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<HTML><HEAD><TITLE>Recipe 2.2. Comparing Floating-Point Numbers (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:30:04Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch02_01.htm"TITLE="2. Numbers"><LINKREL="prev"HREF="ch02_02.htm"TITLE="2.1. Checking Whether a String Is a Valid Number"><LINKREL="next"HREF="ch02_04.htm"TITLE="2.3. Rounding Floating-Point Numbers"></HEAD><BODYBGCOLOR="#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="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch02_02.htm"TITLE="2.1. Checking Whether a String Is a Valid Number"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 2.1. Checking Whether a String Is a Valid Number"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch02_01.htm"TITLE="2. Numbers"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch02_04.htm"TITLE="2.3. Rounding Floating-Point Numbers"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 2.3. Rounding Floating-Point Numbers"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch02-33576">2.2. Comparing Floating-Point Numbers</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch02-pgfId-161">Problem <ACLASS="indexterm"NAME="ch02-idx-1000007988-0"></A><ACLASS="indexterm"NAME="ch02-idx-1000007988-1"></A><ACLASS="indexterm"NAME="ch02-idx-1000007988-2"></A><ACLASS="indexterm"NAME="ch02-idx-1000007988-3"></A></A></H3><PCLASS="para">Floating-point arithmetic isn't precise. You want to compare two floating-point numbers and know if they're equal when carried out to a certain number of decimal places. Most of the time, this is the way you <EMCLASS="emphasis">should</EM> compare floating-point numbers for equality.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch02-pgfId-167">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">sprintf</CODE> to format the numbers to a certain number of decimal places, then compare the resulting strings:</P><PRECLASS="programlisting"># equal(NUM1, NUM2, ACCURACY) : returns true if NUM1 and NUM2 are# equal to ACCURACY number of decimal placessub equal { my ($A, $B, $dp) = @_; return sprintf("%.${dp}g", $A) eq sprintf("%.${dp}g", $B); }</PRE><PCLASS="para">Alternatively, store the numbers as integers by assuming the decimal place.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch02-pgfId-203">Discussion</A></H3><PCLASS="para">You need the <CODECLASS="literal">equal</CODE> routine because most computers' floating-point representations aren't accurate. See the Introduction for a discussion of this issue.</P><PCLASS="para">If you have a fixed number of decimal places, as with currency, you can sidestep the problem by storing your values as integers. Storing <CODECLASS="literal">$3.50</CODE> as <CODECLASS="literal">350</CODE> instead of <CODECLASS="literal">3.5</CODE> removes the need for floating-point values. Reintroduce the decimal point on output:</P><PRECLASS="programlisting">$wage = 536; # $5.36/hour$week = 40 * $wage; # $214.40printf("One week's wage is: \$%.2f\n", $week/100);<CODECLASS="userinput"><B><CODECLASS="replaceable"><I>One week's wage is: $214.40</I></CODE></B></CODE></PRE><PCLASS="para">It rarely makes sense to compare to more than 15 decimal places.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch02-pgfId-223">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">sprintf</CODE> function in <EMCLASS="emphasis">perlfunc </EM>(1) and <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the entry on <CODECLASS="literal">$#</CODE> in the <EMCLASS="emphasis">perlvar </EM>(1) manpage and <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the documentation for the standard Math::BigFloat module (also in <ACLASS="olink"HREF="../prog/ch07_01.htm">Chapter 7</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>); we use <CODECLASS="literal">sprintf</CODE> in <ACLASS="xref"HREF="ch02_04.htm"TITLE="Rounding Floating-Point Numbers">Recipe 2.3</A>; Volume 2, Section 4.2.2 of <CITECLASS="citetitle">The Art of Computer Programming</CITE> <ACLASS="indexterm"NAME="ch02-idx-1000008960-0"></A><ACLASS="indexterm"NAME="ch02-idx-1000008960-1"></A><ACLASS="indexterm"NAME="ch02-idx-1000008960-2"></A><ACLASS="indexterm"NAME="ch02-idx-1000008960-3"></A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch02_02.htm"TITLE="2.1. 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