user-0 提问: 【scanf求助】第一个数据不能由scanf读入

回复:
user: user-0
content: 引用 13 楼 zhao4zhong1 的回复:http://bbs.csdn.net/topics/360055953 Format Specification Fields: scanf and wscanf Functions A format specification has the following form:  % [width] [{h | l | I64 | L}]type  The format argument specifies the interpretation of the input and can contain one or more of the following:   White-space characters: blank (' '); tab ('\t'); or newline ('\n'). A white-space character causes scanf to read, but not store, all consecutive white-space characters in the input up to the next non–white-space character. One white-space character in the format matches any number (including 0) and combination of white-space characters in the input.   Non–white-space characters, except for the percent sign (%). A non–white-space character causes scanf to read, but not store, a matching non–white-space character. If the next character in stdin does not match, scanf terminates.   Format specifications, introduced by the percent sign (%). A format specification causes scanf to read and convert characters in the input into values of a specified type. The value is assigned to an argument in the argument list.  The format is read from left to right. Characters outside format specifications are expected to match the sequence of characters in stdin; the matching characters in stdin are scanned but not stored. If a character in stdin conflicts with the format specification, scanf terminates, and the character is left in stdin as if it had not been read.  When the first format specification is encountered, the value of the first input field is converted according to this specification and stored in the location that is specified by the first argument. The second format specification causes the second input field to be converted and stored in the second argument, and so on through the end of the format string.  An input field is defined as all characters up to the first white-space character (space, tab, or newline), or up to the first character that cannot be converted according to the format specification, or until the field width (if specified) is reached. If there are too many arguments for the given specifications, the extra arguments are evaluated but ignored. The results are unpredictable if there are not enough arguments for the format specification.  Each field of the format specification is a single character or a number signifying a particular format option. The type character, which appears after the last optional format field, determines whether the input field is interpreted as a character, a string, or a number.   The simplest format specification contains only the percent sign and a type character (for example, %s). If a percent sign (%) is followed by a character that has no meaning as a format-control character, that character and the following characters (up to the next percent sign) are treated as an ordinary sequence of characters, that is, a sequence of characters that must match the input. For example, to specify that a percent-sign character is to be input, use %%.  An asterisk (*) following the percent sign suppresses assignment of the next input field, which is interpreted as a field of the specified type. The field is scanned but not stored.我懂了！！！谢谢啦~~~

user: user-0
content: 引用 11 楼 shimachao 的回复:[quote=引用 10 楼 u013643920 的回复:] [quote=引用 9 楼 ForestDB 的回复:] Ctrl-Z回车 这样输入EOF恩恩，成功了。不过我还是想问一下如果我输入的不是EOF或者int型数值而是字符串或者exit命令，它为什么就会一直打印我上一个输入的数据？（那时候我就只能点“X”来关闭窗口了。。。）[/quote] 因为输入不成功，所以a的值没有被檫写[/quote]可是当a的值没有被擦写时，程序为什么就会一直输出呢？它为什么不等待我再次输入呢？你可以说一下当我输入错误的时候，底层具体是在做什么吗？

user: user-1
content: 引用 10 楼 u013643920 的回复:[quote=引用 9 楼 ForestDB 的回复:] Ctrl-Z回车 这样输入EOF恩恩，成功了。不过我还是想问一下如果我输入的不是EOF或者int型数值而是字符串或者exit命令，它为什么就会一直打印我上一个输入的数据？（那时候我就只能点“X”来关闭窗口了。。。）[/quote] 因为输入不成功，所以a的值没有被檫写

user: user-1
content: 引用 12 楼 u013643920 的回复:[quote=引用 11 楼 shimachao 的回复:] [quote=引用 10 楼 u013643920 的回复:] [quote=引用 9 楼 ForestDB 的回复:] Ctrl-Z回车 这样输入EOF恩恩，成功了。不过我还是想问一下如果我输入的不是EOF或者int型数值而是字符串或者exit命令，它为什么就会一直打印我上一个输入的数据？（那时候我就只能点“X”来关闭窗口了。。。）[/quote] 因为输入不成功，所以a的值没有被檫写[/quote]可是当a的值没有被擦写时，程序为什么就会一直输出呢？它为什么不等待我再次输入呢？你可以说一下当我输入错误的时候，底层具体是在做什么吗？[/quote] 看14楼赵老师的回答

user: user-2
content: If a character in stdin conflicts with the format specification, scanf terminates, and the character is left in stdin as if it had not been read.

