In computer science, a value is the representation of some entity that can be manipulated by a program. The members of a type are the values of that type. The "value of a variable" is given by the corresponding mapping in the environment. In languages with assignable variables it becomes necessary to distinguish between the r-value and the l-value of a variable. In declarative languages, values have to be referentially transparent. This means that the resulting value is independent of the location in which a expression needed to compute the value is stored. Only the contents of the location and their interpretation are significant.
Assignment: l-values and r-values
Some languages use the idea of l-values and r-values, deriving from the typical mode of evaluation on the left and right hand side of an assignment statement. An l-value refers to an object that persists beyond a single expression. An r-value is a temporary value that does not persist beyond the expression that uses it. The notion of l-values and r-values was introduced by Combined Programming Language. The notions in an expression of r-value, l-value, and r-value/l-value are analogous to the parameter modes of input parameter, output parameter, and input/output parameter, though the technical details differ between contexts and languages.
R-values and addresses
In many languages, notably the C family, l-values have storage addresses that are programmatically accessible to the running program, meaning that they are variables or de-referenced references to a certain memory location. R-values can be l-values or non-l-values—a term only used to distinguish from l-values. Consider the C expression. When executed, the computer generates an integer value of 13, but because the program has not explicitly designated where in the computer this 13 is stored, the expression is a non l-value. On the other hand, if a C program declares a variable x and assigns the value of 13 to x, then the expression has a value of 13 and is an l-value. In C, the term l-value originally meant something that could be assigned to, but since the reserved word was added to the language, the term is now 'modifiable l-value'. In C++11 a special semantic-glyph exists, to denote the use/access of the expression's address for the compiler only; i.e., the address cannot be retrieved using the address-of operator during the run-time of the program. This type of reference can be applied to all r-values including non-l-values as well as l-values. Some 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. The l-value expression designates an object. A non-modifiable l-value is addressable, but not assignable. A modifiable l-value allows the designated object to be changed as well as examined. An r-value is any expression, a non-l-value is any expression that is not an l-value. One example is an "immediate value" and consequently not addressable..
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. Processors often 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 is the same as or. The byte order of strings may differ between processors, depending on the assembler and computer architecture.
