Add an element
Input
([1, 2], [3], [])
Output
[1, 2, 3]
Start with {1,2}, add 3 => {1,2,3}, nothing removed; return sorted list.
Full lesson preview
Add and remove set elements
Practice adding and removing elements from a set and returning the final contents.
Python practice10 minDictionaries & SetsBeginnerLast updated March 18, 2026
Problem statement
Implement a function that starts from an initial collection of items, adds items from a 'to_add' collection and removes items from a 'to_remove' collection. Use a set for these operations. Use discard when removing to avoid errors if an element is not present. Return the final unique items as a sorted list to keep output deterministic.
Task
Perform additions and removals on a set (using add and discard) and return a deterministic, sorted list of the final elements.
Examples
Input format
Three lists: initial, to_add, to_remove.
Output format
A sorted list with the final unique elements after additions and removals.
Constraints
All items are hashable and of comparable type for sorting. to_add and to_remove may contain duplicates.
Samples
Sample 1
Input
([1, 2], [3], [])
Output
[1, 2, 3]
3 is added to the set.