Practice: Collections¶

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Four practice problems, one per concept step; the numbers line up (Practice-01 goes with Step-01). Build each one yourself in python-practice, then compare with python-practice/solutions. Do this before starting the next section.

Write the code yourself; the stubs only describe the task. Some problems end with a short input() "Write a Program" exercise.

These problems build one connected story: you are setting up the catalog for an online store (products, prices, categories). Each step adds another piece of that catalog.

Step-01: Lists¶

Concepts: lists (append, insert, extend, pop, clear, sort, reverse, indexing, slicing, in / not in, index / count, copy, nested lists, split(), join(), input()).

Problem-1: Catalog Prices.

Write a program that, starting from the product prices 40, 95, 25 with 60 added in:

• Task-1: print the prices sorted from lowest to highest.
• Task-2: print the highest price.

[25, 40, 60, 95]
95

catalog_prices = [40, 95, 25]
catalog_prices.append(60)
# Problem-1 Task-1: print prices sorted low to high
catalog_prices.sort()
print(catalog_prices)
# Problem-1 Task-2: print the highest price
print(catalog_prices[-1])

Problem-2: Split and Join.

Write a program that:

• Task-1: split the product code "TEC-2024-15" on - into a list and print the list.
• Task-2: join the label words "wireless", "gaming", "mouse" with a single space into one string and print it.

['TEC', '2024', '15']
wireless gaming mouse

# Problem-2 Task-1: split the product code on "-"
product_code = "TEC-2024-15"
print(product_code.split("-"))
# Problem-2 Task-2: join the label words with a space
label_words = ["wireless", "gaming", "mouse"]
print(" ".join(label_words))

Problem-3 (Write a Program): Catalog Items.

Write a program that asks for catalog items separated by commas (no spaces), splits the text into a list, prints the list, and then prints count: followed by how many items there are.

Items separated by commas (no spaces): mouse,keyboard,monitor
['mouse', 'keyboard', 'monitor']
count: 3

# Problem-3: read items, split into a list, print list and count
entered = input("Items separated by commas (no spaces): ")
catalog_items = entered.split(",")
print(catalog_items)
print("count:", len(catalog_items))

Problem-4: Add and Remove Items.

Write a program that, starting from the cart items "mouse", "keyboard":

• Task-1: insert "monitor" at index 1 and print the cart.
• Task-2: extend the cart with ["cable", "webcam"] and print the cart.
• Task-3: remove the last item with pop() and print what was removed.
• Task-4: clear the cart and print it.

['mouse', 'monitor', 'keyboard']
['mouse', 'monitor', 'keyboard', 'cable', 'webcam']
webcam
[]

cart = ["mouse", "keyboard"]
# Problem-4 Task-1: insert "monitor" at index 1
cart.insert(1, "monitor")
print(cart)
# Problem-4 Task-2: extend the cart with two items
cart.extend(["cable", "webcam"])
print(cart)
# Problem-4 Task-3: pop the last item and print it
removed = cart.pop()
print(removed)
# Problem-4 Task-4: clear the cart
cart.clear()
print(cart)

Problem-5: Find, Copy, and Reorder.

Write a program that, starting from the order statuses "pending", "shipped", "pending", "delivered":

• Task-1: print the index of "shipped".
• Task-2: print how many times "pending" appears.
• Task-3: make a copy of the list, append "returned" to the copy only, then print the original list and the copy.
• Task-4: reverse the original list and print it.

1
2
['pending', 'shipped', 'pending', 'delivered']
['pending', 'shipped', 'pending', 'delivered', 'returned']
['delivered', 'pending', 'shipped', 'pending']

order_statuses = ["pending", "shipped", "pending", "delivered"]
# Problem-5 Task-1: print the index of "shipped"
print(order_statuses.index("shipped"))
# Problem-5 Task-2: count how many times "pending" appears
print(order_statuses.count("pending"))
# Problem-5 Task-3: copy the list, append to the copy only, print both
statuses_copy = order_statuses.copy()
statuses_copy.append("returned")
print(order_statuses)
print(statuses_copy)
# Problem-5 Task-4: reverse the original list
order_statuses.reverse()
print(order_statuses)

Problem-6: Slice and Membership.

Write a program that, for the price list 15, 30, 45, 60, 75:

• Task-1: print the slice from index 1 up to (but not including) index 4.
• Task-2: print whether 45 is in the list.
• Task-3: print whether 99 is not in the list.

