blocks|key|3098737|text|我假设你有这样的代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3098738|int+n;
while+(someCondition)+{
++++std::cin+>>+n;
++++.....
++++std::cout+<<+someOutput;
}|code-block|syntax|javascript|3098739|当您输入一些无法作为整数读取的内容时，流(std::cin)将进入失败状态，并且只要您不处理输入错误，所有后续的输入尝试都会失败。|3098740|您可以测试输入操作是否成功：|3098741|if+(!(std::cin+>>+n))+//failed+to+read+int|3098742|您可以将流恢复到良好状态，并丢弃未处理的字符(导致输入失败的输入)：|3098743|std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(),+'\n');|3098744|另一种可能是接受字符串变量的输入(很少失败)，并尝试将字符串转换为int。|3098745|这是输入的一个非常常见的问题，在这里和其他地方都应该有大量的线程。|3098746|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

I assume you have code like this:

<pre><code>int n;
while (someCondition) {
 std::cin &gt;&gt; n;
 .....
 std::cout &lt;&lt; someOutput;
}
</code></pre>

When you enter something that cannot be read as an integer, the stream (std::cin) enters a failed state and all following attempts at input fail as long as you don't deal with the input error.

You can test the success of an input operation:

<pre><code>if (!(std::cin &gt;&gt; n)) //failed to read int
</code></pre>

and you can restore the stream to a good state and discard unprocessed characters (the input that caused the input failure):

<pre><code>std::cin.clear();
std::cin.ignore(std::numeric_limits&lt;std::streamsize&gt;::max(), '\n');
</code></pre>

Another possibility is to accept input into a string variable (which rarely fails) and try to convert the string to int.

This is a very common problem with input and there should be tons of threads about it here and elsewhere.

blocks|key|256908|text|如果您显式地将char转换为int，它将使用ASCII代码，否则它不会自动转换为int，因此您将得到奇怪的结果。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|256909|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

if you explicitly cast your char to an int, it'll use the ASCII code, else it's not doing the cast by itself to int, so you get the strange results you are getting.

blocks|key|5212361|text|您可以使用int+istream::get();|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5212362|entityMap^0|5|J|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

You could use <code>int istream::get();</code>

blocks|key|5212522|text|问题是，当您使用C%2B%2B流读入整数变量时，输入方法需要可以组成数字的字符，如'%2B'，'-'，'0‘...+'9’。任何不在此集合中的字符都会终止输入，并从这些字符中创建一个数字。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5212523|要获取字符的ASCII值，需要将数据输入到char变量中。在内部，它具有ASCII值(如果程序不读取16位字符或EBCDIC)。如果想要查看ASCII值，则需要将char变量输出为整数(通常需要在输出之前进行强制转换)。|offset|length|style|CODE|5212524|如果读取整数失败，您的程序需要通知用户输入不正确。程序可能要求用户再次输入数字，然后程序才能处理输入数据。|5212525|entityMap^0|0|L|4|29|4|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]|$D|P|E|Q|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|R|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|K|$]]

The issue is that when you read into an integer variable, using C++ streams, the input method expects characters that can make up a number, such as '+', '-', '0' ... '9'. Any character that is not in this set terminates the input and a number is created from these characters. 

To get the ASCII value of a character, you need to input the data into a <code>char</code> variable. Internally, it has the ASCII value (provided the program in not reading 16-bit chars or EBCDIC). If you want to see the ASCII value, you will need to output the <code>char</code> variable as an integer (generally requiring a cast before output). 

If reading an integer fails, your program needs to inform the User that the input was incorrect. The program may require the User to input the number again before the program can process the input data.

blocks|key|2164773|text|当你做类似这样的事情时：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2164774|int+x;
cin+>>+x;|code-block|syntax|javascript|2164775|您正在指示C%2B%2B期望从键盘读取int。实际上，当您输入非int的内容时，输入流(在本例中为标准输入)由于其错误状态而呈现为不可用。|offset|length|style|CODE|2164776|这就是为什么我个人更喜欢总是读取字符串，并在需要时将其转换为数字。在C%2B%2B标准库的C部分中有一些转换函数可以帮助实现这一点。|2164777|double+cnvtToNumber(const+std::string+&num)
{
++++char+*+ptr;
++++double+toret+=+strtod(+num.c_str(),+&ptr+);

++++if+(+*ptr+!=+0+)+{+//+If+it+got+to+the+end+of+the+string,+then+it+is+correct.
++++++++throw+std::runtime_error(+"input+was+not+a+number"+);
++++}
++++return+toret;
}|2164778|在这里，您正在转换一个数字，并检测转换是否正确。strtod(s)仅在找到字符串的末尾或空格时停止。为了避免尾随空格欺骗函数，您可能需要一个trim函数：|2164779|std::string+&trim(std::string+&str)
{
++++//+Remove+trailing+spaces
++++unsigned+int+pos+=+str.length()+-1;

