language.types.float.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html> <head> <title>Floating point numbers</title> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.types.integer.html">Integers</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.types.string.html">Strings</a></div> <div class="up"><a href="language.types.html">Types</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div><hr /><div id="language.types.float" class="sect1"> <h2 class="title">Floating point numbers</h2> <p class="para"> Floating point numbers (also known as "floats", "doubles", or "real numbers") can be specified using any of the following syntaxes: </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000"><span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #0000BB">1.234</span><span style="color: #007700">; <br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= </span><span style="color: #0000BB">1.2e3</span><span style="color: #007700">; <br /></span><span style="color: #0000BB">$c </span><span style="color: #007700">= </span><span style="color: #0000BB">7E-10</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span></span></code></div> </div> </div> <p class="para"> Formally: </p> <div class="informalexample"> <div class="example-contents"><div class="phpcode"><code><span style="color: #000000">LNUM [0-9]+<br />DNUM ([0-9]*[\.]{LNUM}) | ({LNUM}[\.][0-9]*)<br />EXPONENT_DNUM [+-]?(({LNUM} | {DNUM}) [eE][+-]? {LNUM})</span></code></div> </div> </div> <p class="para"> The size of a float is platform-dependent, although a maximum of ~1.8e308 with a precision of roughly 14 decimal digits is a common value (the 64 bit IEEE format). </p> <div class="warning"><b class="warning">Warning</b> <h1 class="title">Floating point precision</h1> <p class="para"> It is typical that simple decimal fractions like <i>0.1</i> or <i>0.7</i> cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, <i>floor((0.1+0.7)*10)</i> will usually return <i>7</i> instead of the expected <i>8</i>, since the internal representation will be something like <i>7.<span style="text-decoration: overline;">9</span></i>. </p> <p class="para"> This is due to the fact that it is impossible to express some fractions in decimal notation with a finite number of digits. For instance, <i>1/3</i> in decimal form becomes <i>0.<span style="text-decoration: overline;">3</span></i>. </p> <p class="para"> So never trust floating number results to the last digit, and never compare floating point numbers for equality. If higher precision is necessary, the <a href="ref.bc.html" class="link">arbitrary precision math functions</a> and <a href="ref.gmp.html" class="link">gmp</a> functions are available. </p> </div> <div id="language.types.float.casting" class="sect2"> <h3 class="title">Converting to float</h3> <p class="para"> For information on converting <a href="language.types.string.html" class="type string">string</a>s to <a href="language.types.float.html" class="type float">float</a>, see <a href="language.types.string.html#language.types.string.conversion" class="link">String conversion to numbers</a>. For values of other types, the conversion is performed by converting the value to <a href="language.types.integer.html" class="type integer">integer</a> first and then to <a href="language.types.float.html" class="type float">float</a>. See <a href="language.types.integer.html#language.types.integer.casting" class="link">Converting to integer</a> for more information. As of PHP 5, a notice is thrown if an <a href="language.types.object.html" class="type object">object</a> is converted to <a href="language.types.float.html" class="type float">float</a>. </p> </div></div><hr /><div style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.types.integer.html">Integers</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.types.string.html">Strings</a></div> <div class="up"><a href="language.types.html">Types</a></div> <div class="home"><a href="index.html">PHP Manual</a></div></div></body></html>⌨️ 快捷键说明
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