acm.jlu.edu.cn-1552-Identifying Legal Pascal Real Constants

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博客介绍了Pascal中实数常量的语法规则，如须有小数点、指数或两者皆有等。任务是识别合法的Pascal实数常量，给出了输入输出要求，并提供了一段C++代码来实现对输入数据中候选常量的合法性判断。

Pascal requires that real constants have either a decimal point, or an exponent
(starting with the letter e or E, and officially called a scale factor), or both, 
in addition to the usual collection of decimal digits. If a decimal point is
included it must have at least one decimal digit on each side of it. As expected,
a sign (+ or -) may precede the entire number, or the exponent, or both. 

 
Exponents may not include fractional digits. Blanks may precede or follow the
real constant, but they may not be embedded within it. Note that the Pascal
syntax rules for real constants make no assumptions about the range of real values,
and neither does this problem. 
 
Your task in this problem is to identify legal Pascal real constants. 
 
Input and Output
 
Each line of the input data contains a candidate which you are to classify. For each
line of the input, display your finding as illustrated in the example shown below. 
The input terminates with a line that contains only an asterisk in column one. 
 
Sample Input
1.2
   1.
  1.0e-55
  e-12
    6.5E
     1e-12
  +4.1234567890E-99999
   7.6e+12.5
99
*
Sample Output
1.2 is legal.
1. is illegal.
1.0e-55 is legal.
e-12 is illegal.
6.5E is illegal.
1e-12 is legal.
+4.1234567890E-99999 is legal.
7.6e+12.5 is illegal.
99 is illegal.

 
 
#include<iostream>
#include<memory>
using namespace std;

void main()
{
	enum State{PRE_NUM,SIGN1,PRE_POINT,POINT,FRACTION,E,SIGN2,EXP,FOLLOWBLANK,ERROR};
	char ch;
	char str[1000];
	State state;
	while(cin>>ch,ch!='*')
	{
		int idx=0;
		state=PRE_NUM;
		while(ch!='/n')
		{
			switch(state)
			{
			case PRE_NUM:
				if(ch=='+'||ch=='-')
				{
					state=SIGN1;
					break;
				}
				if(ch>='0'&&ch<='9')
				{
					state=PRE_POINT;
					break;
				}
				state=ERROR;
				break;	
			case SIGN1:
				if(ch>='0'&&ch<='9')
				{
					state=PRE_POINT;
				}
				else
					state=ERROR;
				break;
			case PRE_POINT:
				if(ch=='.')
				{
					state=POINT;
					break;
				}
				if(ch=='e'||ch=='E')
				{
					state=E;
					break;
				}
				if(!(ch>='0'&&ch<='9'))
				{
					state=ERROR;
				}
				break;
			case POINT:
				if(ch>='0'&&ch<='9')
				{
					state=FRACTION;
				}
				else
					state=ERROR;
				break;
			case FRACTION:
				if(ch=='e'||ch=='E')
				{
					state=E;
					break;
				}
				if(ch==' ')
				{
					state=FOLLOWBLANK;
					break;
				}
				if(!(ch>='0'&&ch<='9'))
				{
					state=ERROR;
				}
				break;
			case E:
				if(ch=='+'||ch=='-')
				{
					state=SIGN2;
					break;
				}
				if(ch>='0'&&ch<='9')
				{
					state=EXP;
					break;
				}
				state=ERROR;
				break;
			case SIGN2:
				if(ch>='0'&&ch<='9')
				{
					state=EXP;
				}
				else
					state=ERROR;
				break;
			case EXP:
				if(ch==' ')
				{
					state=FOLLOWBLANK;
					break;
				}
				if(!(ch>='0'&&ch<='9'))
						state=ERROR;
				break;
			case FOLLOWBLANK:
				if(ch!=' ')
					state=ERROR;
			default:
				break;
			}
			str[idx++]=ch;
			cin.get(ch);
		}
		str[idx]='/0';
		while(str[--idx]==' ')
			str[idx]='/0';
		cout<<str<<" is ";
		if(state==ERROR||state==PRE_POINT||state==POINT||state==SIGN1||state==SIGN2||state==E)
			cout<<"illegal./n";
		else
			cout<<"legal./n";
		memset(str,0,++idx*sizeof(char)); 
	}
}