A data type in programming languages is an attribute of a datum which tells the computer (and the programmer) something about the kind of datum it is. This involves setting constraints on the datum, such as what values it can take and what operations may be performed upon it.

Overview

In a broad sense, a data type defines a set of values and the allowable operations on those values^[1]. Almost all programming languages explicitly include the notion of data type, though different languages may use different terminology. Most programming languages also allow the programmer to define additional data types, usually by combining multiple elements of other types and defining the valid operations of the new data type. For example, a programmer might create a new data type named "Person" that specifies that data interpreted as Person would include a name and a date of birth.

Common data types may include:

• integers,
• floating-point numbers (decimals), and
• alphanumeric strings.

For example, in the Java programming language, the "int" type represents the set of 32-bit integers ranging in value from -2,147,483,648 to 2,147,483,647, as well as the operations that can be performed on integers, such as addition, subtraction, and multiplication. Colors, on the other hand, are represented by three bytes denoting the amounts each of red, green, and blue, and one string representing that color's name; allowable operations include addition and subtraction, but not multiplication.

A data type can also be thought of as a constraint placed upon the interpretation of data in a type system, describing representation, interpretation and structure of values or objects stored in computer memory. The type system uses data type information to check correctness of computer programs that access or manipulate the data.

Types of data types

Abstract data types (ADT )

Algebraic types

Composite types

Function types

Machine data types

All data in computers based on digital electronics is represented as bits (alternatives 0 and 1) on the lowest level. The smallest addressable unit of data is a group of bits called a byte (usually an octet, which is 8 bits). The unit processed by machine code instructions is called a word (as of 2008, typically 32 or 64 bits). Most instructions interpret the word as a binary number, such that a 32-bit word can represent unsigned integer values from 0 to 2^{32}-1 or signed integer values from -2^{31} to 2^{31}-1. Because of two's complement, the machine language and machine don't need to distinguish between these unsigned and signed data types for the most part.

There is a specific set of arithmetic instructions that use a different interpretation of the bits in word as a floating-point number.

Object types

Pointer and reference types

Primitive types

See also

• Data dictionary
• Type theory for the mathematical models of types
• Type system for different choices in programming language typing

References

Further reading

• Luca Cardelli, Peter Wegner. On Understanding Types, Data Abstraction, and Polymorphism, http://citeseer.ist.psu.edu/cardelli85understanding.html from Computing Surveys, (December, 1985)

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