Lists

Many values under one name

A list holds several values in order, inside square brackets:

balls = ["red", "green", "blue"]
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". Negative indexes count back from the end, so balls[-1] is the last item, "blue". An index past the end, like balls[5], stops the program with an IndexError.

Working with lists

• len(balls) is the number of items, 3.
• balls.append("yellow") adds an item to the end. It changes the list itself.
• "red" in balls is True when "red" is one of the items. not in asks the opposite.
• A slice copies part of a list: angles[1:3] is the items at indexes 1 and 2, [0.5, 1.2]. The item at the end index is left out.

balls = ["red", "green"]
balls.append("blue")
print(balls, len(balls))    # ['red', 'green', 'blue'] 3

Robots keep lists 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 list 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, append it. Either way, return balls.