore 1
   again 1
   agreeing 1
   ah 2
   aidenn 1
   air 1
   all 4
   an 2
   ancient 1
   and 38
   angels 4
   answer 1
   

   Hint: you may need to cast a dict_keys object to a list using list() Here is a solution: raven_words_updated.py.

2. Modify your program to also print the longest word in the file.