begin and end

Learn iterator ranges with begin() and end().

begin and end

Every STL container offers two member functions: begin() returns an iterator to the first element, and end() returns the position one past the last element (past-the-end). Together they define a range: [begin, end).

This range is half-open: begin is included, end is excluded. Loops use it != v.end(), not <. That is because every iterator category supports != but only random access iterators support <. So != is more portable.

Half-open range

The half-open nature of [begin, end) makes empty containers easy. For an empty vector begin() == end() and the loop runs zero times. No special empty check is needed.

end() is never dereferenced. The position it points at is not a valid element. It is used only for comparison. This design is consistent across all STL algorithms such as std::find and std::sort: they all expect a [begin, end) range. That is why this range idea is the foundation of the STL.

Exercise Print with begin and end

Print the vector v = {5, 4, 3, 2, 1} in a while loop using v.begin() and v.end(). Add a space after each element.

Show example solution

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {5, 4, 3, 2, 1};
    auto it = v.begin();
    while (it != v.end()) {
        std::cout << *it << " ";
        ++it;
    }
    return 0;
}

Test yourself

Complete the loop condition

auto it = v.begin();
while (it != v.____()) {
    ++it;
}

Show answer

auto it = v.begin();
while (it != v.end()) {
    ++it;
}

What does the v.end() iterator represent?
```
std::vector<int> v = {1, 2, 3};
auto e = v.end();
```
- The first element
- The position past the last element
- The last element
- An empty container
Show answer

The position past the last element — end() is the position past the last and is never dereferenced.
Match each concept to its description

v.begin()

Iterator to first
v.end()

Past the last
[begin, end)

Half-open range
begin == end

Empty container

Next lesson: Iterator types

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