Inputs

An input is data that enters a program or system so that it can be used. Without inputs, a program could only work with values it already holds, so it could never react to a new customer, a new reading or a new choice.

Where inputs come from

• A user — someone scans a membership card at a till or speaks a command into a microphone.
• A file — data saved earlier, such as a list of prices or a set of saved settings, is read into the program.
• A sensor — a device measures something in the real world and passes that measurement in, such as a soil moisture level or a distance.

In all three cases the data comes from outside the program, which is exactly what makes it an input. Data does not have to come from a person: a reading taken by a sensor is just as much an input as a barcode scanned at a till.

Inputs are not the program's own values

Data that the program has already worked out and stored in a variable is not an input, because it came from inside the program. An input is the new data handed in from outside. When a user types a value in reply to input(), that typed value is the input; the variable it is stored in is only a name the program uses afterwards.

Worked example: watering system

A garden watering system needs three pieces of data before it can choose how long to water for.

Each of these comes from outside: the moisture level was measured rather than calculated, the maximum run time was loaded from a file, and the plant type was chosen by a person. Naming the actual data item, such as the soil moisture percentage rather than a vague label, is what makes an input clear.