1. Introduction to Dictionaries
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Dictionaries

2. Digression — augmented assignment
count = count + 1
remaining = remaining – consumption
speed = speed * factor
dist = dist / 2
What do these have in common?
Answer:– Update of a variable by a quantity
The most basic form of increment or decrement
Shortcut notation:–
count += 1
remaining -= consumption
speed *= factor
dist /= 2
Somewhat like a contraction in English
CS-1004, A-Term 2014
Dictionaries

3. Digression (continued)
Widely used notation in Python, C, C++, Java, and other languages
Occasional subtle differences
Example:–
L[randrange(1,7)] = L[randrange(1,7)] + 1
Does not do the right thing!
Should be:–
idx = randrange(1,7) L[idx] = L[idx] + 1
This does work correctly in all cases:–
L[randrange(1,7)] += 1
Evaluates randrange() only once!
#to update correct list entry
CS-1004, A-Term 2014
Dictionaries

4. Digression (concluded)
Augmented assignment operators:–
+= addition -= subtraction *= multiplication
/= division
//= integer division
%= remainder **= exponentiation
And some we have not used in this class:–
>>= right shift <<= left shift &= bitwise AND ^= bitwise XOR |= bitwise OR
CS-1004, A-Term 2014
Dictionaries

5. Questions?
CS-1004, A-Term 2014
Dictionaries

6. Dictionary Important data type in Python Read and study §11.6
And all of Chapter 11!
Definition:– “Dictionary”
A Python data type for collections, capable of storing and retrieving key-value pairs, …
… where keys and values can be any type, …
… data is unordered!
Called a hash table in most other languages
Not a built-in data type
CS-1004, A-Term 2014
Dictionaries

7. Lab 5 “Write the function PlayManyGames(n) needed by Homework #5.
This must call your previous function PlayOneGame().
For each call to PlayOneGame(), use the result to index into a list and increment the value stored that location in the list.”
A hassle!
Using indexing into list to store values not directly associated with indexes!
Dice value 2  list[0]
Dice value 3  list[1] …
Dice value 12  list[10]
CS-1004, A-Term 2014
Dictionaries

8. Lab 5 and HW 5 (continued) Would really like something like:–
for i in range(2, 13): D[i] = 0
...
D[rollDice()] += 1
… or better yet:–
D[playOneGame()] += 1
Remember that PlayOneGame() returns a pair (win/loss, # of throws)
With dictionaries, we can do this!
CS-1004, A-Term 2014
Dictionaries

9. Using Dictionaries Creating a dictionary Also called a “mapping”
Remember:–
Lists = square brackets — [ ]
Dictionaries = curly brackets — { }
Adding an item to dictionary
D[key] = value
key may number, string, tuple
i.e., anything ‘immutable’
value can be anything at all!
D[key] acts like a variable
Also called a “mapping”
CS-1004, A-Term 2014
Dictionaries

10. Getting item from Dictionary
variable = D[key]
Very fast, efficient “lookup” of key
Uses technique in computing called “hashing”
key is scrambled
Scrambled value used to index into big list
Easy to find by simple search from there
Does not preserve order
Organized internally for speed of access
CS-1004, A-Term 2014
Dictionaries

11. Examples
CS-1004, A-Term 2014
Dictionaries

12. Operators, functions, & methods
key in dict
True if key is in the dictionary, False if not
dict.keys()
List of keys in the dictionary
dict.values()
List of values in dictionary
dict.items()
List of (key,value) pairs
dict.get(key,default)
Returns dict[key] if present, default otherwise
del dict[key]
Deletes key and dict[key] from dictionary
for k in dict:
Loop over all keys in the dictionary
Internal order of storage
CS-1004, A-Term 2014
Dictionaries

13. More Examples
Questions?
CS-1004, A-Term 2014
Dictionaries

14. What can we do with dictionaries
Store stuff!
Easily
So it is easily retrievable
Organize data by different criteria
Pull out non-numeric data …
CS-1004, A-Term 2014
Dictionaries

15. Example – Homework #4 dict is a dictionary of word-count pairs
Indexed by word
Read file, split into words, strip, lower-case
Just as in HW4
For each word of input
If word in dictionary, increment dict[word]
If not, dict[word] = 1
Get list of unique words
uniqueWords = list(dict.keys())
Sort
uniqueWords.sort()
Get counts
for word in uniqueWords(): write(dict[word], word)
CS-1004, A-Term 2014
Dictionaries

16. Questions?
CS-1004, A-Term 2014
Dictionaries