Exercises: Dictionaries

A space club needs a small lookup table for the length of a day in whole Earth hours.

Specification: Define day_hours(planet). Inside the function, make the dictionary {'Mercury': 4223, 'Earth': 24, 'Mars': 25}. Return the value stored at key planet with square-bracket access. Each sample uses a key that exists.

Worked sample calls:

day_hours("Earth") -> 24
day_hours("Mars") -> 25
day_hours("Mercury") -> 4223

A snack table should report zero when a requested snack is missing.

Specification: Define snack_count(snacks, name). Return snacks.get(name, 0). Do not change the input dictionary.

Worked sample calls:

snack_count({"apple": 5, "cracker": 8}, "apple") -> 5
snack_count({"apple": 5, "cracker": 8}, "banana") -> 0
snack_count({}, "cracker") -> 0

A classroom manager needs the total number of supplies across every labelled bin.

Specification: Define total_supplies(supplies). Start total at 0. Loop with for item, amount in supplies.items(): and add each integer amount to total. Return the final total. The two loop names receive one key/value pair per trip.

Worked sample calls:

total_supplies({"pens": 9, "paper": 20, "tape": 2}) -> 31
total_supplies({"clips": 7}) -> 7
total_supplies({}) -> 0

A transit map stores the rider count for each station.

Specification: Define busiest_station(riders) for a non-empty dictionary of non-negative integer counts. Start best_station = "" and best_count = -1. Loop over riders.items(). Replace both best values only when the current count is strictly greater than best_count. Return the winning station. If counts tie, the first tied key wins. Do not call max for this exercise.

Worked sample calls:

busiest_station({"Oak": 12, "Pine": 19, "Elm": 8}) -> "Pine"
busiest_station({"North": 4}) -> "North"
busiest_station({"Red": 7, "Blue": 7, "Gold": 3}) -> "Red"

A word game wants "Cloud" and "cloud" to count as the same word.

Specification: Define word_counts(words). Start with an empty dictionary. For each word, set key = word.lower(). Use the primary missing-key pattern: when key in counts, assign counts[key] = counts[key] + 1; otherwise assign counts[key] = 1. Return the completed dictionary.

Worked sample calls:

word_counts(["Cloud", "rain", "cloud"]) -> {'cloud': 2, 'rain': 1}
word_counts(["GO", "go", "Go", "stop"]) -> {'go': 3, 'stop': 1}
word_counts([]) -> {}

A librarian groups non-empty words by their first letter while keeping the original order within each group.

Specification: Define shelve_by_first(words). Start with an empty dictionary. For each word, use letter = word[0]. Build each list value with exactly this explicit form:

if letter in shelves:
    shelves[letter].append(word)
else:
    shelves[letter] = [word]

Return shelves. Do not use get for this grouping job.

Worked sample calls:

shelve_by_first(["cat", "crow", "dog", "camel"]) -> {'c': ['cat', 'crow', 'camel'], 'd': ['dog']}
shelve_by_first(["sun", "moon", "star"]) -> {'s': ['sun', 'star'], 'm': ['moon']}
shelve_by_first([]) -> {}

A game host wants a leaderboard ordered from the lowest score to the highest.

Specification: Define score_board(scores). Start board as an empty list. Loop over scores.items() and append [score, name] for each pair. Call board.sort() with no arguments, then return board. The sample scores are unique.

Worked sample calls:

score_board({"Mia": 14, "Leo": 9, "Zoe": 18}) -> [[9, 'Leo'], [14, 'Mia'], [18, 'Zoe']]
score_board({"Ivy": 3, "Kai": 11}) -> [[3, 'Ivy'], [11, 'Kai']]
score_board({}) -> []

A signal desk turns letters into Morse code.

Specification: Define to_morse(message). Add a purpose comment. Use this given table: {"A": ".-", "E": ".", "N": "-.", "O": "---", "S": "...", "T": "-"}. For each uppercase letter, append its code to a list. Return " ".join(codes).

