How to Iterate Over Set in C++: Simple Examples and Tips

To iterate over a std::set in C++, you can use a range-based for loop or iterators. The range-based loop is simple and reads each element in ascending order automatically.

Syntax

You can iterate over a std::set using either a range-based for loop or explicit iterators.

• Range-based for loop: Automatically loops through each element.
• Iterator loop: Uses begin() and end() to access elements manually.

cpp

std::set<int> mySet = {1, 2, 3};

// Range-based for loop
for (const auto& element : mySet) {
    // use element
}

// Iterator loop
for (auto it = mySet.begin(); it != mySet.end(); ++it) {
    // use *it
}

Example

This example shows how to create a std::set of integers and print each element using a range-based for loop.

cpp

#include <iostream>
#include <set>

int main() {
    std::set<int> numbers = {10, 5, 20, 15};

    for (const auto& num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output

5 10 15 20 

Common Pitfalls

Common mistakes when iterating over a std::set include:

• Trying to modify elements directly (sets are ordered and elements are const).
• Using a loop that modifies the set while iterating, which invalidates iterators.
• Assuming the order is insertion order; std::set always sorts elements.

cpp

/* Wrong: Trying to modify elements in set */
std::set<int> s = {1, 2, 3};
for (auto& x : s) {
    // x = x + 1; // Error: elements are const
}

/* Correct: Create a new set or copy elements if modification needed */
std::set<int> s2;
for (const auto& x : s) {
    s2.insert(x + 1);
}

Quick Reference

Tips for iterating over std::set:

• Use range-based for loops for simplicity.
• Remember elements are sorted automatically.
• Do not modify elements in place.
• Use iterators if you need more control.

Key Takeaways

Use range-based for loops to iterate over std::set easily and clearly.

Elements in std::set are always sorted in ascending order.

You cannot modify elements directly inside a std::set during iteration.

Use iterators when you need manual control over the iteration process.

Avoid changing the set while iterating to prevent iterator invalidation.