std::vector
- C++ course
- Intermediate
- Lesson 53/116
- 5 min
Learn to use dynamic arrays with std::vector.
std::vector
std::vector is a dynamic array that grows automatically. You add elements to the end with push_back; size() returns the count. Memory is contiguous, so it is cache-friendly.
Example: std::vector<int> v; v.push_back(10); v.push_back(20);. Now v.size() returns 2. v[0] accesses the first element, which is 10. vector is the most commonly used STL container; random access is O(1).
Vector access
You access vector elements with [] or .at(). v[0] is the first element, v[1] the second. .at(0) does bounds checking; it throws on out-of-range access.
Example: v[0] is fast but undefined for out-of-bounds indices. v.at(0) is safer but slightly slower. Looping: for (int i = 0; i < v.size(); ++i) std::cout << v[i];. This prints all elements in order.
Practice Work with vector
Create std::vector<int> v;. Add 10, 20, 30 with push_back. Print v.size() and v[0] on separate lines with std::cout.
Starter code
#include <iostream>
#include <vector>
// create a vector and add three elements
int main() {
// print size and first element
return 0;
}Sample solution
#include <iostream>
#include <vector>
int main() {
std::vector<int> v;
v.push_back(10);
v.push_back(20);
v.push_back(30);
std::cout << v.size() << std::endl;
std::cout << v[0] << std::endl;
return 0;
}Expected output
3 10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does push_back do?
v.push_back(10);- Removes the first element
- Clears all elements
- Adds an element to the end Correct answer
- Fixes the size
Explanation: push_back appends a new element to the vector.
2 Complete the push_back call
v.push_back(____);v.push_back(10);Explanation: v.push_back(10); adds 10 to the vector.
3 Match method to role
| Term | Meaning |
|---|---|
push_back |
Add to end |
size() |
Element count |
v[0] |
First element |
pop_back |
Remove last |