[30, 45, 60]
True
True

price_list = [15, 30, 45, 60, 75]
# Problem-6 Task-1: print the slice from index 1 to 4
print(price_list[1:4])
# Problem-6 Task-2: print whether 45 is in the list
print(45 in price_list)
# Problem-6 Task-3: print whether 99 is not in the list
print(99 not in price_list)

Problem-7: Nested Lists.

Write a program that, starting from the order table order_rows = [["mouse", 25], ["keyboard", 45], ["monitor", 120]] (each inner list is one [product, price] row):

• Task-1: print the whole first row.
• Task-2: print the product name from the first row.
• Task-3: print the price from the last row.

['mouse', 25]
mouse
120

order_rows = [["mouse", 25], ["keyboard", 45], ["monitor", 120]]
# Problem-7 Task-1: print the whole first row
print(order_rows[0])
# Problem-7 Task-2: print the product name from the first row
print(order_rows[0][0])
# Problem-7 Task-3: print the price from the last row
print(order_rows[2][1])

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Step-02: Tuples¶

Concepts: tuples (create, index, len, immutable, count, index, slice, in / not in, packing, one-item tuple, unpack, swap, list inside a tuple).

Problem-1: A Product Row.

Write a program that, for the product row product = ("Mouse", 25, "SKU100") (name, price, sku):

• Task-1: print the name (first item), the sku (last item, negative index), and how many fields it has.
• Task-2: unpack the row into name, price, sku and print Mouse costs 25.

Mouse
SKU100
3
Mouse costs 25

product = ("Mouse", 25, "SKU100")
# Problem-1 Task-1: print name, sku, and field count
print(product[0])
print(product[-1])
print(len(product))
# Problem-1 Task-2: unpack the row and print name costs price
name, price, sku = product
print(name, "costs", price)

Problem-2: Product Ratings.

Write a program that, for the star ratings ratings = (5, 4, 5, 3, 5, 4, 5):

• Task-1: print how many 5-star ratings there are, and the position of the first 3-star rating.
• Task-2: print the first three ratings (a slice); then the last rating two ways: as a one-item tuple (ratings[-1:]) and as a plain number (ratings[-1]).
• Task-3: check that a 3-star rating is in the ratings (in), and that a 1-star rating is not (not in).

4
3
(5, 4, 5)
(5,)
5
True
True

ratings = (5, 4, 5, 3, 5, 4, 5)
# Problem-2 Task-1: count 5-star and find position of the first 3-star
print(ratings.count(5))
print(ratings.index(3))
# Problem-2 Task-2: slice the first three, then the last two ways
print(ratings[0:3])
print(ratings[-1:])
print(ratings[-1])
# Problem-2 Task-3: membership checks with in and not in
print(3 in ratings)
print(1 not in ratings)

Problem-3: Build Tuples.

Write a program that:

• Task-1: build a product row row from the name Keyboard and the price 45, writing no brackets around them (packing), and print it.
• Task-2: make a one-item tuple holding just the price 45 (a trailing comma is required), and a plain (45) with no comma; print both type names.

('Keyboard', 45)
tuple int

# Problem-3 Task-1: pack a row without brackets
row = "Keyboard", 45
print(row)
# Problem-3 Task-2: one-item tuple vs plain value, print both type names
one_price = (45,)
not_a_tuple = (45)
print(type(one_price).__name__, type(not_a_tuple).__name__)

Problem-4: Fix Swapped Prices.

Write a program that, for prices = (45, 25) (the mouse and keyboard prices, entered the wrong way round):

• Task-1: unpack prices into mouse and keyboard, then swap them in one line.
• Task-2: since a tuple cannot change, pack the corrected order into a new tuple new_prices and print it.
• Task-3: print the original prices tuple to show it did not change.

(25, 45)
(45, 25)

prices = (45, 25)
# Problem-4 Task-1: unpack then swap in one line
mouse, keyboard = prices
mouse, keyboard = keyboard, mouse
# Problem-4 Task-2: build a new corrected tuple and print it
new_prices = (mouse, keyboard)
print(new_prices)
# Problem-4 Task-3: print the original to show it is unchanged
print(prices)

Problem-5: An Order's Item List.

Write a program that, for the order order = ("SKU100", ["mouse", "pad"]) (a fixed sku and a changeable list of items):

• Task-1: add "cable" to the items list and print the order.
• Task-2: clear the items list, then add "keyboard" and "monitor", and print the order.
• Task-3: print the sku (order[0]) to show the fixed part is unchanged.

