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<HTML><HEAD><TITLE>Chapter 11 -- Creating Reports</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 11</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>Creating Reports</FONT></H1><HR><P><CENTER><B><FONT SIZE=5>CONTENTS</FONT></B></CENTER><UL><LI><A HREF="#WhatsaFormatStatement">What's a Format Statement?</A><UL><LI><A HREF="#ExampleUsingFieldLines">Example: Using Field Lines</A><LI><A HREF="#ExampleReportHeadings">Example: Report Headings</A><LI><A HREF="#ExampleUsingFuNCtionsintheValueLine">Example: Using FuNCtions in the Value Line</A><LI><A HREF="#ExampleChangingFormats">Example: Changing Formats</A><LI><A HREF="#ExampleUsingLongPiecesofTextinReports">Example: Using Long Pieces of Text in Reports</A><LI><A HREF="#ExampleWritingtoaFileInsteadofISTDOUTI">Example: Writing to a File Instead of <I>STDOUT</I></A></UL><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>Perl has a few special features that let you create simple reports.The reports can have a header area where you can place a title,page number, and other information that stays the same from onepage to the next. Perl will track how many lines have been usedin the report and automatically generate new pages as needed.<P>Compared to learning about regular expressions, learning how tocreate reports will be a breeze. There are only a few tricky parts,which I'll be sure to point out.<P>This chapter starts out by using the print() fuNCtion to displaya CD collection and then gradually moves from displaying the datato a fully formatted report. The data file shown in Listing 11.1is used for all of the examples in this chapter. The format ispretty simple: the CD album's title, the artist's name, and thealbum's price.<HR><BLOCKQUOTE><B>Listing 11.1 FORMAT.DAT-The Data File<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>The Lion King!Tumbleweed Connection!Elton John!123.32Photographs & Memories!Jim Croce!4.95Heads & Tales!Harry Chapin!12.50</PRE></BLOCKQUOTE><HR><P>You'll find that Perl is very handy for small text-based datafiles like this. You can create them in any editor and use anyfield delimiter you like. In this file, I used an exclamationpoint to delimit the field. However, I could just as easily haveused a caret, a tilde, or any other character.<P>Now that we have some data, let's look at Listing 11.2, whichis a program that reads the data file and displays the information.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Open the </I><TT><I>FORMAT.DAT</I></TT><I>file.<BR><I>Read all the file's lines and place them in the </I></I><TT><I>@lines</I></TT><I>array. Each line becomes a different element in the array.<BR>Close the file.<BR>Iterate over the </I><TT><I>@lines</I></TT><I>array. </I><TT><I>$_</I></TT><I> isset to a different array element each time through the loop.<BR>Remove the linefeed character from the end of the string.<BR>Split the string into three fields using the exclamation pointas the delimiter. Place each field into the </I><TT><I>$album</I></TT><I>,</I><TT><I>$artist</I></TT><I>, and</I><TT><I>$price</I></TT><I> variables.<BR>Print the variables.</I></BLOCKQUOTE><HR><BLOCKQUOTE><B>Listing 11.2 11LIST02.PL-A Program to Read and Displaythe Data File<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>open(FILE, "<format.dat");@lines = <FILE>;close(FILE);foreach (@lines) { chop; ($album, $artist, $price) = (split(/!/)); print("Album=$album Artist=$artist Price=$price\n");}</PRE></BLOCKQUOTE><HR><P>This program displays:<BLOCKQUOTE><PRE>Use of uninitialized value at 11lst02.pl line 8.Album=The Lion King Artist= Price=Album=Tumbleweed Connection Artist=Elton John Price=123.32Album=Photographs & Memories Artist=Jim Croce Price=4.95Album=Heads & Tales Artist=Harry Chapin Price=12.50</PRE></BLOCKQUOTE><P>Why is an error being displayed on the first line of the output?If you said that the split() fuNCtion was returning the undefinedvalue when there was no matching field in the input file, youwere correct. The first input line was the following:<BLOCKQUOTE><PRE>The Lion King!