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printf [C++ Reference]
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<li class="level2"><div class="li"><span class="li"><a href="#printf" class="toc">printf</a></span></div>
<ul class="toc">
<li class="level3"><div class="li"><span class="li"><a href="#formatting_codes" class="toc">Formatting Codes</a></span></div></li>
<li class="level3"><div class="li"><span class="li"><a href="#formatting_modifiers" class="toc">Formatting Modifiers</a></span></div></li></ul>
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<h2><a name="printf" id="printf">printf</a></h2>
<div class="level2">
<p>
Syntax:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#include <cstdio></span>
<span class="kw4">int</span> <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html"><span class="kw3">printf</span></a><span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy0">*</span>format<span class="sy0">,</span> ... <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
The <code>printf</code> function prints output to <code>stdout</code>, according to <code>format</code> and other
arguments passed to <code>printf</code>. The string <code>format</code> consists of two types of items:
characters that will be printed to the screen, and format commands that
define how the other arguments to <code>printf</code> are displayed. Basically, you
specify a format string that has text in it, as well as “special” characters
that map to the other arguments of <code>printf</code>. For example, this code
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">char</span> name<span class="br0">[</span><span class="nu0">20</span><span class="br0">]</span> <span class="sy0">=</span> <span class="st0">"Bob"</span><span class="sy0">;</span>
<span class="kw4">int</span> age <span class="sy0">=</span> <span class="nu0">21</span><span class="sy0">;</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html"><span class="kw3">printf</span></a><span class="br0">(</span> <span class="st0">"Hello %s, you are %d years old<span class="es1">\n</span>"</span><span class="sy0">,</span> name<span class="sy0">,</span> age <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
displays the following output:
</p>
<pre class="code">
Hello Bob, you are 21 years old</pre>
<p>
The <code>%s</code> means, “insert the first argument, a string, right here.” The <code>%d</code>
indicates that the second argument (an integer) should be placed there.
</p>
<p>
The return value of <code>printf</code> is the number of characters printed, or a negative
number if an error occurred.
</p>
</div>
<h3><a name="formatting_codes" id="formatting_codes">Formatting Codes</a></h3>
<div class="level3">
<p>
There are different %-codes for different variable types, as well as options to limit
the length of the variables and whatnot.
</p>
<table class="inline">
<tr class="row0">
<th class="col0">Code</th><th class="col1">Format</th>
</tr>
<tr class="row1">
<td class="col0">%c</td><td class="col1">character</td>
</tr>
<tr class="row2">
<td class="col0">%d</td><td class="col1">signed integers</td>
</tr>
<tr class="row3">
<td class="col0">%i</td><td class="col1">signed integers</td>
</tr>
<tr class="row4">
<td class="col0">%I64d</td><td class="col1">long long (8B integer), <acronym title="Microsoft">MS</acronym>-specific</td>
</tr>
<tr class="row5">
<td class="col0">%I64u</td><td class="col1">unsigned long long (8B integer), <acronym title="Microsoft">MS</acronym>-specific</td>
</tr>
<tr class="row6">
<td class="col0">%e</td><td class="col1">scientific notation, with a lowercase “e”</td>
</tr>
<tr class="row7">
<td class="col0">%E</td><td class="col1">scientific notation, with a uppercase “E”</td>
</tr>
<tr class="row8">
<td class="col0">%f</td><td class="col1">floating point</td>
</tr>
<tr class="row9">
<td class="col0">%g</td><td class="col1">use %e or %f, whichever is shorter</td>
</tr>
<tr class="row10">
<td class="col0">%G</td><td class="col1">use %E or %f, whichever is shorter</td>
</tr>
<tr class="row11">
<td class="col0">%o</td><td class="col1">octal</td>
</tr>
<tr class="row12">
<td class="col0">%s</td><td class="col1">a string of characters</td>
</tr>
<tr class="row13">
<td class="col0">%u</td><td class="col1">unsigned integer</td>
</tr>
<tr class="row14">
<td class="col0">%x</td><td class="col1">unsigned hexadecimal, with lowercase letters</td>
</tr>
<tr class="row15">
<td class="col0">%X</td><td class="col1">unsigned hexadecimal, with uppercase letters</td>
</tr>
<tr class="row16">
<td class="col0">%p</td><td class="col1">a pointer</td>
</tr>
<tr class="row17">
<td class="col0">%n</td><td class="col1">the argument shall be a pointer to an integer into which is placed the number of characters written so far</td>
</tr>
</table>
</div>
<h3><a name="formatting_modifiers" id="formatting_modifiers">Formatting Modifiers</a></h3>