Worked sample calls:

to_morse("NOTE") -> "-. --- - ."
to_morse("SOS") -> "... --- ..."

A history display converts Roman numerals into whole numbers.

Specification: Define roman_to_int(s). Add a purpose comment. Use {"I": 1, "V": 5, "X": 10, "L": 50, "C": 100}. Visit each position by index. If its value is smaller than the next symbol’s value, subtract it; otherwise add it. The last symbol is added. Return the total. Use an if/elif/else chain to distinguish a smaller next value, the last position, and an ordinary addition.

Worked sample calls:

roman_to_int("XLII") -> 42
roman_to_int("LXIX") -> 69

A word artist turns letter counts into bars.

Specification: Define tally_chart(word). Add a purpose comment. Count each letter in a dictionary. Visit its keys in first-appearance order and append each row as f"{ch} " + "#" * count to a list. Return "\n".join(rows); no trailing spaces.

Worked sample calls:

tally_chart("papaya") -> "p ##\na ###\ny #"

A decoder needs labels for short codes.

Specification: Define invert(mapping). The input values are unique. Loop over mapping.items() and assign each old value as a new key with the old key as its value. Return the new dictionary.

Worked sample calls:

invert({"cold": "C", "warm": "W", "hot": "H"}) -> {'C': 'cold', 'W': 'warm', 'H': 'hot'}

A word puzzle compares letter supplies.

Specification: Define is_anagram(first, second). Build a separate letter-count dictionary for each word, then compare the dictionaries with ==. Use the same tally method on both words; return the Boolean result.

Worked sample calls:

is_anagram("listen", "silent") -> True
is_anagram("cider", "crumb") -> False

A checkout table adds item prices, including repeated purchases.

Specification: Define cart_total(prices, items). Start a total at zero; for each item in the list, add prices[item]. Return the total. All listed items have a price.

Worked sample calls:

cart_total({"pen": 3, "pad": 5, "ink": 12}, ["pen", "ink", "pen"]) -> 18

A check digit flags a mistyped number.

Specification: Define luhn_valid(s). Scan the digit string from right to left using s[len(s) - 1 - i]. Convert each digit with int. Double every second digit from the right (positions 1, 3, ...); if a doubled value exceeds 9, subtract 9. Add values to a total and return whether total % 10 == 0. Do not use a negative slice step.

Worked sample calls:

luhn_valid("79927398713") -> True
luhn_valid("79927398710") -> False

Several records may update the same player total.

Specification: Define parse_records(records). Each string is name score. Split each record and convert parts[1] with int. If the name already exists, add the score to its stored total; otherwise store the first score. Return the dictionary.

Worked sample calls:

parse_records(["Rin 12", "Oto 7", "Rin 5"]) -> {'Rin': 17, 'Oto': 7}

A word shelf groups words of equal length.

Specification: Define group_by_length(words). Use word lengths as dictionary keys and lists of words as values. For each word, use an explicit membership test: append to an existing group, or assign [word] to a new group. Keep word order.

Worked sample calls:

group_by_length(["elk", "ibex", "yak", "hare", "gnu"]) -> {3: ['elk', 'yak', 'gnu'], 4: ['ibex', 'hare']}

A typesetter needs the most common letter.

Specification: Define top_letter(word) for a non-empty word with one unique most frequent letter. Tally its letters with a conditional increment, then scan the pairs with a strict > comparison to keep the leader. Return its letter.

Worked sample calls:

top_letter("bookkeeper") -> "e"

A poll report converts counts into percentages.

Specification: Define percentages(votes) for a non-empty dictionary with a positive total. First total all counts with a loop. Then build a new dictionary whose value for each key is count / total * 100. Return it.

Worked sample calls:

percentages({"yes": 6, "no": 2}) -> {'yes': 75.0, 'no': 25.0}

A pocket glossary changes known words and keeps unknown ones.

