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CHAPTER 9. FUNCTIONS AND PARAMETERS use .net vs 2010 data matrix ecc200 generation toproduce data matrix barcode for .net iPad Function Syntax Fixed versus Variable Argument Functions Many language Data Matrix barcode for .NET s require that a function be de ned with a xed number of parameters of speci ed types. The required parameters and types for prede ned functions are speci ed in the syntax of the language.

The elements required for a control statement are similarly de ned by the language syntax. The parser requires every statement and function call to be well-formed: it counts the arguments, checks their types, and binds them to the de ned dummy parameter names. If either the number or the type of arguments is wrong, the program is not well-formed and the translator gives an error comment.

If some element is misplaced or missing, the translator cannot, in general, know the intended meaning of the program. For example, if an END statement is missing, it is impossible in many languages for a translator to guess, accurately, where it was supposed to be. Because the parser rejects programs that are not well-formed, being well-formed can be seen as a syntactic requirement.

But the syntax merely re ects semantic requirements: the code that de nes the semantics of these statements and functions is not prepared to handle a variable number or type of argument. A function uses a certain number of parameters; that is, it refers to their names. There are several ways in which there might be a mismatch between the number of arguments passed and the number of parameters that are actually used, as follows: Unused arguments: Parameters might be named and passed but not referred to anywhere in the function.

In a procedural language, an unused argument might as well be omitted. In functional languages, however, there are situations in which this is useful. (Consider the lambda calculus functions T and F , de ned in Section 4.

3.4, which each discard one parameter.) Optional parameters: A parameter might be named and used, but no corresponding argument passed.

This can be meaningful if default values are de ned for missing arguments, and the argument-parameter correspondence is unambiguously speci ed. Inde nite-length argument lists: Parameters might be passed but not named. These can be useful if some way is supplied by the language for referring to them.

Optional parameters. Several modern languages support optional parameters. If the number of actual arguments passed is smaller than the number of parameters called for, some parameters cannot be given a meaning in the normal way.

If this is permitted, the meaning of such an omission must be de ned by default or explicitly within the function. Every parameter must have a meaning. 9.1. FUNCTION SYNTAX Exhibit 9.1. Data Matrix barcode for .

NET An Ada function with a default value for one parameter. Imagine that this function is a small routine inside a chef s program for guring out ingredient quantities when the quantity of a recipe must be changed. The usually case is to double the recipe, so the program has an optional parameter with a default value of 2.

0. function CONVERT ( ) begin CONVERT(3.75) CONVERT(3.

75, .5) quantity:real, proportion: real := 2.0 return real is return quantity*proportion.

end CONVERT;. Legal and mea VS .NET Data Matrix 2d barcode ningful calls on CONVERT: -- Double the amount, result is 7.5.

-- Halve the amount, result is 1.675..

if and when i t is actually used in a computation. One solution, used in Ada, is that parameters may be de ned to have default values. If an actual argument is supplied, its value is used for that parameter, otherwise the default value is used [Exhibit 9.

1].. Parameter Correspondence When calling a function with optional parameters, arguments may (or may not) be omitted. This complicates the problem of matching up argument values and parameter names, and makes the simple positional correspondence syntax inadequate. The problem can be handled in three ways: Permit the user to use adjacent commas to hold the place for an omitted argument.

Require all arguments that are supplied to precede the ones that are omitted. Use correspondence-by-keyword for all arguments after the rst one that is omitted, or for all optional arguments. All three ways have been used.

For example, command line interpreters commonly use the rst and third conventions, and Ada supports the second and third. The Ada CONVERT function from Exhibit 9.1 had one optional parameter.

Because it was the last parameter, we could use positional argument correspondence to call the CONVERT function. If a function has several optional parameters, however, which could be included or omitted independently, positional correspondence cannot be used. Exhibit 9.

2 shows how correspondenceby-keyword can be used to specify the correct parameter values. The rule in Ada is that all positional arguments in a call must precede all keyword arguments. Pros and Cons.

Whether or not a language should support optional parameters reduces to the usual question of values. Adding syntax and semantic mechanisms to support correspondence-bykeyword does complicate a language and its compiler. Sometimes the e ect of optional parameters.

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