活用scanf

scanf()是C语言中用于读入格式化数据（formatted data）的函数。

我们可能对scanf()的一般用法已经了然，而至于scanf()读入数据的机制恐怕并不十分清楚。

下面我将比较详细地介绍scanf()的工作机制，并指出其丰富且强大的格式化方式。

内容来自这个链接

Function

int scanf ( const char * format, ... );

Read formatted data from stdin 

（务必注意 formatted data这两个词）

Reads data from stdin and stores them according to the parameter format into the locations pointed by the additional arguments.
The additional arguments should point to already allocated objects of the type specified by their corresponding format specifier within the format string.

Parameters

format

C string that contains a sequence of characters that control how characters extracted from the stream are treated:

• Whitespace character: the function will read and ignore any whitespace characters encountered before the next non-whitespace character (whitespace characters include spaces, newline and tab characters -- see isspace). A single whitespace in the format string validates any quantity of whitespace characters extracted from the stream (including none).
• Non-whitespace character, except format specifier (%): Any character that is not either a whitespace character (blank, newline or tab) or part of a format specifier (which begin with a % character) causes the function to read the next character from the stream, compare it to this non-whitespace character and if it matches, it is discarded and the function continues with the next character of format. If the character does not match, the function fails, returning and leaving subsequent characters of the stream unread.
• Format specifiers: A sequence formed by an initial percentage sign (%) indicates a format specifier, which is used to specify the type and format of the data to be retrieved from the stream and stored into the locations pointed by the additional arguments.

A format specifier for scanf follows this prototype:

%[*][width][length]specifier

Where the specifier character at the end is the most significant component, since it defines which characters are extracted, their interpretation and the type of its corresponding argument:

specifier	Description	Characters extracted
i	Integer	Any number of digits, optionally preceded by a sign (+ or -). Decimal digits assumed by default (0-9), but a 0 prefix introduces octal digits (0-7), and 0xhexadecimal digits (0-f). Signed argument.
d or u	Decimal integer	Any number of decimal digits (0-9), optionally preceded by a sign (+ or -). d is for a signed argument, and u for an unsigned.
o	Octal integer	Any number of octal digits (0-7), optionally preceded by a sign (+ or -). Unsigned argument.
x	Hexadecimal integer	Any number of hexadecimal digits (0-9, a-f, A-F), optionally preceded by 0x or 0X, and all optionally preceded by a sign (+ or -). Unsigned argument.
f, e, g	Floating point number	A series of decimal digits, optionally containing a decimal point, optionally preceeded by a sign (+ or -) and optionally followed by the e or E character and a decimal integer (or some of the other sequences supported by strtod). Implementations complying with C99 also support hexadecimal floating-point format when preceded by 0x or 0X.
a
c	Character	The next character. If a width other than 1 is specified, the function reads exactly width characters and stores them in the successive locations of the array passed as argument. No null character is appended at the end.
s	String of characters	Any number of non-whitespace characters, stopping at the first whitespace character found. A terminating null character is automatically added at the end of the stored sequence.
p	Pointer address	A sequence of characters representing a pointer. The particular format used depends on the system and library implementation, but it is the same as the one used to format %p in fprintf.
[characters]	Scanset	Any number of the characters specified between the brackets. A dash (-) that is not the first character may produce non-portable behavior in some library implementations.
[^characters]	Negated scanset	Any number of characters none of them specified as characters between the brackets.
n	Count	No input is consumed. The number of characters read so far from stdin is stored in the pointed location.
%	%	A % followed by another % matches a single %.

Except for n, at least one character shall be consumed by any specifier. Otherwise the match fails, and the scan ends there.

The format specifier can also contain sub-specifiers: asterisk (*), width and length (in that order), which are optional and follow these specifications:

sub-specifier	description
*	An optional starting asterisk indicates that the data is to be read from the stream but ignored (i.e. it is not stored in the location pointed by an argument).
width	Specifies the maximum number of characters to be read in the current reading operation (optional).
length	One of hh, h, l, ll, j, z, t, L (optional). This alters the expected type of the storage pointed by the corresponding argument (see below).

This is a chart showing the types expected for the corresponding arguments where input is stored (both with and without a lengthsub-specifier):

	specifiers
length	d i	u o x	f e g a	c s [] [^]	p	n
(none)	int*	unsigned int*	float*	char*	void**	int*
hh	signed char*	unsigned char*				signed char*
h	short int*	unsigned short int*				short int*
l	long int*	unsigned long int*	double*	wchar_t*		long int*
ll	long long int*	unsigned long long int*				long long int*
j	intmax_t*	uintmax_t*				intmax_t*
z	size_t*	size_t*				size_t*
t	ptrdiff_t*	ptrdiff_t*				ptrdiff_t*
L			long double*

Note: Yellow rows indicate specifiers and sub-specifiers introduced by C99.

... (additional arguments)

Depending on the format string, the function may expect a sequence of additional arguments, each containing a pointer to allocated storage where the interpretation of the extracted characters is stored with the appropriate type.
There should be at least as many of these arguments as the number of values stored by the format specifiers. Additional arguments are ignored by the function.
These arguments are expected to be pointers: to store the result of a scanf operation on a regular variable, its name should be preceded by the reference operator (&) (see example).

用法举例 1

Codeforces 78A

题目大意：

从stdin读入3行由小写英文字母 (lowercase Latin letters) 与空格 (space) 组成的字符串，统计每行中元音字母 （a, e, i, o, u）出现的次数，看是否依次为5, 7, 5。

问题归结为如何读入包含空格的一整行，根据上述题目中的要求我们将问题具体描述为：

如何读入含有空格的一整行字符串，起始空格（leading spaces）可忽略。

#include<bits/stdc++.h>
using namespace std;
const char *vow="aeiou";
int count_vow(char *s){
	int res=;
	for(int i=; s[i]; i++)
		for(int j=; vow[j]; j++){
			if(s[i]==vow[j]){
				res++;
				break;
			}
		}
	return res;
}
int main(){
	//freopen("in", "r", stdin);
	char s[];
	int ok[]={, , };
	for(int i=; i<; i++){
		scanf(" %[^\n]", s);
		if(count_vow(s)!=ok[i]){
			puts("NO");
			return ;
		}
	}
	puts("YES");
	return ;
}