C++ Vectors

A list that can grow

A raw array has a size the compiler must know: int xs[4]. If you do not know how many values you will get, that type is the wrong tool. vector from the standard library starts empty (or with a count) and grows as you push_back.

Include <vector>. The type is vector<int>, vector<string>, or vector of whatever element type you need. StudyGrid compiles this as C++17.

push_back and size

push_back adds one element at the end. size() returns how many you have. Size is a number of elements, not bytes. It can change after each push.

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> nums;
  nums.push_back(8);
  nums.push_back(3);
  nums.push_back(5);
  cout << nums.size() << endl;
  return 0;
}

Output is 3. You never wrote a capacity in the type. The vector asked the heap for space as needed.

Index with [i]

The first element is still index 0. nums[i] reads or writes that slot, the same idea as a raw array. Valid indices run from 0 to size() - 1. Walking off the end is undefined behavior, just as it is with arrays.

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> nums;
  nums.push_back(8);
  nums.push_back(3);
  nums.push_back(5);
  for (int i = 0; i < nums.size(); i++) {
    cout << nums[i] << endl;
  }
  return 0;
}

nums[1] = 9; would replace 3. There is also at(i), which throws if the index is out of range. [] does not check. Use [] when you already know i is in range.

Range-for

When you only need the values, not the index, a range-based for loop is shorter. Each step bindsn to the next element.

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> nums;
  nums.push_back(8);
  nums.push_back(3);
  nums.push_back(5);
  for (int n : nums) {
    cout << n << endl;
  }
  return 0;
}

Write for (int& n : nums) if you need to change each element. Writefor (const int& n : nums) if the elements are large and you only read them. Forint, a copy is cheap.

Strings work the same way

The element type is a template argument. A list of names is vector<string>. Include<string> as well.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
  vector<string> names;
  names.push_back("Ada");
  names.push_back("Linus");
  cout << names.size() << endl;
  for (const string& name : names) {
    cout << name << endl;
  }
  return 0;
}

Vector versus raw array

Keep raw arrays for tiny fixed tables (days in a week, a 3-D point). For everything you collect while the program runs, start with vector.

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> temps = {12, 9, 15, 11};
  int total = 0;
  for (int t : temps) {
    total += t;
  }
  cout << "count: " << temps.size() << endl;
  cout << "sum: " << total << endl;
  cout << "mean: " << total / static_cast<double>(temps.size()) << endl;
  return 0;
}

What the vector owns

When the vector variable goes out of scope, it frees its buffer. You do not call delete. That is why the next chapter tells beginners to prefer containers over bare new.