Programs and Programming

A program is a set of instructions. When you write down directions to your house for a friend, you are writing a program. Your friend “executes” that program by following each instruction in turn.

Every program is written in terms of a few basic operations that its reader already understands. For example, the set of operations that your friend can understand might include the following: “Turn left at Darwin Street,” “Go forward three blocks,” and “If you get to the gas station, turn around—you’ve gone too far.”

Computers are similar but have a different set of operations. Some operations are mathematical, like “Take the square root of a number,” whereas others include “Read a line from the file named data.txt” and “Make a pixel blue.”

The most important difference between a computer and an old-fashioned calculator is that you can “teach” a computer new operations by defining them in terms of old ones. For example, you can teach the computer that “Take the average” means “Add up the numbers in a sequence and divide by the sequence’s size.” You can then use the operations you have just defined to create still more operations, each layered on top of the ones that came before. It’s a lot like creating life by putting atoms together to make proteins and then combining proteins to build cells, combining cells to make organs, and combining organs to make a creature.

Defining new operations and combining them to do useful things is the heart and soul of programming. It is also a tremendously powerful way to think about other kinds of problems. As Professor Jeannette Wing wrote in Computational Thinking [Win06], computational thinking is about the following:

• Conceptualizing, not programming. Computer science isn’t computer programming. Thinking like a computer scientist means more than being able to program a computer: it requires thinking at multiple levels of abstraction.

• A way that humans, not computers, think. Computational thinking is a way humans solve problems; it isn’t trying to get humans to think like computers. Computers are dull and boring; humans are clever and imaginative. We humans make computers exciting. Equipped with computing devices, we use our cleverness to tackle problems we wouldn’t dare take on before the age of computing and build systems with functionality limited only by our imaginations.

• For everyone, everywhere. Computational thinking will be a reality when it becomes so integral to human endeavors it disappears as an explicit philosophy.

We hope that by the time you have finished reading this book, you will see the world in a slightly different way.