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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:46 2005 by faqproc version 2.7 --><!-- from source file stdio.sgml dated Wed Dec 21 13:07:57 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/stdio/scanfjam.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 12.19</title><link href="scanfc.html" rev=precedes><link href="scanfprobs.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="scanfc.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="scanfprobs.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 12.19</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I figured I could use <TT>scanf</TT> more safelyif I checked its return valueto make sure that the usertyped the numeric values I expect:<pre> int n; while(1) { printf("enter a number: "); if(scanf("%d", &n) == 1) break; printf("try again: "); } printf("you typed %d\n", n);</pre>but sometimes it seems to go into an infinite loop.<a href="achtung.html" rel=subdocument>[footnote]</a>Why?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>When <TT>scanf</TT> is attempting to convert numbers,any non-numeric characters it encounters terminate theconversion<em>and are left on the input stream</em>.Therefore, unless some other steps are taken,unexpected non-numeric input ``jams'' <TT>scanf</TT>again and again:<TT>scanf</TT> never gets pastthe bad character(s)toencounterlater, valid data.If the user types a characterlike `<TT>x</TT>'in response tothe code above,the codewill loop printing``try again'' forever,butitwon't give the user a chance to try.</p><p>You may be wondering why <TT>scanf</TT>leavesunmatchable characters on the input stream.Supposeyou had a compact data file containing lines consisting of anumber and an alphabetic code string,without intervening whitespace, like<pre> 123CODE</pre>You might want to parse this data file with <TT>scanf</TT>,using the format string <TT>"%d%s"</TT>.But if the <TT>%d</TT> conversion did not leave the unmatchedcharacter on the input stream,<TT>%s</TT> would incorrectly read <TT>"ODE"</TT>instead of <TT>"CODE"</TT>.(The problem is a standard onein lexicalanalysis: when scanning an arbitrary-length numeric constant oralphanumeric identifier, you never know where it ends untilyou've read ``too far.''This isone reason that <TT>ungetc</TT> exists.)</p><p>See also question<a href="scanfprobs.html">12.20</a>.</p><p>References:ISO Sec. 7.9.6.2<br>H&S Sec. 15.8 pp. 357-64<br></p><!-- aend --><p><hr><a href="scanfc.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="scanfprobs.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/stdio/scanfjam.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --></html>⌨️ 快捷键说明
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