Data Types

Imagine being asked to store the item 17. Before a program can work with it, the program needs to know what kind of data it is handling. Is it a whole number, a number with a decimal point, a single symbol, or a sequence of text? A data type supplies that category. Throughout this article, you will trace how a value is named, located, and classified, so that the idea of a data type becomes concrete.

Programs need a way to refer to the data they use. A variable provides that way: it is a memory location given a name. Because the location has a name, the program can refer to the data through that name rather than needing to know the location itself.

The diagram shows the three parts of the relationship. The name score identifies a location. That location holds the value 17. The name and the location are not the same thing: the name is how the program refers to the storage, and the location is the storage that actually holds the data.

The programmer refers to a variable by its name, not by its memory location. This keeps the programmer focused on meaningful names such as score rather than on the physical location of the storage.

The compiler is responsible for keeping track of the memory location associated with that name. The programmer writes the name, and the compiler arranges for that name to refer to the correct storage. The name is the part the programmer works with; the location is the part the compiler manages.

The important step is to classify the value before thinking about how a variable might use it. The type is the category that the program uses to interpret what is stored in the location. The computer uses different representations for different data types, so the type determines how the stored data is handled.

Two common categories of numeric data are distinguished in this material. Integers and floating-point numbers are distinct, even when their written values appear closely related.

The visible difference between 17 and 17.0 is small, but the computer treats them as different kinds of data: the first is an integer and the second is a floating-point number. The integer 17 and the floating-point number 17.0 belong to different data types and are represented differently inside the computer.

A programmer may see two numbers that look almost identical, but the computer does not treat every written number as the same kind of value. This distinction is one reason a variable's type matters: it tells the program what sort of data is being stored.

Characters are a different kind of stored data from numbers. A character represents an individual symbol or letter. A string goes further: it is a sequence of characters and can therefore contain multiple characters.

In the diagram, the single character A is one symbol. The string Hello is a sequence of five characters, shown here in order. The source describes a string as a sequence of characters that can contain multiple characters; the ordering of its characters is what distinguishes it from a single symbol.