Specification: Define translate(line, dictionary). Split the line into words. For each word, append dictionary.get(word, word) to a result list. Return " ".join(result).

Worked sample calls:

translate("a red hen", {"red": "rojo", "hen": "gallina"}) -> "a rojo gallina"

A stock report asks for an absent key.

Specification: The broken code is:

stock = {"nails": 40}
print(stock["screws"])

KeyError: 'screws'

Define safe_stock(stock, name) to return stock.get(name, 0). Use it to repair the printed lookup without changing the dictionary.

Worked sample calls:

safe_stock({"nails": 40}, "screws") -> 0
print(safe_stock({"nails": 40}, "screws")) -> 0

Trace what two assignments do to a dictionary.

Specification: Predict the output of this code, then define updated_dict() that performs the same assignments and returns the dictionary:

d = {"a": 1}
d["a"] = d["a"] + 5
d["b"] = 2
print(d)

This code runs without an error — predict what it prints.

Worked sample calls:

updated_dict() -> {'a': 6, 'b': 2}
print(updated_dict()) -> {'a': 6, 'b': 2}

# Define updated_dict(), then try all sample calls.

A catalog prints its labels alphabetically.

Specification: Define sorted_keys(mapping). Append every key to a list, call .sort() on that list with no arguments, and return it. Do not call sorted(mapping).

Worked sample calls:

sorted_keys({"quince": 4, "lime": 9, "date": 2}) -> ['date', 'lime', 'quince']

A supply list selects items below a cutoff.

Specification: Define low_stock(stock, limit). Loop through keys in dictionary order, append each key whose value is less than limit, and return the list.

Worked sample calls:

low_stock({"bolts": 3, "gears": 12, "springs": 1}, 5) -> ['bolts', 'springs']

A cashier needs names absent from a price table.

Specification: Define missing_prices(prices, items). Visit items in list order; append an item when it is not in the price dictionary. Return the list.

Worked sample calls:

missing_prices({"pen": 3, "pad": 5}, ["pad", "cap", "pen", "mug"]) -> ['cap', 'mug']

A tournament needs the names whose scores meet a cutoff.

Specification: Define qualifiers(scores, minimum). Start an empty list named passing. Loop over scores.items(). If a score is at least minimum, append its name to passing. Return the list. Keep the dictionary's visit order.

Worked sample calls:

qualifiers({"Ana": 16, "Bo": 7, "Cy": 12}, 10) -> ['Ana', 'Cy']
qualifiers({"Dee": 5, "Eli": 6}, 6) -> ['Eli']
qualifiers({"Fox": 4}, 9) -> []

A headline tool needs both a case-insensitive tally and its most common word.

Specification: Define word_winner(words) for a non-empty list with one unique most-common lowercase word. First build counts with a loop. This time use the shortcut assignment counts[key] = counts.get(key, 0) + 1 after setting key = word.lower(). Then use a separate strict find-extreme loop over counts.items(). Return an f-string in the form "word: count".

Worked sample calls:

word_winner(["Red", "blue", "red"]) -> "red: 2"
word_winner(["owl", "fox", "FOX", "fox"]) -> "fox: 3"
word_winner(["solo"]) -> "solo: 1"

A receiver turns separated Morse codes into letters.

Specification: Define from_morse(message). Use the same given code table as Morse Encoder: {"A": ".-", "E": ".", "N": "-.", "O": "---", "S": "...", "T": "-"}. Invert the table with a loop. Split the Morse message on spaces, look up each code, and build the returned word.

Worked sample calls:

from_morse("- . -.") -> "TEN"

A report card shows averages in name order.

Specification: Define report_card(scores) where each name maps to a non-empty list of scores. Collect names into a list and call .sort(). For each name, compute its average and append the row f"{name} {average}". Return "\n".join(rows) with no trailing spaces.

Worked sample calls:

report_card({"Pax": [70, 75, 80], "Juno": [90, 80]}) -> "Juno 85.0\nPax 75.0"