Dictionary Problems

Counting, grouping, and lookup, the three superpowers of dicts

Dictionaries are the Swiss Army knife of Python. These three patterns, counting, grouping, and lookup, solve an enormous range of problems.

Pattern 1: Frequency CounterCount how often each item appears. The foundation of many algorithms.

Frequency counterpython

# Count word frequencies
words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
counts = {}
for word in words:
    counts[word] = counts.get(word, 0) + 1
print(counts)   # {'apple': 3, 'banana': 2, 'cherry': 1}

# Find top item
most_common = max(counts, key=counts.get)
print(f"Most common: {most_common}")   # apple

Pattern 2: GroupingGroup items by a shared property.

Grouping patternpython

# Group words by their first letter
words = ["apple", "ant", "banana", "bear", "cherry"]
groups = {}
for word in words:
    key = word[0]
    if key not in groups:
        groups[key] = []
    groups[key].append(word)
print(groups)
# {'a': ['apple', 'ant'], 'b': ['banana', 'bear'], 'c': ['cherry']}

Pattern 3: Lookup TablePre-compute results and store them for O(1) access instead of recalculating.

Lookup tablepython

# Instead of recalculating, pre-compute
squares = {n: n**2 for n in range(1, 11)}
print(squares[7])   # 49, instant lookup
print(squares[10])  # 100

🌍In the Real World...

In log analysis: count requests per endpoint, group errors by type, map IP addresses to hostnames. In infrastructure: map resource IDs to metadata, group instances by region. Dict patterns are everywhere.

🎯

Phase Complete!

Problem Solving

Describe the 6-step problem-solving framework in your own words. Which step do you find hardest? How do you plan to overcome it?

0/500

Practice Exercises

0/3 solved

Exercise 1 of 3medium

⏱ 00:00

Top 3 Errors

Given a list of error messages, find the top 3 most frequent ones.
Given: errors = ["TimeoutError", "ConnectionError", "TimeoutError", "ValueError", "TimeoutError", "ConnectionError", "ValueError", "ValueError", "IndexError"]
Expected output:TimeoutError: 3ValueError: 3ConnectionError: 2

solution.py

1 / 3

Exercise 2 of 3medium

⏱ 00:00

Group by Length

Group words by their length.
Given: words = ["hi", "cat", "dog", "it", "elephant", "bee"]Expected output:2: ['hi', 'it']3: ['cat', 'dog', 'bee']8: ['elephant']

solution.py

2 / 3

Exercise 3 of 3medium

⏱ 00:00

Two Sum with Lookup

Using a lookup dict, find if any two numbers sum to the target in ONE pass.
Given: nums = [3, 5, 2, 9, 7], target = 12Expected output: Found: 5 + 7 = 12

solution.py

3 / 3

Solve all 3 exercises to unlock completion

Previous Next