reveal.js

# Dictionaries

---

CS 65 // 2021-03-30

## Administrivia
- Midterm course evaluations
    + Please take the time to fill out [this anonymous questionnaire](https://docs.google.com/forms/d/e/1FAIpQLSeF4te7AXCmSb2tsFGi_YaKpsxHaGUtwtAj_Vrol0J5a6B1bA/viewform?usp=sf_link)
- 
 Still grading Assignment 4 and Exam 2

## Extra Credit Activities
- [Drake's Got Talent](https://calendar.drake.edu/event/drakes_got_talent?utm_source=newsletter&utm_medium=email&utm_content=https%3A//d31hzlhk6di2h5.cloudfront.net/20210330/37/d7/7f/18/d0461f71e982682339189262_352x354.jpeg&utm_campaign=Staff-March-30#.YGJQSGRKg-Q)
    + Thursday, April 1 at 5pm
    + Helmick Commons
- 
 Drake Relays
    + Relays Reveal 2021
        * Wednesday, March 31st at 6pm
        * Olmsted Center in the Parents Hall
    + 
 Other events during the week of April 12 to 24th

## Assignment 6
- Questions about anything?

# Associative Data

## Associative Data
- Lists allow us to store **sequential data**
    + i.e. data that can be strictly ordered
- 
 The strict ordering of the elements in a list allows us to easily access elements **by their index**
    + `t[0]`, `t[1]`, `t[2]`, ...
- 
 A list gives us a mapping of **indices** to values
- 
 What if we want to map **strings** to values?

## Associative Data Examples
- A **dialer app** on your cell phone
    + Want to store contact information
    + Want to **lookup** phone numbers by **names**
    + Want to *add*, *remove*, and *change* contact info
- 
 A **library app** that maps author names to books they've written
- 
 A **student meal plan app** that maps a Drake ID to the number of meals / Bulldog Bucks remaining

# Dictionaries

## The `dict` Type
- **Purpose**
    + To conveniently manipulate associative data
- **Express**
    + `{}`, `dict()`
    + `{key1:value1, key2:value2, ...}`

## The `dict` Type
- **Operations**
    + Lookup values by their key
    + 
 Change values by their key
    + 
 Deleted key/value pairs
    + 
 Number of key/value pairs

## Self Check 1
1. Why can't we use a `list` object as a key in a dictionary?
2. 
 How can we iterate over a dictionary with a `for` loop?

## Dictionary Exercise
- Create a `dict` that maps **email addresses** to **names**
    + Eric Manley (eric.manley@drake.edu)
    + Meredith Moore (meredith.moore@drake.edu)
- 
 Write an expression that adds an entry for
    + Adam Case (adam.case@drake.edu)
- 
 Delete the entry for **Eric Manley**
- 
 Write a loop that iterates over all the key/value pairs and prints them

## Self Check 2
- Write a function called `has_value(d, value)` that takes two parameters and returns `True` if the dictionary `d` contains the value `value` and returns `False` otherwise.

#  Word Frequencies

## Word Frequencies
- Suppose we want to analyze the frequency of words an author uses in their books
- 
 Let's write a function that constructs a dictionary that maps each word in a file to the number of times it appears in the text
- 
 We can then run our program on various books on **Project Gutenberg**

## Word Frequencies

```py
def parse_words(filename):
    f = open(filename)
    list_of_words = []
    for line in f:
        words_on_line = line.strip().split()
        for word in words_on_line:
            if word.isalpha():
                list_of_words.append(word.lower())
    f.close()
    return list_of_words
```

## Word Frequencies

```py
def count_words(list_of_words):
    freq = {}
    for word in list_of_words:
        if word in freq:
            freq[word] += 1
        else:
            freq[word] = 1

return freq
```

#  Hashtables

## Hashtables
- Dictionaries are implemented a **hashtable**
- 
 These are beyond the scope of this course, but they provides very efficient access to operations like:
    + 
 Add (key, value) pair
    + 
 Lookup by key
    + 
 Modify the value for a certain key
    + 
 Delete (key, value) pairs
- 
 Stay tuned for CS 66 where you'll learn more!

#  Choosing Keys

## Choosing Good Keys
- Suppose we want to store some associative data
- 
 What should be the "key" of the data?
    + 
 **books**: title, author, ISBN, year, publisher
    + 
 **players**: name, uniform number, team, position
    + 
 **computers**: manufacturer, model number, processor, memory, disk size
- 
 Are some "key" choices better than others? Why or why not?

## Choosing Good Keys
- **books**: title, author, ISBN, year, publisher
- **players**: name, uniform number, team, position
- **computers**: manufacturer, model number, processor, memory, disk size

## Choosing Good Keys
- Keys should be **convenient** for your application
- 
 Keys should **uniquely identify** the entry---otherwise key collisions might occur causing data to be overwritten!
- 
 Keys should be **immutable**---this helps ensure that lookups are consistent
- 
 Don't use graphics objects as keys!

# Assignment Time