Value (computer science): Difference between revisions
Made summary/explanation of "referential transparency" less misleading, clearer an more readible. |
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A value is the [[normal form (term rewriting)|normal form]] of an expression, which means that an expression that is a value can not be [[reduce]]d to a simpler expression. For example, the value of the expression "1 + 2" is "3", which cannot any longer be reduced to a simpler expression. |
A value is the [[normal form (term rewriting)|normal form]] of an expression, which means that an expression that is a value can not be [[reduce]]d to a simpler expression. For example, the value of the expression "1 + 2" is "3", which cannot any longer be reduced to a simpler expression. |
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==In C: lvalues and rvalues== |
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Some languages use the idea of '''lvalues''' and '''rvalues'''. Lvalues are values that have [[memory address|addresses]], meaning they are variables or dereferenced references to a certain memory location. Rvalues are non-lvalues—a term only used to distinguish from lvalues. In C, the term lvalue originally meant something that could be assigned to (coming from left-value, indicating it was on the left side of the assignment operator), but since 'const' was added to the language, this now is termed a 'modifiable lvalue'. |
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The lvalue expression designates (refers to) an object. A non-modifiable lvalue is addressable, but not assignable. A modifiable lvalue allows the designated object to be changed as well as examined. An rvalue is any expression that is not an lvalue. It refers to a data value which is "immediate value" (look below) and consequently not addressable. |
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The notion of lvalues and rvalues was introduced by [[Combined Programming Language|CPL]]. |
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== In assembly language == |
== In assembly language == |
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Revision as of 09:40, 23 September 2009
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In computer science, a value is a sequence of bits that is interpreted according to some data type. It is possible for the same sequence of bits to have different values, depending on the type used to interpret its meaning. For instance, the value could be an integer or floating point value, or a string.
Some kinds of value are common to most programming languages (e.g., various kinds of number representations), while others are less commonly supported (e.g., Pascal supports a set type).
In declarative (high-level) languages, values have to be referentially transparent. This means that the resulting value is independent of the fact in which location a (sub-)expression needed to compute the value is stored. Only the contents of the location (the bits, whether they are 1 or 0) and their interpretation are significant. A value is the normal form of an expression, which means that an expression that is a value can not be reduced to a simpler expression. For example, the value of the expression "1 + 2" is "3", which cannot any longer be reduced to a simpler expression.
fuck encyclopedia
In assembly language
A value can be virtually any kind of data by a given data type, for instance a string, a digit, a single letter.
Most processors provide one or more instructions which take an "immediate value", sometimes referred to as "immediate" for short. An immediate value is stored as part of the instruction which employs it, usually to load into, add to, or subtract from, a register. The other parts of the instruction are the opcode, and destination. The latter may be implicit. (A non-immediate value may reside in a register, or be stored elsewhere in memory, requiring the instruction to contain a direct or indirect address [e.g. index register address] to the value.)
Some processors support more than one size of immediate data, e.g. 8 or 16 bit, employing a unique opcode and mnemonic for each instruction variant. If a programmer supplies a data value that will not fit, the assembler issues an "Out of range" error message. Most assemblers allow an immediate value to be expressed as ASCII, decimal, hexadecimal, octal, or binary data. Thus, the ASCII character "'A'" is the same as "65" or "0x41". The byte order of strings may differ between processors, depending on the assembler and computer architecture.