Modules added to software projects will contain one or more subroutines. - These subroutines can be either procedures or functions.
- In most languages the syntax used to call or execute subroutines written from scratch, and subroutines included from outside sources, is similar or in most cases identical.
- However, there are differences in the way a procedure call and a function call are implemented within the source code of all languages.
Procedure calls - The name given to a procedure is a special kind of identifier.
- This identifier does not have a data type, rather it is used by the programming language to locate the subprogram within the source code.
- In most languages, the use of a procedure’s identifier causes control to execute the associated procedure.
- In essence, the procedure’s name, together with any parameters, is the statement used to execute the procedure.
- For example, if a Visual Basic procedure has been written called DoThis it can be called using the statement Call DoThis or more simply by the statement DoThis.
Function calls - Functions are similar in most respects to procedures.
- However, the identifier used to name a function also has a data type.
- This data type defines the type of data that will be returned by the function.
- Functions should be designed to return a single data item via this return value.
- This is why they are called functions. In mathematics, a function is defined to be a relationship where each set of inputs returns a single unique output.
- Subroutines written as functions should adhere to this definition.
- Unfortunately many programming languages do not enforce this rule.
- Function calls form part of expressions.
- For example, 5 + Cos(34) is an expression that calls the Cosine function using the value 34 as its parameter.
- The result is returned via the Cos identifier.
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