</PRE></BLOCKQUOTE><P>There are no entries for the <TT>Artist</TT>or <TT>Price</TT> fields. Therefore,the <TT>$artist </TT>and <TT>$price</TT>variables were assigned the undefined value, which resulted inPerl complaining about uninitialized values. You can avoid thisproblem by assigning the empty string to any variable that hasthe undefined value. Listing 11.3 shows a program that does this.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Open the </I><TT><I>FORMAT.DAT</I></TT><I>file, read all the lines into </I><TT><I>@lines</I></TT><I>,and then close the file.<BR><I>Iterate over the </I></I><TT><I>@lines</I></TT><I>array.<BR>Remove the linefeed character.<BR>Split the string into three fields.<BR>If any of the three fields is not present in the line, providea default value of an empty string.<BR>Print the variables.</I></BLOCKQUOTE><HR><BLOCKQUOTE><B>Listing 11.3 11LST03.PL-How to Avoid the UninitializedError When Using the <I>Split()</I> FuNCtion<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>open(FILE, "<format.dat");@lines = <FILE>;close(FILE);foreach (@lines) { chop; ($album, $artist, $price) = (split(/!/)); $album = "" if !defined($album); These lines assign null $artist = "" if !defined($artist); strings if no info is $price = "" if !defined($price); present in the record. print("Album=$album Artist=$artist Price=$price\n");}The first four lines this program displays are the following:Album=The Lion King Artist= Price=Album=Tumbleweed Connection Artist=Elton John Price=123.32Album=Photographs & Memories Artist=Jim Croce Price=4.95Album=Heads & Tales Artist=Harry Chapin Price=12.50</PRE></BLOCKQUOTE><HR><P>The error has been eliminated, but it is still very hard to readthe output because the columns are not aligned. The rest of thischapter is devoted to turning this jumbled output into a report.<P>Perl reports have <I>heading</I> and have <I>detail lines</I>.A heading is used to identify the report title, the page number,the date, and any other information that needs to appear at thetop of each page. Detail lines are used to show information abouteach record in the report. In the data file being used for theexamples in this chapter (refer to Listing 11.1), each CD hasits own detail line.<P>Headings and detail lines are defined by using format statements,which are discussed in the next section.<H2><A NAME="WhatsaFormatStatement"><FONT SIZE=5 COLOR=#FF0000>What's a Format Statement?</FONT></A></H2><P>Perl uses <I>formats</I> as guidelines when writing report information.A format is used to tell Perl what static text is needed and wherevariable information should be placed. Formats are defined byusing the <TT>format</TT> statement.The syntax for the <TT>format</TT>statement is<BLOCKQUOTE><PRE>format FORMATNAME = FIELD_LINE VALUE_LINE</PRE></BLOCKQUOTE><P>The <TT>FORMATNAME</TT> is usuallythe same name as the file handle that is used to accept the reportoutput. The section "Example: Changing Formats," laterin this chapter, talks about using the <TT>format</TT>statement where the <TT>FORMATNAME</TT>is different from the file handle. If you don't specify a <TT>FORMATNAME</TT>,Perl uses <TT>STDOUT</TT>. The <TT>FIELD_LINE</TT>part of the format statement consists of text and field holders.A <I>field holder</I> represents a given line width that Perlwill fill with the value of a variable. The <TT>VALUE_LINE</TT>line consists of a comma-delimited list of expressions used tofill the field holders in <TT>FIELD_LINE</TT>.<P>Report headings, which appear at the top of each page, have thefollowing format:<BLOCKQUOTE><PRE>format FORMATNAME_TOP = FIELD_LINE VALUE_LINE</PRE></BLOCKQUOTE><P>Yes, the only differeNCe between a detail line and a heading isthat <TT>_TOP</TT> is appended tothe <TT>FORMATNAME</TT>.<BR><p><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%><TR><TD><B>Note</B></TD></TR><TR><TD><BLOCKQUOTE>The location of <TT>format</TT> statements is unimportant because they define only a format and never are executed. I feel that they should appear either at the beginning of a program or the end of a program, rarely in the middle. Placing <TT>format</TT> statements in the middle of your program might make them hard to find when they need to be changed. Of course, you should be consistent where you place them.