Primitive data type
|This article needs additional citations for verification. (March 2015) (Learn how and when to remove this template message)|
||This article needs attention from an expert in Computer science. (March 2015)|
- a basic type is a data type provided by a programming language as a basic building block. Most languages allow more complicated composite types to be recursively constructed starting from basic types.
- a built-in type is a data type for which the programming language provides built-in support.
In most programming languages, all basic data types are built-in. In addition, many languages also provide a set of composite data types. Opinions vary as to whether a built-in type that is not basic should be considered "primitive".
Depending on the language and its implementation, primitive data types may or may not have a one-to-one correspondence with objects in the computer's memory. However, one usually expects operations on basic primitive data types to be the fastest language constructs there are. Integer addition, for example, can be performed as a single machine instruction, and some processors offer specific instructions to process sequences of characters with a single instruction. In particular, the C standard mentions that "a 'plain' int object has the natural size suggested by the architecture of the execution environment". This means that
int is likely to be 32 bits long on a 32-bit architecture. Basic primitive types are almost always value types.
Most languages do not allow the behavior or capabilities of primitive (either built-in or basic) data types to be modified by programs. Exceptions include Smalltalk, which permits all data types to be extended within a program, adding to the operations that can be performed on them or even redefining the built-in operations.
Classic basic primitive types may include:
- Character (
- Integer (
byte) with a variety of precisions;
- Floating-point number (
- Fixed-point number (
fixed) with a variety of precisions and a programmer-selected scale.
- Boolean, logical values true and false.
- Reference (also called a pointer or handle), a small value referring to another object's address in memory, possibly a much larger one.
More sophisticated types which can be built-in include:
- Tuple in ML, Python
- List in Lisp
- Complex number in Fortran, C (C99), Lisp, Python, Perl 6, D
- Rational number in Lisp, Perl 6
- Associative array in various guises, in Lisp, Perl, Python, Lua, D
- First-class function, closure, continuation in languages that support functional programming such as Lisp, ML, Perl 6, D and C# 3.0
Specific primitive data types
An integer data type can hold a whole number, but no fraction. Integers may be either signed (allowing negative values) or unsigned (nonnegative values only). Typical sizes of integers are:
|Size (bytes)||Size (bits)||Names||Signed range||Unsigned range|
|1 byte||8 bits||Byte, octet, char||−128 to +127||0 to 255|
|2 bytes||16 bits||Word (x86), short int||−32,768 to +32,767||0 to 65,535|
|4 bytes||32 bits||Double Word (x86), int||−2,147,483,648 to +2,147,483,647||0 to 4,294,967,295|
|8 bytes||64 bits||long int (C in 64-bit linux), long long (C), long (Java, the signed integer variant only)||−9,223,372,036,854,775,808 to +9,223,372,036,854,775,807||0 to 18,446,744,073,709,551,615|
|unlimited / 8||unlimited||Bignum||–( ( 2unlimited ) / 2 ) to +( ( 2unlimited ) / 2 )||0 to 2unlimited|
Literals for integers consist of a sequence of digits. Most programming languages disallow use of commas for digit grouping, although Fortran (77, 90, and above, fixed form source but not free form source) allows embedded spaces, and Perl, Ruby, Java, Julia and D allow embedded underscores for clarity (e.g. 10_000_000). Negation is indicated by a minus sign (-) before the value. Examples of integer literals are:
A boolean type, typically denoted "bool" or "boolean", is typically a logical type that can be either "true" or "false". Although only one bit is necessary to accommodate the value set "true" and "false", programming languages typically implement boolean types as one or more bytes.
Many languages (e.g. Java, Pascal and Ada) implement booleans adhering to the concept of boolean as a distinct logical type. Languages, though, may implicitly convert booleans to numeric types at times to give extended semantics to booleans and boolean expressions or to achieve backwards compatibility with earlier versions of the language. In C++, e.g., boolean values may be implicitly converted to integers, according to the mapping false → 0 and true → 1 (for example,
true + true would be a valid expression evaluating to 2). The boolean type
bool in C++ is considered an integral type and is a cross between numeric type and a logical type.
A floating-point number represents a limited-precision rational number that may have a fractional part. These numbers are stored internally in a format equivalent to scientific notation, typically in binary but sometimes in decimal. Because floating-point numbers have limited precision, only a subset of real or rational numbers are exactly representable; other numbers can be represented only approximately.
Literals for floating point numbers include a decimal point, and typically use "e" or "E" to denote scientific notation. Examples of floating-point literals are:
A fixed-point number represents a limited-precision rational number that may have a fractional part. These numbers are stored internally in a scaled-integer form, typically in binary but sometimes in decimal. Because fixed-point numbers have limited precision, only a subset of real or rational numbers are exactly representable; other numbers can be represented only approximately. Fixed-point numbers also tend to have a more limited range of values than floating point, and so the programmer must be careful to avoid overflow in intermediate calculations as well as the final results.
Characters and strings
A character type (typically called "char") may contain a single letter, digit, punctuation mark, symbol, formatting code, control code, or some other specialized code (e.g., a byte order mark). In C,
Characters may be combined into strings. The string data can include numbers and other numerical symbols but will be treated as text.
Strings are implemented in various ways, depending on the programming language. The simplest way to implement strings is to create them as an array of characters, followed by a delimiting character used to signal the end of the string, usually NUL. These are referred to as null-terminated strings, and are usually found in languages with a low amount of hardware abstraction, such as C and Assembly. While easy to implement, null terminated strings have been criticized for causing buffer overflows. Most high-level scripting languages, such as Python, Ruby, and many dialects of BASIC, have no separate character type; strings with a length of one are normally used to represent single characters. Some languages, such as C++ and Java, have the capability to use null-terminated strings (usually for backwards-compatibility measures), but additionally provide their own class for string handling (
java.lang.String, respectively) in the standard library.
Literals for characters and strings are usually surrounded by quotation marks: sometimes, single quotes (') are used for characters and double quotes (") are used for strings.
Examples of character literals in C syntax are:
- '\t' (tab character)
Examples of string literals in C syntax are:
- "Hello World"
Numeric data type ranges
Each numeric data type has its maximum and minimum value known as the 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 data type is usually able to store values (where is the number of bits that contribute to the value). 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 (such as Ada and Pascal) 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.
- Fog, Agner (2010-02-16). "Calling conventions for different C++ compilers and operating systems: Chapter 3, Data Representation" (PDF). Retrieved 2010-08-30.
- Owens, Sean R. (2009-11-05). "Java and unsigned int, unsigned short, unsigned byte, unsigned long, etc. (Or rather, the lack thereof)". Retrieved 2010-09-05.