MACROMEDIA COLDFUSION 4.5-CFML LANGUAGE Manuale Utente Scaricare il pdf (Pagina 28)

28 CHAPTER 30 Extending ColdFusion with CFX

Listing 30.5 (continued)

// Output optional debug info

if ( pRequest->Debug() )

{

pRequest->WriteDebug( “Debug info...” ) ;

}

// Catch Cold Fusion exceptions & re-raise them

catch( CCFXException* e )

{

pRequest->ReThrowException( e ) ;

}

// Catch ALL other exceptions and throw them as

// Cold Fusion exceptions (DO NOT REMOVE! --

// this prevents the server from crashing in

// case of an unexpected exception)

catch( ... )

{

pRequest->ThrowException(

“Error occurred in tag CFX_COMPUTESTANDARDDEVIATION”,

“Unexpected error occurred while processing tag.” ) ;

}

Writing the C++ Code

At this point, you could compile and register the new tag using the steps described in the next sec-

tion. If you do so, the code generated by the Wizard will simply display a “Hello, World” type of

message when the tag is actually used in a ColdFusion page. Of course, to actually give your new

tag its functionality, you need to remove the line that outputs the “Hello, World” message and add

your own C++ code that accomplishes whatever task you want the tag to perform.

The code example we’ll be discussing in this chapter creates a CFX tag called

<CFX_Compute

StandardDeviation>

. The tag accepts a query recordset and computes the standard deviation of the

numbers in one of the query’s columns. Tools like Excel provide built-in functions for computing

standard deviations and other common statistical tasks, but CFML does not. The problem can be

solved fairly easily with C++, so it makes for a good CFX tag example.

NOTE

If you’re not familiar with what a standard deviation is, it’s a statistical measure commonly used for analyzing data. Basically, the stan-

dard deviation can be thought of as a way of describing the similarity of a set of numbers. You’re no doubt already familiar the concept

of a mathematical average (also called the mean), right? Well, the standard deviation measures how much the individual numbers

tend to deviate from the average. A set of numbers that fluctuates wildly from number to number will have a higher standard deviation;

numbers that fluctuate less will have a lower standard deviation. If all the numbers are the same (which is another way of saying that

they all conform exactly to the average), the standard deviation is zero. In any case, the standard deviation (and the related concept

of variance) forms the foundation of many strategies for analyzing trends in data, so this CFX tag can put you on the path of creating

pages that perform relatively sophisticated data analysis.

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