Iterator arithmetic

Learn arithmetic on random access iterators.

Iterator arithmetic

Random access iterators support pointer arithmetic. it + n gives the position n elements forward, it - n gives n elements back. it2 - it1 gives the distance between two iterators as a count of elements. it[n] is shorthand for *(it + n).

These operations work only on random access iterators. Using + on a forward or bidirectional iterator causes a compile error. Containers like std::vector and std::array support this arithmetic; std::list does not.

std::distance

There are two ways to find the distance between two iterators. On random access iterators it2 - it1 gives the distance directly. For other categories use std::distance(it1, it2), which advances step by step.

To find the index of an element use std::distance(v.begin(), it). For example after auto it = std::find(v.begin(), v.end(), 30); the call std::distance(v.begin(), it) gives the index of 30. This is the standard way to convert an iterator to an index.

Exercise Access the third element via offset

For v = {7, 14, 21, 28}, take an iterator with v.begin() and print the third element using *(it + 2) with std::cout.

Show example solution

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {7, 14, 21, 28};
    auto it = v.begin();
    std::cout << *(it + 2);
    return 0;
}

Test yourself

Match each operation to its result

it + 3

Forward offset
it - 2

Backward offset
it2 - it1

Distance
it[n]

nth element
Which operation works only on random access iterators?
```
// this operation does not work on every iterator
```
- Take offset with it + 5
- Dereference with *it
- Advance with ++it
- Compare with it != end
Show answer

Take offset with it + 5 — it + 5 works only on random access iterators.

Compute the index

auto idx = std::____(v.begin(), it);

Show answer

auto idx = std::distance(v.begin(), it);

Next lesson: Summary - Iterators

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