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><P>A typical <TT>format</TT> statementmight look like this:<BLOCKQUOTE><PRE>format = The total amount is $@###.## $total</PRE></BLOCKQUOTE><P>The at character @ is used to start a field holder. In this example,the field holder is seven characters long (the at sign and decimalpoint count, as well as the pound signs #). The next section,"Example: Using Field Lines," goes into more detailabout field lines and field holders.<P><TT>Format</TT> statements are usedonly when invoked by the write() fuNCtion. The write() fuNCtiontakes only one parameter: a file handle to send output to. Likemany things in Perl, if no parameter is specified, a default isprovided. In this case, STDOUT will be used when no FORMATNAMEis specified. In order to use the preceding format, you simplyassign a value to $total and then call the write() fuNCtion. Forexample:<BLOCKQUOTE><PRE>$total = 243.45write();$total = 50.00write();</PRE></BLOCKQUOTE><P>These lines will display:<BLOCKQUOTE><PRE> The total amount is $ 243.45 The total amount is $ 50.50</PRE></BLOCKQUOTE><P>The output will be sent to STDOUT. Notice that the decimal pointsare automatically lined up when the lines are displayed.<H3><A NAME="ExampleUsingFieldLines">Example: Using Field Lines</A></H3><P>The field lines of a <TT>format</TT>statement control what is displayed and how. The simplest fieldline contains only static text. You can use <I>static</I> or uNChangingtext as labels for variable information, dollar signs in frontof amounts, a separator character such as a comma between firstand last name, or whatever else is needed. However, you'll rarelyuse just static text in your format statement. Most likely, you'lluse a mix of static text and field holders.<P>You saw a field holder in action in the last section in whichI demonstrated sending the report to STDOUT. I'll repeat the formatstatement here so you can look at it in more detail:<BLOCKQUOTE><PRE>format = The total amount is $@###.## $total</PRE></BLOCKQUOTE><P>The character sequeNCe <TT>The total amountis $</TT> is static text. It will not change no matterhow many times the report is printed. The character sequeNCe @###.##,however, is a field holder. It reserves seven spaces in the linefor a number to be inserted. The third line is the value line;it tells Perl which variable to use with the field holder. Table11.1 contains a list of the different format characters you canuse in field lines.<BR><P><CENTER><B>Table 11.1 Field Holder Formats</B></CENTER><p><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%><TR><TD WIDTH=133><CENTER><I>Format Character</I></CENTER></TD><TD WIDTH=457><CENTER><I>Description</I></CENTER></TD></TR><TR><TD WIDTH=133><CENTER><TT>@</TT></CENTER></TD><TD WIDTH=457>This character represents the start of a field holder.</TD></TR><TR><TD WIDTH=133><CENTER><TT><</TT></CENTER></TD><TD WIDTH=457>This character indicates that the field should be left-justified.</TD></TR><TR><TD WIDTH=133><CENTER><TT>></TT></CENTER></TD><TD WIDTH=457>This character indicates that the field should be right-justified.</TD></TR><TR><TD WIDTH=133><CENTER><TT>|</TT></CENTER></TD><TD WIDTH=457>This character indicates that the field should be centered.</TD></TR><TR><TD WIDTH=133><CENTER><TT>#</TT></CENTER></TD><TD WIDTH=457>This character indicates that the field will be numeric. If used as the first character in the line, it indicates that the entire line is a comment.</TD></TR><TR><TD WIDTH=133><CENTER><TT>.</TT></CENTER></TD><TD WIDTH=457>This character indicates that a decimal point should be used with numeric fields.</TD></TR><TR><TD WIDTH=133><CENTER><TT>^</TT></CENTER></TD><TD WIDTH=457>This character also represents the start of a field holder. Moreover, it tells Perl to turn on word-wrap mode. See the section "Example: Using Long Pieces of Text in Reports" later in this chapter for more information about word-wrapping.</TD></TR><TR><TD WIDTH=133><CENTER><TT>~</TT></CENTER>⌨️ 快捷键说明
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