MACROMEDIA COLDFUSION 4.5-CFML LANGUAGE Manuale Utente Scaricare il pdf (Pagina 33)

33Writing CFX Tags with Visual C++

NOTE

Queries cannot be passed using any attribute name other than query; therefore, only one query can be passed to any given CFX

tag. In contrast, CFX tags are free to return as many queries as they wish, simply by calling

pRequest->AddQuery() multiple

times (using a different query name each time).

Once the tag’s attributes have been reasonably validated, execution continues to the next portion of

the code (beginning with the series of

double declarations). After deﬁning a few local variables, this

portion of the code uses two

for loops to perform the necessary mathematical calculations. Each of

the loops iterates from 1 to the number of rows in the query; within the loops, the value of the

Row

variable indicates the row currently being processed. The ﬁrst thing each of these loops does is to

populate a string variable called

thisVal with the current value of the column speciﬁed in the tag’s

COLUMN attribute. It’s worth noting that values retrieved from queries are always retrieved as strings;

if you want to work with values as numbers, you need to use

atof() or some similar function to

convert the string to the desired numeric data type.

Speaking of which, the ﬁrst loop uses an

if test to make sure that the length of the current row’s

value is not zero (that is, to make sure that the value is not an empty string). If it is, that probably

means that a

NULL is occupying that row of the query. ColdFusion represents null values as empty

strings whenever such a value is accessed as string; since

GetData() always accesses all data as strings,

you will always get an empty string for any null value in a query.

If the value is not an empty string, the tag adds the current numeric value to the

dTotal variable and

increments the

iValueCount variable by one. When the ﬁrst for loop is complete, dTotal will hold

the total (sum) of all numbers in the data set, and

iValueCount will hold the number of data points

included in the total (that is, the number of rows that were not skipped because of suspected null

values).

If, on the other hand, a particular value is determined to be an empty string, the code checks to

see if the tag has been called with a

DEBUG attribute. If so, it uses pRequest->WriteDebug() to display

a debugging message whenever an empty string is found in the data set. See the “Generating Debug

Output” section, later in this chapter, to see what the debugging messages look like.

In any case, as long as at least one valid data point was found in the query, the next line of code

computes the mathematical average (mean) of the data points by dividing the

dTotal by iValueCount.

The average is stored in the variable called

dMean (which is not meant to imply that it’s demeaning

to be average). Next, another

for loop is used to iterate over the set of numbers again, this time

subtracting each value from the mean and squaring the result. After this loop ﬁnishes, the average

of these results is computed by dividing by the number of data points; this is called the variance.

The square root of the variance is then computed with the standard

sqrt() function; this is the

standard deviation.

The tag has now completed its work, so the only thing left to do is to return the standard deviation

to the calling ColdFusion page using

pRequest->SetVariable(). Since SetVariable() only accepts

string values, the standard deviation must ﬁrst be converted to a string using the standard

gcvt()

function.

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MACROMEDIA COLDFUSION 4.5-CFML LANGUAGE Manuale Utente Pagina 33

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