user: user-2
content: http://bbs.csdn.net/topics/360055953 Format Specification Fields: scanf and wscanf Functions A format specification has the following form:  % [width] [{h | l | I64 | L}]type  The format argument specifies the interpretation of the input and can contain one or more of the following:   White-space characters: blank (' '); tab ('\t'); or newline ('\n'). A white-space character causes scanf to read, but not store, all consecutive white-space characters in the input up to the next non–white-space character. One white-space character in the format matches any number (including 0) and combination of white-space characters in the input.   Non–white-space characters, except for the percent sign (%). A non–white-space character causes scanf to read, but not store, a matching non–white-space character. If the next character in stdin does not match, scanf terminates.   Format specifications, introduced by the percent sign (%). A format specification causes scanf to read and convert characters in the input into values of a specified type. The value is assigned to an argument in the argument list.  The format is read from left to right. Characters outside format specifications are expected to match the sequence of characters in stdin; the matching characters in stdin are scanned but not stored. If a character in stdin conflicts with the format specification, scanf terminates, and the character is left in stdin as if it had not been read.  When the first format specification is encountered, the value of the first input field is converted according to this specification and stored in the location that is specified by the first argument. The second format specification causes the second input field to be converted and stored in the second argument, and so on through the end of the format string.  An input field is defined as all characters up to the first white-space character (space, tab, or newline), or up to the first character that cannot be converted according to the format specification, or until the field width (if specified) is reached. If there are too many arguments for the given specifications, the extra arguments are evaluated but ignored. The results are unpredictable if there are not enough arguments for the format specification.  Each field of the format specification is a single character or a number signifying a particular format option. The type character, which appears after the last optional format field, determines whether the input field is interpreted as a character, a string, or a number.   The simplest format specification contains only the percent sign and a type character (for example, %s). If a percent sign (%) is followed by a character that has no meaning as a format-control character, that character and the following characters (up to the next percent sign) are treated as an ordinary sequence of characters, that is, a sequence of characters that must match the input. For example, to specify that a percent-sign character is to be input, use %%.  An asterisk (*) following the percent sign suppresses assignment of the next input field, which is interpreted as a field of the specified type. The field is scanned but not stored.

user: user-0
content: 引用 9 楼 ForestDB 的回复:Ctrl-Z回车 这样输入EOF恩恩，成功了。不过我还是想问一下如果我输入的不是EOF或者int型数值而是字符串或者exit命令，它为什么就会一直打印我上一个输入的数据？（那时候我就只能点“X”来关闭窗口了。。。）

user: user-3
content: Ctrl-Z回车 这样输入EOF

user: user-0
content: 引用 2 楼 user-7 的回复:读进去了 你没有打印而已 随后又开始循环读和打印请问下我想退出循环应该怎么做？我试过输入exit，eof，可是最后程序都会不断打印我输入的最后一个整数？为什么？

user: user-0
content: 引用 6 楼 zhao4zhong1 的回复:不要使用 while (条件) 更不要使用 while (组合条件) 要使用 while (1) {  if (条件1) break;  //...  if (条件2) continue;  //...  if (条件3) return;  //... } 因为前两种写法在语言表达意思的层面上有二义性，只有第三种才忠实反映了程序流的实际情况。 典型如： 下面两段的语义都是当文件未结束时读字符 while (!feof(f)) {  a=fgetc(f);  //...  b=fgetc(f);//可能此时已经feof了！  //... } 而这样写就没有问题： while (1) {  a=fgetc(f);  if (feof(f)) break;  //...  b=fgetc(f);  if (feof(f)) break;  //... } 类似的例子还可以举很多。 ]受教了，虽然没有直接解决我的问题，但是学到了新东西！！！谢了！受教了，虽然没有直接解决我的问题，但是学到了新东西！！！谢了！

user: user-2
content: 不要使用 while (条件) 更不要使用 while (组合条件) 要使用 while (1) {  if (条件1) break;  //...  if (条件2) continue;  //...  if (条件3) return;  //... } 因为前两种写法在语言表达意思的层面上有二义性，只有第三种才忠实反映了程序流的实际情况。 典型如： 下面两段的语义都是当文件未结束时读字符 while (!feof(f)) {  a=fgetc(f);  //...  b=fgetc(f);//可能此时已经feof了！  //... } 而这样写就没有问题： while (1) {  a=fgetc(f);  if (feof(f)) break;  //...  b=fgetc(f);  if (feof(f)) break;  //... } 类似的例子还可以举很多。

user: user-4
content: 把循环之前的那个scanf删掉就行了

user: user-5
content: 你while循环之前不是scanf了吗？？但是你又没输出~~~

user: user-6
content: 读过了，只是你后面的循环重新输入了。

user: user-7
content: 读进去了 你没有打印而已 随后又开始循环读和打印

user: user-0