Software / Building Blocks

How a Computer Follows Instructions

A program gives a computer instructions. It works with the values you supply and the rules you write, so a small mismatch between your intention and your rule can change the result.

• Values are the material: a cue name, a duration, or whether a prop is available.
• Variables give values names so later instructions can use them.
• Control flow chooses what runs next through conditions and loops.
• Functions package a task behind a name. A function that checks a cue can be called for every prop and tested with several examples.

Trace a cue decision to see those ideas work together.

Bring the rule from Logic: a cue must be available and known to be silent. Here is that decision in JavaScript. You can trace it on paper; the code is also ready to run in a JavaScript console.

function canPlay(cue) {
  return cue.available === true && cue.sound === false;
}

const lantern = { available: true, sound: false };
canPlay(lantern); // true

=== checks an exact value; && requires both checks to pass. The lantern returns true. That result approves a cue; it does not switch on a lamp or run a show.

1. Change available to false. Predict the result before checking it.
2. Restore availability and change sound to true. You have made the bell's case.
3. Remove the sound field. Decide whether incomplete information should count as permission.

Keep: the function and an input/output pair for each change. Give the whole cue collection a shape, or turn your predictions into repeatable checks.