Data type
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A data type is a constraint placed upon the interpretation of data in a type system in computer programming. Common types of data in programming languages include primitive types (such as integers, floating point numbers or characters), tuples, records, algebraic data types, abstract data types, reference types, classes and function types. A data type describes representation, interpretation and structure of values manipulated by algorithms or objects stored in computer memory or other storage device. The type system uses data type information to check correctness of computer programs that access or manipulate the data.
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[edit] 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 2006, 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 232 − 1 or signed integer values from − 231 to 231 − 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.
[edit] Primitive data types
Programming languages provide some primitive data types as basic building blocks for programs and more specialized composite types. Typically primitive data types include various integer, floating-point and string types. Although basic building blocks, arrays, records and references for relations between pieces of data might not be included in primitive data types as they can be seen as a collection of several primitive values.
[edit] Number types
[edit] Composite types
These can include the following (depending on the programming language):
- RECORD - a set of variable types - Example: a row in a database table
- TABLE - an indexed column in a database
- NESTED TABLE - unconstrained, single dimension array of a single or composite type
- VARRAY - fixed size collection of same type variable
[edit] Integer Numbers
An integer number can hold a whole number, but no fraction. For the value to be negative, you must place a hyphen symbol (-) before the value. You must not use commas as part of the number as this would generate a syntax error. Examples of integer numbers:
- 42
- 10000
- -233000
- -100
[edit] Real Numbers:
A real number can hold a whole number or a fractional number that uses a decimal point. For the value to be negative, you must place a hyphen symbol (-) before the value. Examples of real numbers:
- 20.0005
- 99.9
- -5000.12
- -9999.9991
[edit] Strings:
String data is non-numerical, and is used to store characters and words. All strings consist of characters enclosed within double quotation marks. The string data can include numbers and other numerical symbols but will be treated as text. Examples of strings are:
- "A"
- "Hello World"
- "Telephone"
- "I am 6000 years old"
- "1.2.3.4.5.6.7.8.9"
- "Yahya Adam Patel! Jero"
Each string can consist of as many characters as the memory allows. You can also have a string with no data whatsoever, represented by an empty pair of double quotation marks.
[edit] Numeric datatype ranges
Each numeric datatype has a maximum and minimum value known as the range. It is important to know the ranges, especially when dealing with smaller types, as you can only store numbers that lie within that range. Attempting to store a number outside the range may lead to compiler/runtime errors, or to incorrect calculations (due to truncation) depending on the language being used.
The range of a variable is based on the number of bytes used to save the value, and an integer datatype is usually[1] able to store 2n values (where n is the no. of bits). For other data types (e.g. floating point values) the range is more complicated and will vary depending on the method used to store it. There are also some types that do not use entire bytes, e.g. a boolean that requires a single bit, and represents a binary value (although in practice a byte is often used, with the remaining 7 bits being redundant). Some programming languages[2] also allow the opposite direction, that is, the programmer defines the range and precision needed to solve a given problem and the compiler chooses the most appropriate integer or floating point type automatically.
The following table lists a set of common of numeric datatypes and their ranges:[3]
| Data Type | Size | Range |
|---|---|---|
| Integer types | ||
| Boolean | 1 bit (though often stored as 1 byte) | 0 to 1 |
| Byte | 8 bits | 0 to 255 |
| Word | 2 bytes | 0 to 65535 |
| Double Word | 4 bytes | 0 to 4,294,967,295 |
| Integer | 4 bytes | –2,147,483,648 to 2,147,483,647 |
| Double Integer | 8 bytes | –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 |
| Floating point types | ||
| Real | 4 bytes | 1E-37 to 1E+37 (6 decimal digits) |
| Double Float | 8 bytes | 1E-307 to 1E+308 (15 decimal digits) |
[edit] Data structures
[edit] Abstract types
[edit] See also
- Type theory for the mathematical models of types
- Type system for different choices in programming language typing
[edit] Notes
- ^ There are situations where one or more bits are reserved for other functions, e.g. parity checking.
- ^ Ada, Pascal to name two.
- ^ Note that the size of most data types may vary between platforms and languages. The values listed here are the most commonly used sizes in use today.
[edit] References
- Luca Cardelli, Peter Wegner. On Understanding Types, Data Abstraction, and Polymorphism, [1] from Computing Surveys, (December, 1985)
