W6 Tutorial

Coming Up

  • Writing nice code
  • Dictionaries
  • Sets
  • None and sort() vs .sorted()

Writing Nice Code

Notes on Writing Nice Code
  • ✅ Do
    • Be clear and efficient
    • Write scalable code
      • think "can this code support bigger inputs?"
    • Use loops
    • Abstraction
  • ❌ Don't
    • Have lots of repetition

Problem 1

Do the following code snippets do the same thing? What are some advantages and disadvantages of each snippet? What if we needed a hundred different types of tool?

Example A:

print("We need some saws")
print("We need some hammers")
print("We need some cogs")
print("We need some nails")

Example B:

def get_str(part):
    return f"We need some {part}"
print(get_str("saws"))
print(get_str("hammers"))
print(get_str("cogs"))
print(get_str("nails"))

Example C:

def get_str(part):
    return f"We need some {part}"
parts = ("saws", "hammers", "cogs", "nails")
for part in parts:
    print(get_str(part))

Problem 2

Consider the following while loop and two conversions to for loops. Are the two for loops equivalent? Why might you choose one over the other?

Original While Loop:

count = 0
items = ('eggs', 'spam', 'more eggs')
while count < len(items):
    print(f"we need to buy more {items[count]}")
    count += 1

For Loop A:

items = ('eggs', 'spam', 'more eggs')
for count in range(len(items)):
    print(f"we need to buy more {items[count]}")

For Loop B:

items = ('eggs', 'spam', 'more eggs')
for item in items:
    print(f"we need to buy more {item}")

Discussion 1

Dictionary Operations

Notes on Dictionary Operations
  • Accessing
    • d[key] access value associated to key - returns KeyError if key doesn't exist
    • d.get(key) returns value associated with key - returns None if key doesn't exist
    • key in d test for presence of key
  • Adding
    • d[new_key] = new_value adding a new key
    • d.copy() makes a shallow copy of dictionary
  • Deleting
    • d.pop(key) deletes key-value pair
    • del d[key] deletes key-value pair
    • d.clear() deletes all key-value pairs from the dictionary
  • Iterables
    • d.keys() returns iterable collection of keys
    • d.values() returns iterable collection of values
    • d.items() returns iterable collection of 2-tuples (key, value)
    • list(d) same as list(d.keys())

Discussion 2

What is the difference between using the .pop() method on a dictionary and using it on a list?

Sets

Notes on Set Operations

Discussion 3

In what situations would we use a “set”? How does it differ from other “containers” such as lists and dictionaries?

Discussion 4

What special operations can we perform on sets? How do we add and remove items from them?

Exercise 3

Evaluate the following given the assignment d = {"R": 0, "G": 255, "B": 0, "other": {"opacity": 0.6}}. Specify whether the value of d changes as a result. Assume d is reset to its original value each time.

(a) "R"in d

(b) d["R"]

(c) d["R"] = 255

(d) d["A"]

(e) d["A"] = 50

(f) d.pop("G")

(g) d["other"]["blur"] = 0.1

(h) d.items()

Exercise 4

Evaluate the following given the assignment s1 = {1, 2, 4} and s2 = {3, 4, 5}. If s1 or s2 change as a result, give their new value. Assume s1 and s2 are reset to their original values each time.

(a) s1.add(7)

(b) s1.add(2)

(c) s2.remove(5)

(d) s1 & s2

(e) s1.union(s2)

(f) s1 - s2

Problem 1

Write a function which takes a string as input and prints the frequency of each character in the string using a dictionary. freq_counts('booboo') should print:

b 2
o 4

Problem 2

Write a function which takes two lists as input and returns a list containing the numbers which they both have in common. in_common([1, 2, 4], [3, 4, 5]) should return [4].

Problem 3

Write a function which takes a dictionary and returns a sorted list containing the unique values in that dictionary. unique_values({'a': 1, 'b': 0, 'c': 0}) should return [0, 1].

Problem 4

Write a function which takes a string, a character and an integer threshold and returns True if the character appears in the string with a frequency above the threshold, False if it appears at or below the threshold, and None if it doesn’t appear at all. above_thresh('I like the letter e', 'e', 3) should return True.