Scenario: The Blushing Snowlady
Several snow people are outdoors, on a snow-covered landscape. A snowman is trying to meet a snow woman who is talking with a friend (another snow woman.) The snowman tries and tries to get her attention. He calls out "Ahem" and blinks his eyes at her. She turns to look at the snowman and blushes. But, alas, she is not interested in meeting him. She gives him a cold shoulder and turns back to talk with her friend. He gives up and turns away.
Design
A story board is the design approach we will use to create a solution to a problem, or plan a list of actions to perform a task, as specified in the scenario. At Pixar, Disney, and other major animation studios, animators break down a long scenario into sequences of many short scenarios. For each scenario, a story board is created to depict the sequence of scenes. The story board may consist of dozens of scene sketches, drawn by animation artists or generated by computer animation specialists using computer software. This approach of designing a solution to a problem by breaking it down into sub-problems is not unique to computer programmers and animators. Playwrights, for example, break their plays down into individual acts, and the acts into individual scenes! Engineers break down complicated systems (e.g., jet airplanes and claw hammers) or devices (e.g., microcircuits) into component parts to make the problem more manageable.
Design: The Written Algorithm (or story board)
Do in order
snowman looks at the snowwoman
snowman calls out to the snowwoman
Do together
snowman blinks his eyes at the snowwoman
snowwoman turns to see who is calling her.
Do together
snowwoman blushes (her head turns red)
snowwoman turns back to her friends
snowwoman’s face turns back to white
snowman turns away (gives up)
Coding Notes
Sequential versus simultaneous actions
From our story board, it is clear that the first two actions must occur in a specific sequence – the snowman turns to look at the snowwoman and then the snowman calls out “Ahem” to the snowwoman. We can tell Alice to Do these instructions in order. But, other actions occur simultaneously (at the same time). For example, the snowman blinks his eyes at the snowwoman at the same time as the snowwoman turns to face the snowman. To have these actions occur at the same time, Alice must be told to Do these actions together. Do in order and Do together are part of the Alice language. We call them control statements because we use these statements to tell Alice how to carry out the instructions in a program.