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 <h4 class="subsection">12.14.1 Formatted Input Basics</h4>

 <p>Calls to <code>scanf</code> are superficially similar to calls to
 <code>printf</code> in that arbitrary arguments are read under the control of
 a template string.  While the syntax of the conversion specifications in
 the template is very similar to that for <code>printf</code>, the
 interpretation of the template is oriented more towards free-format
 input and simple pattern matching, rather than fixed-field formatting.
 For example, most <code>scanf</code> conversions skip over any amount of
 &ldquo;white space&rdquo; (including spaces, tabs, and newlines) in the input
 file, and there is no concept of precision for the numeric input
 conversions as there is for the corresponding output conversions.
 Ordinarily, non-whitespace characters in the template are expected to
 match characters in the input stream exactly, but a matching failure is
 distinct from an input error on the stream.
 <a name="index-conversion-specifications-_0028_0040code_007bscanf_007d_0029-1076"></a>
 Another area of difference between <code>scanf</code> and <code>printf</code> is
 that you must remember to supply pointers rather than immediate values
 as the optional arguments to <code>scanf</code>; the values that are read are
 stored in the objects that the pointers point to.  Even experienced
 programmers tend to forget this occasionally, so if your program is
 getting strange errors that seem to be related to <code>scanf</code>, you
 might want to double-check this.

    <p>When a <dfn>matching failure</dfn> occurs, <code>scanf</code> returns immediately,
 leaving the first non-matching character as the next character to be
 read from the stream.  The normal return value from <code>scanf</code> is the
 number of values that were assigned, so you can use this to determine if
 a matching error happened before all the expected values were read.
 <a name="index-matching-failure_002c-in-_0040code_007bscanf_007d-1077"></a>
 The <code>scanf</code> function is typically used for things like reading in
 the contents of tables.  For example, here is a function that uses
 <code>scanf</code> to initialize an array of <code>double</code>:

 <pre class="smallexample">     void
      readarray (double *array, int n)
      {
        int i;
        for (i=0; i&lt;n; i++)
          if (scanf (" %lf", &amp;(array[i])) != 1)
            invalid_input_error ();
      }
 </pre>
    <p>The formatted input functions are not used as frequently as the
 formatted output functions.  Partly, this is because it takes some care
 to use them properly.  Another reason is that it is difficult to recover
 from a matching error.

    <p>If you are trying to read input that doesn't match a single, fixed
 pattern, you may be better off using a tool such as Flex to generate a
 lexical scanner, or Bison to generate a parser, rather than using
 <code>scanf</code>.  For more information about these tools, see <a href="../flex/index.html#Top">Top</a>, and <a href="../bison/index.html#Top">Top</a>.

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	Up: <a rel="up" accesskey="u" href="Formatted-Input.html#Formatted-Input">Formatted Input</a>
	<hr>
	</div>

	<h4 class="subsection">12.14.1 Formatted Input Basics</h4>

	<p>Calls to <code>scanf</code> are superficially similar to calls to
	<code>printf</code> in that arbitrary arguments are read under the control of
	a template string. While the syntax of the conversion specifications in
	the template is very similar to that for <code>printf</code>, the
	interpretation of the template is oriented more towards free-format
	input and simple pattern matching, rather than fixed-field formatting.
	For example, most <code>scanf</code> conversions skip over any amount of
	“white space” (including spaces, tabs, and newlines) in the input
	file, and there is no concept of precision for the numeric input
	conversions as there is for the corresponding output conversions.
	Ordinarily, non-whitespace characters in the template are expected to
	match characters in the input stream exactly, but a matching failure is
	distinct from an input error on the stream.
	<a name="index-conversion-specifications-_0028_0040code_007bscanf_007d_0029-1076"></a>
	Another area of difference between <code>scanf</code> and <code>printf</code> is
	that you must remember to supply pointers rather than immediate values
	as the optional arguments to <code>scanf</code>; the values that are read are
	stored in the objects that the pointers point to. Even experienced
	programmers tend to forget this occasionally, so if your program is
	getting strange errors that seem to be related to <code>scanf</code>, you
	might want to double-check this.

	<p>When a <dfn>matching failure</dfn> occurs, <code>scanf</code> returns immediately,
	leaving the first non-matching character as the next character to be
	read from the stream. The normal return value from <code>scanf</code> is the
	number of values that were assigned, so you can use this to determine if
	a matching error happened before all the expected values were read.
	<a name="index-matching-failure_002c-in-_0040code_007bscanf_007d-1077"></a>
	The <code>scanf</code> function is typically used for things like reading in
	the contents of tables. For example, here is a function that uses
	<code>scanf</code> to initialize an array of <code>double</code>:

	<pre class="smallexample"> void
	readarray (double *array, int n)
	{
	int i;
	for (i=0; i<n; i++)
	if (scanf (" %lf", &(array[i])) != 1)
	invalid_input_error ();
	}
	</pre>
	<p>The formatted input functions are not used as frequently as the
	formatted output functions. Partly, this is because it takes some care
	to use them properly. Another reason is that it is difficult to recover
	from a matching error.

	<p>If you are trying to read input that doesn't match a single, fixed
	pattern, you may be better off using a tool such as Flex to generate a
	lexical scanner, or Bison to generate a parser, rather than using
	<code>scanf</code>. For more information about these tools, see <a href="../flex/index.html#Top">Top</a>, and <a href="../bison/index.html#Top">Top</a>.

	</body></html>