Iterator arithmetic
- C++ course
- Intermediate
- Lesson 63/116
- 5 min
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.
Practice 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.
Starter code
#include <iostream>
#include <vector>
int main() {
std::vector<int> v = {7, 14, 21, 28};
// Take an iterator with begin(), access via offset
return 0;
}Sample 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;
}Expected output
21
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each operation to its result
| Term | Meaning |
|---|---|
it + 3 |
Forward offset |
it - 2 |
Backward offset |
it2 - it1 |
Distance |
it[n] |
nth element |
2 Which operation works only on random access iterators?
// this operation does not work on every iterator- Take offset with it + 5 Correct answer
- Dereference with *it
- Advance with ++it
- Compare with it != end
Explanation: it + 5 works only on random access iterators.
3 Compute the index
auto idx = std::____(v.begin(), it);auto idx = std::distance(v.begin(), it);Explanation: std::distance converts an iterator to an index.