A visitor counter wants unique cities rather than every repeated observation.
What You Need to Understand
A set stores distinct hashable values and supports fast average membership tests. It does not promise a stable iteration order and cannot be accessed by numeric index.
When to Use It
A visitor counter wants unique cities rather than every repeated observation.
Step-by-Step Python Example
cities = {"Haifa", "Tel Aviv", "Haifa"}
cities.add("Eilat")
print(len(cities))
print("Eilat" in cities)How the Example Works
Repeated city names collapse to one set element, so the size reflects unique cities.
Example Output
3
True
2A Second Example
Compare this smaller variation with the main example, then experiment with your own values.
codes = ["P1", "P2", "P1"]
print(len(set(codes)))Important Behavior and Edge Cases
Never depend on the printed ordering of a set. Values must be hashable; a plain list cannot be inserted as one set member.
Try It Yourself
Make a set from a list of product codes and count unique codes.
Common Beginner Mistakes
- Never depend on the printed ordering of a set. Values must be hashable; a plain list cannot be inserted as one set member.
- Depending on set iteration order or using a set when duplicates and original positions must be preserved.
Key Takeaways
- A set stores distinct hashable values and supports fast average membership tests.
- Repeated city names collapse to one set element, so the size reflects unique cities.
- Never depend on the printed ordering of a set.