++++while(+str[+pos+]+==+'+'+)+{
++++++++--pos;
++++}

++++if+(+pos+<+(+str.length()+-1+)+)+{
++++++++str.erase(+pos+%2B+1+);
++++}

++++//+Remove+spaces+at+the+beginning
++++pos+=+0;
++++while(+str[+pos+]+==+'+'+)+{
++++++++%2B%2Bpos;
++++}

++++if+(+pos+<+1+)+{
++++++++str.erase(+0,+pos+);
++++}

++++return+str;
}|2164780|现在，您可以安全地从控制台(或任何其他输入流)读取：|2164781|int+main()
{
++++std::string+entry;

++++try+{
++++++++std::getline(+std::cin,+entry+);
++++++++int+age+=+cnvtToNumber(+trim(+entry+)+);

++++++++std::cout+<<+age+<<+std::endl;
++++}+catch(const+std::exception+&e)+{
++++++++std::cerr+<<+e.what()+<<+std::endl;
++++}
}|2164782|当然，如果你总是读取double，然后转换它们，你可能会失去精度。如果你想进一步研究，有一些特定的整数函数(strtol(s))。|2164783|entityMap^0|0|0|F|3|S|3|0|0|0|O|9|0|0|0|0|A|6|1I|9|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@$I|15|J|16|K|L]|$I|17|J|18|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|19|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|1A|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|1B|8|@$I|1C|J|1D|K|L]]|9|@]|A|$]]|$1|S|3|T|5|D|7|1E|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1F|8|@]|9|@]|A|$]]|$1|W|3|X|5|D|7|1G|8|@]|9|@]|A|$E|F]]|$1|Y|3|Z|5|6|7|1H|8|@$I|1I|J|1J|K|L]|$I|1K|J|1L|K|L]]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]]|11|$]]

When you are doing something like:

<pre><code>int x;
cin &gt;&gt; x;
</code></pre>

You are instructing C++ to expect to read an <code>int</code> from keyboard. Actually, when you enter something not an <code>int</code>, the input stream (standard input, in this case) renders unusable due to its error state.

That's why I personally prefer to always read strings, and convert them to numbers if needed. There are conversion functions in the C part of the C++ standard library that can help with this.

<pre><code>double cnvtToNumber(const std::string &amp;num)
{
 char * ptr;
 double toret = strtod( num.c_str(), &amp;ptr );

 if ( *ptr != 0 ) { // If it got to the end of the string, then it is correct.
 throw std::runtime_error( "input was not a number" );
 }
 return toret;
}
</code></pre>

Here you are converting a number, and detecting if the conversion was right. <code>strtod(s)</code> will only stop when finding the end of the string, or a space. In order to avoid trailing spaces fooling the function, you could need a trim function:

<pre><code>std::string &amp;trim(std::string &amp;str)
{
 // Remove trailing spaces
 unsigned int pos = str.length() -1;

 while( str[ pos ] == ' ' ) {
 --pos;
 }

 if ( pos &lt; ( str.length() -1 ) ) {
 str.erase( pos + 1 );
 }

 // Remove spaces at the beginning
 pos = 0;
 while( str[ pos ] == ' ' ) {
 ++pos;
 }

 if ( pos &lt; 1 ) {
 str.erase( 0, pos );
 }

 return str;
}
</code></pre>

Now you can safely read from console (or whatever other input stream):

<pre><code>int main()
{
 std::string entry;

 try {
 std::getline( std::cin, entry );
 int age = cnvtToNumber( trim( entry ) );

 std::cout &lt;&lt; age &lt;&lt; std::endl;
 } catch(const std::exception &amp;e) {
 std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
 }
}
</code></pre>

Of course you can lose precision if you always read <code>double</code>'s and then convert them. There are specific functions for integers (<code>strtol(s)</code>), in case you want to investigate further.

what happens when you cin>> letter to int variable? I tried simple code to add 2 int numbers, first read them, than add them. But when I enter letter, it just fails and prints tons of numbers to screen. But what causes this error? I mean, I expected it to load and use ASCII code of that letter.

C++ character to int

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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当你将字母cin>>到整型变量时会发生什么？我试着用简单的代码将两个整数相加，先读取它们，然后再相加。但当我输入字母时，它就会失败，并在屏幕上打印成吨的数字。但是是什么导致了这个错误呢？我是说，我希望它能加载并使用那封信的ASCII码。

问整型的C++字符
EN

回答 5

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