std::vector (quick recap and operations)

std::vector (quick recap and operations)

std::vector is the default go-to container: an array that grows while the program runs. It keeps its elements in a row and remembers its own size.

Create one

#include <vector>

std::vector<int> num = {1, 2, 3};

There is no fixed size to promise up front. The element type and an optional initializer list are all you need.

Add and count

num.push_back(4);             // num is now {1, 2, 3, 4}
std::cout << num.size();      // 4

push_back appends at the back. size() tells you how many elements are in the vector right now.

Read an element

num[0]        // the first element, fast but unchecked
num.at(1)     // the second element, checks the index

at(i) throws an exception if you read out of range; [] does not check at all.

Remove elements

num.erase(num.begin());        // remove the first element
num.erase(num.begin() + 2);   // remove the element at index 2

erase needs an iterator so it knows which position to delete. More on iterators soon.

Why is it the default?

• Instant random access by index.
• Cheap to add or remove at the back with push_back and pop_back.
• Knows its size, checks bounds on demand, and grows by itself.
• It is the style choice that keeps most simple code simple.

TL;DR

• std::vector<int> num = {1, 2, 3}; builds a growable array.
• num.push_back(4) adds at the back, num.size() counts elements.
• num[0] is unchecked; num.at(0) checks the index.
• num.erase(num.begin()) removes the first element.
• vector is the default container for the growable sequence.