<title>printf format identifiers.</title>
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<h1>printf format identifiers.</h1>
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<p>
printf formatting is controlled by 'format identifiers' which, are shown
below in their simplest form.
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<pre>
%d %i Decimal signed integer.
%o Octal integer.
%x %X Hex integer.
%u Unsigned integer.
%c Character.
%s String. See below.
%f double
%e %E double.
%g %G double.
%p pointer.
%n Number of characters written by this printf.
No argument expected.
%% %. No argument expected.
</pre>
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These identifiers actually have upto 6 parts as shown in the table below.
They MUST be used in the order shown.
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<td><a href="#percent">%</a></td>
<td><a href="#flags">Flags</a></td>
<td><a href="#width">Minimum field width</a></td>
<td><a href="#period">Period</a></td>
<td><a href="#precision">Precision. Maximum field width</a></td>
<td><a href="#id">Argument type</a></td>
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<td>Required</td>
<td>Optional</td>
<td>Optional</td>
<td>Optional</td>
<td>Optional</td>
<td>Required</td>
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<a name=percent>
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<h2>%</h2>
The % marks the start and therfore is manatory.
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<a name=flags>
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<h2>Flags</h2>
The format identifers can be altered from their default function by
applying the following <b>flags</b>:
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- Left justify.
0 Field is padded with 0's instead of blanks.
+ Sign of number always O/P.
blank Positive values begin with a blank.
# Various uses:
%#o (Octal) 0 prefix inserted.
%#x (Hex) 0x prefix added to non-zero values.
%#X (Hex) 0X prefix added to non-zero values.
%#e Always show the decimal point.
%#E Always show the decimal point.
%#f Always show the decimal point.
%#g Always show the decimal point trailing
zeros not removed.
%#G Always show the decimal point trailing
zeros not removed.
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<ul>
<li>The flags must follow the <a href="#percent">%</a>.
<li>Where it makes sense, more than one flag can be used.
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Here are a few more examples.
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printf(" %-10d \n", number);
printf(" %010d \n", number);
printf(" %-#10x \n", number);
printf(" %#x \n", number);
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<a name=width>
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<h2>Minimum field width.</h2>
By default the width of a field will be the minimum required to hold
the data. If you want to increase the field width you can use the
following syntax.
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<pre>
main()
{
int number = 5;
char *pointer = "little";
printf("Here is a number-%4d-and a-%10s-word.\n", number, pointer);
}
/*********************************
*
* Program result is:
*
* Here is a number- 5-and a- little-word.
*
*********************************/
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As you can see, the data is right justified within the field. It can
be left justified by using the <a href="#flags">- flag</a>.
A <a href="#precision">maximum string</a> width can also be specified.
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The width can also be given as a variable as shown below.
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main()
{
int number=5;
printf("---%*d----\n", 6, number);
}
/*********************************
*
* Program result is:
*
* ---- 5---
*
*********************************/
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The * is replaced with the supplied <b>int</b> to provide the ability to
dynamically specify the field width.
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<a name=period>
<h2>Period</h2>
If you wish to specify the <a href="#precision">precision</a> of an argument,
it MUST be prefixed with the period.
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<a name=precision>
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<h2>Precision</h2>
The Precision takes different meanings for the different format types.
<h3>Float Precision</h3>
<pre>
%8.2f
</pre>
This says you require a total field of 8 characters, within the 8
characters the last 2 will hold the decimal part.
<pre>
%.2f
</pre>
The example above requests the minimum field width and the last two
characters are to hold the decimal part.
<h3>Character String Maximum field width</h3>
The precision within a string format specifies the maximum
field width.
<pre>
%4.8s
</pre>
Specifies a minimum width of 4 and a maximum width
of 8 characters. If the string is greater than 8 characters,
it will be cropped down to size.
<p>
<img src=../../GRAPHICS/computer.gif>
<a href=../EXAMPLES/sprintf1.c>Here is a little program</a>
that shows an alternative to <a href=strncpy.html>strncpy</a>.
<h3>* Precision</h3>
As with the 'width' above, the precision does not have to be hard
coded, the * symbol can be used and an integer supplied to give its
value.
<p>
<hr>
<a name=id>
<h2>Format Identifiers</h2>
The format identifier describes the expected data. The identifier is the
character that ends
Here is a list of the format identifers as used in 'printf' ,'sprintf'
,'fprintf' and 'scanf'.
<ol>
<li>Except for '%' and 'n', all the identifiers expect to extract an argument
from the <b>printf</b> parameter list.
<li>All of the parmameters should be the value
to be inserted. EXCEPT %s, this expects a <a href="../CONCEPT/pointers.html">
pointer</a> to be passed.
</ol>
<h2>An example.</h2>
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<pre>
main()
{
int number=5;
char *pointer="little";
printf("Here is a number %d and a %s word.\n", number, pointer);
}
/*********************************
*
* Program result is:
*
* Here is a number 5 and a little word.
*
*********************************/
</pre>
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<address>Martin Leslie
</address><p>
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