Problem No 6
Count item occurrences with a dictionary
Easy≈ 10 minute session
Lesson guide
What this Python exercise practices
Count item occurrences with a dictionary is a beginner practice lesson that focuses on functions, parameters, return values. It is designed to be solved in about 10 minutes with examples, starter code, and test feedback.
Prerequisites
- Python variables
- Function parameters
- Return values
Difficulty and time
- Level
- Beginner
- Estimated time
- 10 minutes
Practice path
Summary
Count how many times each item appears in an iterable using a dictionary.
Problem statement
Given an iterable (e.g., list or string), count the frequency of each element and return a dictionary where keys are the elements and values are their counts. The order of keys in the result should reflect the order each key first appears in the input.
Task
Create a function that returns a dictionary mapping each distinct item to the number of times it appears in the input sequence. Preserve the order of first appearance.
Examples
List with repeated items
Input
count_items(['a', 'b', 'a'])
Output
{'a': 2, 'b': 1}
Explanation
The function returns counts: 'a' appears twice and 'b' once. 'a' appears before 'b', so it comes first in the dictionary.
Input format
A single iterable (commonly a list or string) passed to count_items(seq).
Output format
A dictionary mapping each distinct item to its occurrence count.
Constraints
Elements of the iterable must be hashable. The function should work for empty iterables and should preserve the order of first occurrence for keys.
Samples
Sample input 0
count_items(['x', 'x', 'y'])
Sample output 0
{'x': 2, 'y': 1}
Explanation 0
x appears twice and y once; x appears before y in the result because it appeared first in the input.
AI assistant
Ask me anything!
Need help? I can explain the core idea behind this problem, review your current code, and give targeted hints. Use “Teach Theory” for the concept, “Get AI hint” for a quick scaffold nudge, or ask a specific question below.
Chat history is temporary and will not be saved.
Free preview includes 1 Teach Theory response and 1 AI hint per unlocked preview lesson.