W6: Dictionaries, Sets, None and Sorting

• Google Colab
• Google Slides

Coming Up

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

E1. 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))

E2. 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}")

d = {
    "name": "Jane",
    "hobbies": ["coding", "basketball"]
}
print(f"Original dictionary: {d}\n")

print(list(d.items())[0])

for item in d.items():
    print(item)


# # adding a new key
# d["age"] = 30
# print(f"After adding new key: {d}\n")

# # updating existing key
# d["name"] = "Alex"
# d["hobbies"].append("reading")
# print(f"After updating: {d}\n")

# # deleting
# pop_return = d.pop("hobbies")
# print(f'pop_return: {pop_return}')
# del d["name"]
# print(f"After deletions: {d}")

D2. What is the difference between using the .pop() method on a dictionary and using it on a list?

Sets

D3. In what situations would we use a “set”? How does it differ from other “containers” such as lists and dictionaries?

You can type your answer to D3 here...

D4. What special operations can we perform on sets? How do we add and remove items from them?

You can type your answer to D4 here...

E3. 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.

You can type your answer to E3 here...

(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()`

E4. 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.

You can type your answer to E4 here...

(a) `s1.add(7)`

(b) `s1.add(2)`

(c) `s2.remove(5)`

(d) `s1 & s2`

(e) `s1.union(s2)`

(f) `s1 - s2`

Coding Problems

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.