<div class="level3">
<p>
An integer placed between a % sign and the format command acts as a minimum
field width specifier, and pads the output with spaces or zeros to make it long
enough. If you want to pad with zeros, place a zero before the minimum field
width specifier:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="sy0">%</span>012d</pre>
<p>
You may also specify the minimum field width in an int variable if instead of a
number you put the * sign:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">int</span> width <span class="sy0">=</span> <span class="nu0">12</span><span class="sy0">;</span>
<span class="kw4">int</span> age <span class="sy0">=</span> <span class="nu0">100</span><span class="sy0">;</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html"><span class="kw3">printf</span></a><span class="br0">(</span><span class="st0">"%*d"</span><span class="sy0">,</span> width<span class="sy0">,</span> age<span class="br0">)</span><span class="sy0">;</span></pre>
<p>
You can also include a precision modifier, in the form of a <code>.N</code> where <code>N</code> is some
number, before the format command:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="sy0">%</span>012.4d</pre>
<p>
The precision modifier has different meanings depending on the format command
being used:
</p>
<ul>
<li class="level1"><div class="li"> With <code>%e</code>, <code>%E</code>, and <code>%f</code>, the precision modifier lets you specify the number of decimal places desired. For example, <code>%12.6f</code> will display a floating number at least 12 digits wide, with six decimal places.</div>
</li>
<li class="level1"><div class="li"> With <code>%g</code> and <code>%G</code>, the precision modifier determines the maximum number of significant digits displayed.</div>
</li>
<li class="level1"><div class="li"> With <code>%s</code>, the precision modifier simply acts as a maximum field length, to complement the minimum field length that precedes the period.</div>
</li>
</ul>
<p>
As with field width specifier, you may use an int variable
to specify the precision modifier by using the * sign:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy0">*</span> msg <span class="sy0">=</span> <span class="st0">"Hello printf"</span><span class="sy0">;</span>
<span class="kw4">int</span> string_size <span class="sy0">=</span> strlen <span class="br0">(</span>msg<span class="br0">)</span><span class="sy0">;</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html"><span class="kw3">printf</span></a><span class="br0">(</span><span class="st0">"msg: %.*s"</span><span class="sy0">,</span> string_size<span class="sy0">,</span> msg<span class="br0">)</span><span class="sy0">;</span></pre>
<p>
All of <code>printf</code>'s output is right-justified, unless you place a minus sign
right after the % sign. For example,
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="sy0">%-</span><span class="nu17">12.4f</span></pre>
<p>
will display a floating point number with a minimum of 12 characters, 4 decimal
places, and left justified.
</p>
<p>
You may modify the <code>%d</code>, <code>%i</code>, <code>%o</code>, <code>%u</code>, and <code>%x</code> type
specifiers with the letter <code>l</code> and the letter <code>h</code> to specify <code>long</code> and <code>short</code> data
types (e.g. <code>%hd</code> means a <code>short</code> integer).
</p>
<p>
The <code>%e</code>, <code>%f</code>, and <code>%g</code> type specifiers can have the letter <code>l</code> before them to indicate that a double follows.
The <code>%g</code>, <code>%f</code>, and <code>%e</code> type specifiers can be preceded with the character <code>#</code> to ensure that
the decimal point will be present, even if there are no decimal digits.
</p>
<p>
The use of the <code>#</code> character with the <code>%x</code> type specifier indicates that the hexidecimal
number should be printed with the <code>0x</code> prefix.
</p>
<p>
The use of the <code>#</code> character
with the <code>%o</code> type specifier indicates that the octal value should be displayed
with a <code>0</code> prefix.
</p>
<p>
Inserting a plus sign <code>+</code> into the type specifier will force positive values to
be preceded by a <code>+</code> sign. Putting a space character ' ' there will force
positive values to be preceded by a single space character.
</p>
<p>
You can also include <a href="../../escape_sequences.html" class="wikilink1" title="escape_sequences">constant escape sequences</a> in the output string.
</p>
<p>
Related Topics: <a href="../../c/io/fprintf.html" class="wikilink1" title="c:io:fprintf">fprintf</a>, <a href="../../c/io/puts.html" class="wikilink1" title="c:io:puts">puts</a>, <a href="../../c/io/scanf.html" class="wikilink1" title="c:io:scanf">scanf</a>, <a href="../../c/io/sprintf.html" class="wikilink1" title="c:io:sprintf">sprintf</a>
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