('SKU100', ['mouse', 'pad', 'cable'])
('SKU100', ['keyboard', 'monitor'])
SKU100

order = ("SKU100", ["mouse", "pad"])
# Problem-5 Task-1: append to the inner items list and print the order
order[1].append("cable")
print(order)
# Problem-5 Task-2: clear the list, add two items, print the order
order[1].clear()
order[1].append("keyboard")
order[1].append("monitor")
print(order)
# Problem-5 Task-3: print the sku to show the fixed part is unchanged
print(order[0])

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Step-03: Dictionaries¶

Concepts: dictionaries (create, look up by key, add/update, .update(), len, .get() with a default, empty then fill, .keys()/.values()/.items(), in / not in, del / .pop(), a list value, a nested dict, list of dicts, input()).

Problem-1: Build and Grow a Product Record.

Write a program that, for a product dictionary with "name" set to "Wireless Mouse" and "price" set to 25:

• Task-1: print the name.
• Task-2: update the price to 20.
• Task-3: add a "brand" of "Logix".
• Task-4: use .update() to set "stock" to 100 and "price" to 22 in one call.
• Task-5: print how many fields the product has, then print the whole product.

Wireless Mouse
4
{'name': 'Wireless Mouse', 'price': 22, 'brand': 'Logix', 'stock': 100}

product = {"name": "Wireless Mouse", "price": 25}
# Problem-1 Task-1: print the name
print(product["name"])
# Problem-1 Task-2: update the price to 20
product["price"] = 20
# Problem-1 Task-3: add a "brand" of "Logix"
product["brand"] = "Logix"
# Problem-1 Task-4: use .update() to set stock and price at once
product.update({"stock": 100, "price": 22})
# Problem-1 Task-5: print the field count, then the whole product
print(len(product))
print(product)

Problem-2: Safe Lookup and an Empty Cart.

Write a program that:

• Task-1: for a product dictionary with "name" set to "Mouse" and "price" set to 25, print the value for the missing key "weight", falling back to "unknown".
• Task-2: build an empty cart dictionary, add "item1" set to "Mouse" and "item2" set to "Keyboard", then print the cart.

unknown
{'item1': 'Mouse', 'item2': 'Keyboard'}

# Problem-2 Task-1: safe lookup of a missing key with a fallback
product = {"name": "Mouse", "price": 25}
print(product.get("weight", "unknown"))
# Problem-2 Task-2: build an empty cart, add two items, print it
cart = {}
cart["item1"] = "Mouse"
cart["item2"] = "Keyboard"
print(cart)

Problem-3: Inspect a Record.

Write a program that, for a product dictionary with "name" set to "Mouse", "price" set to 25, and "brand" set to "Logix":

• Task-1: print the list of keys.
• Task-2: print the list of values.
• Task-3: print the list of (key, value) pairs.

['name', 'price', 'brand']
['Mouse', 25, 'Logix']
[('name', 'Mouse'), ('price', 25), ('brand', 'Logix')]

product = {"name": "Mouse", "price": 25, "brand": "Logix"}
# Problem-3 Task-1: print the list of keys
print(list(product.keys()))
# Problem-3 Task-2: print the list of values
print(list(product.values()))
# Problem-3 Task-3: print the list of (key, value) pairs
print(list(product.items()))

Problem-4 (Write a Program): Price Lookup.

Write a program that, for a prices dictionary with "mouse" set to 25 and "keyboard" set to 45, asks for a product name (typed in ANY capitalization) and prints that product's price, or Not Found when the product is not in the dictionary.

Enter product name for price check: Keyboard
45

# Problem-4: read a product name and print its price or "Not Found"
prices = {"mouse": 25, "keyboard": 45}
product_name = input("Enter product name for price check: ")
print(prices.get(product_name.lower(), "Not Found"))

Problem-5: Remove Fields, Then Check.

Write a program that, for a product dictionary with "name" set to "Mouse", "price" set to 25, and "old" set to True:

• Task-1: remove "old" with del.
• Task-2: remove "price" with .pop(), keeping its value; print the product, then the removed price.
• Task-3: print whether "name" is a key in the product, and whether "price" is not a key.

{'name': 'Mouse'}
25
True
True

product = {"name": "Mouse", "price": 25, "old": True}
# Problem-5 Task-1: remove "old" with del
del product["old"]
# Problem-5 Task-2: pop "price" keeping its value, print product then price
removed_price = product.pop("price")
print(product)
print(removed_price)
# Problem-5 Task-3: check key membership with in and not in
print("name" in product)
print("price" not in product)

Problem-6: Records That Hold More.

