Parameter (computer programming)
 | This article may be too technical for most readers to understand. (October 2008) |
In computer programming, a parameter is a variable that takes on the meaning of a corresponding argument passed in a call to a subroutine. In the most common case, call-by-value, a parameter acts within the subroutine as a local (isolated) copy of the argument, but in other cases, e.g. call-by-reference, the argument supplied by the caller can be affected by actions within the called subroutine (as discussed in evaluation strategy).
Nearly all programming languages support subroutine parameters. The semantics for how parameters can be declared and how the arguments get passed to the parameters of subroutines are defined by the language, but the details of how this is represented in any particular computer system depends on the calling conventions of that system.
In lambda calculus, each function has exactly one parameter. What is thought of as functions with multiple parameters is usually represented in lambda calculus as a function which takes the first argument, and returns a function which takes the rest of the arguments; this is a transformation known as currying. Some programming languages, like ML and Haskell, follow this scheme. In these languages, every function has exactly one parameter, and what may look like the definition of a function of multiple parameters, is actually syntactic sugar for the definition of a function that returns a function, etc. Function application is left-associative in these languages as well as in lambda calculus, so what looks like an application of a function to multiple arguments, is correctly evaluated as the function applied to the first argument, then the resulting function applied to the second argument, etc.
Parameters and arguments
A parameter represents a value that the procedure expects you to supply when you call it. The procedure's declaration defines its parameters.
You can define a procedure with no parameters, one parameter, or more than one. The part of the procedure definition that specifies the parameters is called the parameter list.
An argument represents the value you supply to a procedure parameter when you call the procedure. The calling code supplies the arguments when it calls the procedure. The part of the procedure call that specifies the arguments is called the argument list.
Although parameters are also commonly referred to as arguments, arguments are more properly thought of as the actual values or references assigned to the parameter variables when the subroutine is called at runtime. When discussing code that is calling into a subroutine, any values or references passed into the subroutine are the arguments, and the place in the code where these values or references are given is the parameter list. When discussing the code inside the subroutine definition, the variables in the subroutine's parameter list are the parameters, while the values of the parameters at runtime are the arguments.
Many programmers use parameter and argument interchangeably, depending on context to distinguish the meaning. In practice, distinguishing between the two terms is usually unnecessary in order to use them correctly or communicate their use to other programmers. Alternatively, the words actual and formal can be used to distinguish between an argument and a parameter, respectively. For example, the equivalent terms actual argument and actual parameter may be used instead of argument; and formal argument and formal parameter may be used instead of parameter.
To better understand the difference, consider the following subroutine written in C:
int sum(int addend1, int addend2)
{
return addend1 + addend2;
}
The subroutine sum has two parameters, named addend1 and addend2. It adds the values passed into the parameters, and returns the result to the subroutine's caller (using a technique automatically supplied by the C compiler).
The code which calls the sum subroutine might look like this:
int sumValue;
int value1 = 40;
int value2 = 2;
sumValue = sum(value1, value2);
The variables value1 and value2 are initialized with values. The variables are not arguments or parameters.
At runtime, the values assigned to these variables are passed to the subroutine sum. In the sum subroutine, the parameters addend1 and addend2 are evaluated, yielding the arguments 40 and 2, respectively. The values of the arguments are added, and the result is returned to the caller, where it is assigned to the variable sumValue.
Datatypes
In strongly-typed programming languages that are explicitly typed, each parameter's type is specified in the subroutine's declaration. Languages using type inference attempt to discover the types automatically from the function's body and usage, while weakly-typed programming languages defer type resolution to run-time.
Some languages use a special keyword (e.g. void) to indicate that the subroutine has no parameters; in formal type theory, such functions take an empty parameter list (whose type is not void, but rather unit).
Argument passing
The exact mechanism for assigning arguments to parameters, called argument passing, depends upon the evaluation strategy used for that parameter (typically call-by-value), which may be specified using keywords.
Default arguments
Some programming languages such as Python, C++, and Windows PowerShell allow for a default argument to be explicitly or implicitly given in a subroutine's declaration. This allows the caller to omit that argument when calling the subroutine. If the default argument is explicitly given, then that value is used if it is not provided by the caller. If the default argument is implicit (sometimes by using a keyword such as Optional) then the language provides a well-known value (such as null, Empty, zero, an empty string, etc.) if a value is not provided by the caller.
PowerShell example:
function doc($g = 1.21) {
"$g gigawatts? $g gigawatts? Great Scott!"
}
PS> doc 1.21 gigawatts? 1.21 gigawatts? Great Scott!
PS> doc 88 88 gigawatts? 88 gigawatts? Great Scott!
Default arguments can be seen as a special case of the variable-length argument list.
Variable-length parameter lists
Some languages allow subroutines to be defined to accept a variable number of arguments. For such languages, the subroutines must iterate through the list of arguments.
PowerShell example:
function marty {
$args | foreach { "back to the year $_" }
}
PS> marty 1985 back to the year 1985
PS> marty 2015 1985 1955 back to the year 2015 back to the year 1985 back to the year 1955
Named parameters
Some programming languages allow subroutines to have named parameters. This allows the calling code to be more self-documenting. It also provides more flexibility to the caller, often allowing the order of the arguments to be changed, or for arguments to be omitted as needed.
PowerShell example:
function jennifer($young, $old) {
"Young Jennifer: I'm $young!"
"Old Jennifer: I'm $old!"
}
PS> jennifer 'old' 'young' Young Jennifer: I'm old! Old Jennifer: I'm young!
PS> jennifer -old 'young' -young 'old' Young Jennifer: I'm old! Old Jennifer: I'm young!