1.1: What is a program?

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A program is a sequence of instructions that specifies how to perform a computation. The computation might be something mathematical, such as solving a system of equations or finding the roots of a polynomial, but it can also be a symbolic computation, such as searching and replacing text in a document or something graphical, like processing an image or playing a video.

The details look different in different languages, but a few basic instructions appear in just about every language:

input:: Get data from the keyboard, a file, the network, or some other device.
output:: Display data on the screen, save it in a file, send it over the network, etc.
math:: Perform basic mathematical operations like addition and multiplication.
conditional execution:: Check for certain conditions and run the appropriate code.
repetition:: Perform some action repeatedly, usually with some variation.

Believe it or not, that’s pretty much all there is to it. Every program you’ve ever used, no matter how complicated, is made up of instructions that look pretty much like these. So you can think of programming as the process of breaking a large, complex task into smaller and smaller subtasks until the subtasks are simple enough to be performed with one of these basic instructions.