From aa3b2dd31a47926c1f9a7d4c11a1aee528959939 Mon Sep 17 00:00:00 2001
From: Hartmut Holzgraefe <hholzgra@php.net>
Date: Thu, 17 Aug 2000 19:24:36 +0000
Subject: [PATCH] some notes on float precision

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@30424 c90b9560-bf6c-de11-be94-00142212c4b1
---
 functions/math.xml |  8 ++++----
 language/types.xml | 30 ++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/functions/math.xml b/functions/math.xml
index 3532eee2e3..355c2e9eee 100644
--- a/functions/math.xml
+++ b/functions/math.xml
@@ -307,8 +307,8 @@ $binary = base_convert ($hexadecimal, 16, 2);
      get the old behaviour.
     </simpara>
     <simpara>
-     See also <function>floor</function> and
-     <function>round</function>.
+     See also <function>floor</function> and <function>round</function>
+		 and the <link linkend="warn.float-precision">float precision note</link>.
     </simpara>
    </refsect1>
   </refentry>
@@ -478,8 +478,8 @@ $binary = base_convert ($hexadecimal, 16, 2);
      get the old behaviour.
     </simpara>
     <simpara>
-     See also <function>ceil</function> and
-     <function>round</function>.
+     See also <function>ceil</function> and <function>round</function> 
+		 and the <link linkend="warn.float-precision">float precision note</link>.
     </simpara>
    </refsect1>
   </refentry>
diff --git a/language/types.xml b/language/types.xml
index b29f6c09ae..c5ccb45c7a 100644
--- a/language/types.xml
+++ b/language/types.xml
@@ -62,6 +62,9 @@ $a = 0123; # octal number (equivalent to 83 decimal)
 $a = 0x12; # hexadecimal number (equivalent to 18 decimal)
      </programlisting>
     </informalexample>
+		The size of an integer is platform-dependant, although a 
+		maximum value of about 2 billion is the usual value 
+    (thats 32 bits signed).
    </para>
   </sect1>
 
@@ -75,7 +78,34 @@ $a = 0x12; # hexadecimal number (equivalent to 18 decimal)
 $a = 1.234; $a = 1.2e3;
      </programlisting>
     </informalexample>
+		The size of a floating point number is platform-dependant, 
+		although a maximum of ~1.8e308 with a precision of roughly 14 
+		decimal digits is a common value (thats 64 bit IEEE format).
    </para>
+	 <warning id="warn.float-precision">
+		<para>
+		 It is quite usual that simple decimal fractions like 0.1 or 0.7 
+		 cannot be converted into their internal binary counterparts
+		 without a little lack of precision. This can lead to confusing
+		 results as for example <literal>floor((0.1+0.7)*10)</literal>
+		 will usualy return <literal>7</literal> instead of the expexted
+		 <literal>8</literal> as the result of the inner expression 
+		 really something like <literal>7.9999999999...</literal>.
+		</para>
+		<para>
+		 This is similar to the impossibility of writing down the result
+		 of <literal>1/3</literal> exactly in decimal form with a limit 
+		 numer of digits as it is <literal>0.3333333...</literal>.
+		</para>
+		<para>
+		 So never trust floating number results to the last digit and
+		 never compare floating point numbers for equalness. If you
+		 really need higher precision for decimal calculations or
+		 in general you should use the
+		 <link linkend="ref.bc">arbitrary precision math functions</link>
+		 instead.
+		</para>
+	 </warning>
   </sect1>
 
   <sect1 id="language.types.string">