In addition to speed and accuracy, one quality that makes computers powerful is their ability to evaluate information and make small decisions quickly: a thermostat checks the temperature continuously and turns on heating or cooling as soon as the temperature goes below or above a certain number; sensors on new cars react and apply brakes more quickly than we can when another car suddenly stops ahead; spam filters turn away dozens of emails to keep our inboxes clean.
In each of these cases, the computer checks a set of conditions: Is the temperature too cold? Is there something in the path of the car? Does the email look like spam?
In Chapter 4, we saw a statement that uses a condition to make a decision: the