Vectors

Many values under one name

A std::vector holds several values of one type, in order. The type of the items goes in angle brackets:

std::vector<std::string> balls = {"red", "green", "blue"};
std::vector<double> angles = {0.0, 0.5, 1.2, -0.3};

Each item has a position called its index, counting from 0: balls[0] is "red" and balls[2] is "blue". balls.front() and balls.back() are the first and last items.

C++ doesn't check balls[5]: reading past the end gives garbage or crashes the program. balls.at(5) checks, and stops the program with an std::out_of_range error instead. Use .at() while you're unsure of an index.

Working with vectors

• balls.size() is the number of items, 3.
• balls.push_back("yellow") adds an item to the end. It changes the vector itself.
• std::ranges::count(balls, "red"), from #include <algorithm>, counts how many items equal "red". Zero means it isn't there.
• return {a, b}; builds and returns a new vector from its items, when the function's return type is a vector.

std::vector<std::string> balls = {"red", "green"};
balls.push_back("blue");
std::cout << balls.size() << " " << balls.back() << "\n";   // 3 blue

std::cout << balls doesn't compile: print the items one at a time. The next step's loops make that easy.

Robots keep vectors everywhere: the four joint angles of the arm, the balls on the table, the points along a path.

Your task

1. Write first_and_last(items) so it returns a new vector holding the first and the last item of items. For {"red", "green", "blue"} it returns {"red", "blue"}.
2. Write add_ball(balls, color). If color isn't in balls yet, add it to the end. Either way, return balls.