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<html><head><title>Formatting Data with sprintf (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch15_02.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch15_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">15.3. Formatting Data with sprintf</h2><p><a name="INDEX-1007" /> <a name="INDEX-1008" />The<tt class="literal">sprintf</tt> function takes the same arguments as<tt class="literal">printf</tt> (except for the optional filehandle, ofcourse), but it returns the requested string instead of printing it.This is handy if you want to store a formatted string into a variablefor later use, or if you want more control over the result than<tt class="literal">printf</tt> alone would provide:</p><blockquote><pre class="code">my $date_tag = sprintf "%4d/%02d/%02d %2d:%02d:%02d", $yr, $mo, $da, $h, $m, $s;</pre></blockquote><p>In that example, <tt class="literal">$date_tag</tt> gets something like<tt class="literal">"2038/01/19 3:00:08"</tt>. The format string (the firstargument to <tt class="literal">sprintf</tt>) used a leading zero on someof the format number, which we didn't mention when we talkedabout <tt class="literal">printf</tt> formats in <a href="ch06_01.htm">Chapter 6, "I/O Basics"</a>. The leading zero on the format number meansto use leading zeroes as needed to make the number as wide asrequested. Without a leading zero in the formats, the resultingdate-and-time string would have unwanted leading spaces instead ofzeroes, looking like <tt class="literal">"2038/ 1/19 3: 0: 8"</tt>.</p><a name="lperl3-CHP-15-SECT-3.1" /><div class="sect2"><h3 class="sect2">15.3.1. Using sprintf with "Money Numbers"</h3><p><a name="INDEX-1009" /> <a name="INDEX-1010" /> <a name="INDEX-1011" />One popular use for<tt class="literal">sprintf</tt> is when a number needs to be renderedwith a certain number of places after the decimal point, such as whenan amount of money needs to be shown as <tt class="literal">2.50</tt> andnot <tt class="literal">2.5</tt> -- and certainly not as<tt class="literal">2.49997</tt>! That's easy to accomplish with the<tt class="literal">"%.2f"</tt> format:</p><blockquote><pre class="code">my $money = sprintf "%.2f", 2.49997;</pre></blockquote><p>The full implications of rounding are numerous and subtle, but inmost cases you should keep numbers in memory with all of theavailable accuracy, rounding off only for output.</p><p>If you have a "money number" that may be large enough toneed commas to show its size, you might find it handy to use asubroutine like this one.<a href="#FOOTNOTE-337">[337]</a></p><blockquote class="footnote"> <a name="FOOTNOTE-337" /><p>[337]Yes, we know that noteverywhere in the world are commas used to separate groups of digits,not everywhere are the digits grouped by threes, and not everywherethe currency symbol appears as it does for U.S. dollars. But this isa good example anyway, so there!</p> </blockquote><blockquote><pre class="code">sub big_money { my $number = sprintf "%.2f", shift @_; # Add one comma each time through the do-nothing loop 1 while $number =~ s/^(-?\d+)(\d\d\d)/$1,$2/; # Put the dollar sign in the right place $number =~ s/^(-?)/$1\$/; $number;}</pre></blockquote><p>This subroutine uses some techniques you haven't seen yet, butthey logically follow from what we've shown you. The first lineof the subroutine formats the first (and only) parameter to haveexactly two digits after the decimal point. That is, if the parameterwere the number <tt class="literal">12345678.9</tt>, now our<tt class="literal">$number</tt> is the string<tt class="literal">"12345678.90"</tt>. </p><p>The next line of code uses a<tt class="literal">while</tt><a name="INDEX-1012" /> modifier. As we mentioned when wecovered that modifier in <a href="ch10_01.htm">Chapter 10, "More Control Structures"</a>, that canalways be rewritten as a traditional <tt class="literal">while</tt> loop:</p><blockquote><pre class="code">while ($number =~ s/^(-?\d+)(\d\d\d)/$1,$2/) { 1;}</pre></blockquote><p>What does that say to do? It says that, as long as the substitutionreturns a true value (signifying success), the loop body should run.But the loop body does nothing! That's okay with Perl, but ittells us that the purpose of that statement is to do the conditionalexpression (the substitution), rather than the useless loop body. Thevalue <tt class="literal">1</tt> is traditionally used as this kind of aplaceholder, although any other value would be equallyuseful.<a href="#FOOTNOTE-338">[338]</a> Thisworks just as well as the loop above:</p><blockquote class="footnote"> <a name="FOOTNOTE-338" /><p>[338]Which is to say, useless. By the way, in caseyou're wondering, Perl optimizes away the constant expressionso it doesn't even take up any runtime.</p> </blockquote><blockquote><pre class="code">'keep looping' while $number =~ s/^(-?\d+)(\d\d\d)/$1,$2/;</pre></blockquote><p>So, now we know that the substitution is the real purpose of theloop. But what is the substitution doing? Remember that<tt class="literal">$number</tt> will be some string like<tt class="literal">"12345678.90"</tt> at this point. The pattern willmatch the first part of the string, but it can't get past thedecimal point. (Do you see why it can't?) Memory<tt class="literal">$1</tt> will get <tt class="literal">"12345"</tt>, and<tt class="literal">$2</tt> will get <tt class="literal">"678"</tt>, so thesubstitution will make <tt class="literal">$number</tt> into<tt class="literal">"12345,678.90"</tt> (remember, it couldn't matchthe decimal point, so the last part of the string is left untouched).</p><p>Do you see what the dash is doing near the start of that pattern?(Hint: The dash is allowed at only one place in the string.)We'll tell you at the end of this section, in case youhaven't figured it out.</p><p>We're not done with that substitution statement yet. Since thesubstitution succeeded, the do-nothing loop goes back to try again.This time, the pattern can't match anything from the commaonward, so <tt class="literal">$number</tt> becomes<tt class="literal">"12,345,678.90"</tt>. The substitution thus adds acomma to the number each time through the loop.</p><p>Speaking of the loop, it's still not done. Since the previoussubstitution was a success, we're back around the loop to tryagain. But this time, the pattern can't match at all, since ithas to match at least four digits at the start of the string, so nowthat is the end of the loop.</p><p>Why couldn't we have simply used the <tt class="literal">/g</tt>modifier to do a "global" search-and-replace, to save thetrouble and confusion of the <tt class="literal">1 while</tt>? Wecouldn't use that because we're working backwards fromthe decimal point, rather than forward from the start of the string.Putting the commas in a number like this can't be done simplywith the <tt class="literal">s///g</tt> substitution alone.<a href="#FOOTNOTE-339">[339]</a></p><blockquote class="footnote"><a name="FOOTNOTE-339" /><p>[339]At least, it can't be done without some more-advancedregular expression techniques than we've shown you so far.Those darn Perl developers keep making it harder and harder to writePerl books that use the word "can't."</p></blockquote><p>So, did you figure out the dash? It's allowing for a possibleminus-sign at the start of the string. The next line of code makesthe same allowance, putting the dollar-sign in the right place sothat <tt class="literal">$number</tt> is something like<tt class="literal">"$12,345,678.90"</tt>, or perhaps<tt class="literal">"-$12,345,678.90"</tt> if it's negative. Notethat the dollar sign isn't necessarily the first character inthe string, or that line would be a lot simpler. Finally, the lastline of code returns our nicely formatted "money number,"ready to be printed in the annual<a name="INDEX-1013" /> <a name="INDEX-1014" /> <a name="INDEX-1015" />report.<a name="INDEX-1016" /> <a name="INDEX-1017" /> </p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch15_02.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch15_04.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">15.2. Manipulating a Substring with substr</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">15.4. Advanced Sorting</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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