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In computer programming, a subroutine is a sequence of program instructions that perform a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Subprograms may be defined within programs, or separately in libraries that can be used by multiple programs. In different programming languages, a subroutine may be called a procedure, a function, a routine, a method, or a subprogram. The generic term callable unit is sometimes used. As the name ''subprogram'' suggests, a subroutine behaves in much the same way as a computer program that is used as one step in a larger program or another subprogram. A subroutine is often coded so that it can be started (called) several times and from several places during one execution of the program, including from other subroutines, and then branch back (''return'') to the next instruction after the ''call'' once the subroutine's task is done. Maurice Wilkes, David Wheeler, and Stanley Gill are credited with the invention of this concept, which they termed a ''closed subroutine'', contrasted with an ''open subroutine'' or macro. Subroutines are a powerful programming tool, and the syntax of many programming languages includes support for writing and using them. Judicious use of subroutines (for example, through the structured programming approach) will often substantially reduce the cost of developing and maintaining a large program, while increasing its quality and reliability. Subroutines, often collected into libraries, are an important mechanism for sharing and trading software. The discipline of object-oriented programming is based on objects and methods (which are subroutines attached to these objects or object classes). In the compiling method called threaded code, the executable program is basically a sequence of subroutine calls. ==Main concepts== The content of a subroutine is its body, which is the piece of program code that is executed when the subroutine is called or invoked. A subroutine may be written so that it expects to obtain one or more data values from the calling program (its parameters or formal parameters). The calling program provides actual values for these parameters, called arguments. Different programming languages may use different conventions for passing arguments: The subroutine may return a computed value to its caller (its return value), or provide various result values or output parameters. Indeed, a common use of subroutines is to implement mathematical functions, in which the purpose of the subroutine is purely to compute one or more results whose values are entirely determined by the parameters passed to the subroutine. (Examples might include computing the logarithm of a number or the determinant of a matrix.) A subroutine call may also have side effects such as modifying data structures in a computer memory, reading from or writing to a peripheral device, creating a file, halting the program or the machine, or even delaying the program's execution for a specified time. A subprogram with side effects may return different results each time it is called, even if it is called with the same arguments. An example is a random number function, available in many languages, that returns a different pseudo-random number each time it is called. The widespread use of subroutines with side effects is a characteristic of imperative programming languages. A subroutine can be coded so that it may call itself recursively, at one or more places, to perform its task. This method allows direct implementation of functions defined by mathematical induction and recursive divide and conquer algorithms. A subroutine whose purpose is to compute one boolean-valued function (that is, to answer a yes/no question) is sometimes called a predicate. In logic programming languages, often all subroutines are called predicates, since they primarily determine success or failure. For example, any type of function is a subroutine but not main(). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「subroutine」の詳細全文を読む スポンサード リンク
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