Write a program that:

• Task-1: for a product dictionary with "name" set to "Mouse" and "tags" set to the list ["wireless", "usb"], print the first tag, then append "ergonomic" and print the tags.
• Task-2: for a product dictionary with "name" set to "Mouse" and "seller" set to the dictionary {"city": "Delhi", "rating": 5}, print the seller's city.
• Task-3: for a catalog list holding {"name": "Mouse", "price": 25} and {"name": "Keyboard", "price": 45}, print the first product's name, then the second product's price.

wireless
['wireless', 'usb', 'ergonomic']
Delhi
Mouse
45

# Problem-6 Task-1: a list value, so read the first tag, append one, print tags
product = {"name": "Mouse", "tags": ["wireless", "usb"]}
print(product["tags"][0])
product["tags"].append("ergonomic")
print(product["tags"])
# Problem-6 Task-2: a nested dict value, so read the seller's city
product = {"name": "Mouse", "seller": {"city": "Delhi", "rating": 5}}
print(product["seller"]["city"])
# Problem-6 Task-3: a list of product dicts, so read a name and a price
catalog = [{"name": "Mouse", "price": 25}, {"name": "Keyboard", "price": 45}]
print(catalog[0]["name"])
print(catalog[1]["price"])

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Step-04: Sets¶

Concepts: sets (create, unique, len, sorted(), empty set() vs {}, set(list) dedup, in / not in, .add(), .update(), .discard(), union |, intersection &, difference -, symmetric difference ^, input()). A set has no order, so every set is printed via sorted().

Problem-1: Unique Categories.

Write a program that, for the category list ["audio", "video", "audio", "gaming", "video"]:

• Task-1: build a set from the list, then print how many UNIQUE categories there are.
• Task-2: print the unique categories in sorted order.

3
['audio', 'gaming', 'video']

categories = ["audio", "video", "audio", "gaming", "video"]
# Problem-1 Task-1: build a set and print the unique count
unique = set(categories)
print(len(unique))
# Problem-1 Task-2: print the unique categories sorted
print(sorted(unique))

Problem-2: The Empty-Set Trap.

Write a program that:

• Task-1: make an empty cart with set() and print its type.
• Task-2: print the type of {} to show it is a dictionary, not a set.

<class 'set'>
<class 'dict'>

# Problem-2 Task-1: make an empty set and print its type
empty_cart = set()
print(type(empty_cart))
# Problem-2 Task-2: print the type of {} to show it is a dict
print(type({}))

Problem-3: Grow and Shrink a Tag Set.

Write a program that, for the tag set {"audio", "video"}:

• Task-1: add "gaming" with .add(), bulk-add ["mobile", "audio"] with .update(), discard "video" and discard "toys" (not present), then print the sorted tags.
• Task-2: print whether "audio" is in the tags, and whether "video" is not in the tags.

['audio', 'gaming', 'mobile']
True
True

tags = {"audio", "video"}
# Problem-3 Task-1: add one tag, then bulk-add more
tags.add("gaming")
tags.update(["mobile", "audio"])
# Problem-3 Task-2: discard two tags, print the sorted tags
tags.discard("video")
tags.discard("toys")
print(sorted(tags))
# Problem-3 Task-3: membership checks with in and not in
print("audio" in tags)
print("video" not in tags)

Problem-4 (Write a Program): Brand Check.

Write a program that, for the brand set {"sony", "bose", "jbl"}, asks for a brand name (typed in ANY capitalization) and prints True if the store carries it, else False.

Enter a brand to check: Sony
True

# Problem-4: read a brand and print whether the store carries it
brands = {"sony", "bose", "jbl"}
brand = input("Enter a brand to check: ")
print(brand.lower() in brands)

Problem-5: Category Set Math.

Write a program that, for electronics = {"audio", "video", "gaming"} and clearance = {"video", "gaming", "toys"}:

• Task-1: print the sorted union (categories in either set).
• Task-2: print the sorted intersection (in both).
• Task-3: print the sorted difference (only in electronics).
• Task-4: print the sorted symmetric difference (in exactly one set).

['audio', 'gaming', 'toys', 'video']
['gaming', 'video']
['audio']
['audio', 'toys']

electronics = {"audio", "video", "gaming"}
clearance = {"video", "gaming", "toys"}
# Problem-5 Task-1: print the sorted union
print(sorted(electronics | clearance))
# Problem-5 Task-2: print the sorted intersection
print(sorted(electronics & clearance))
# Problem-5 Task-3: print the sorted difference (only in electronics)
print(sorted(electronics - clearance))
# Problem-5 Task-4: print the sorted symmetric difference
print(sorted(electronics ^ clearance))

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