这个程序是不是一种特殊的递归？
#include <iostream>
#include <string>
#include <map>
#include <cctype>
using namespace std;
enum Token_value
{
NAME,NUMBER,END,
PLUS = '+',MINUS = '-',MUL = '*',DIV = '/',
PRINT = ';',ASSIGN = '=',LP = '(',RP = ')'
};
Token_value curr_tok = PRINT;
map< string, double >table;
int no_of_errors;
double number_value;
string string_value;
double expr( bool get );
double error( const string& s )
{
++no_of_errors;
cerr << "error: " << s << endl;
return 1;
}
Token_value get_token()
{
char ch;
do
{
if( !cin.get( ch ) ) return curr_tok;
}while( ch != '\n' && isspace( ch ) );
switch( ch )
{
case ';':
case '\n':
return curr_tok = PRINT;
case '*':
case '/':
case '+':
case '-':
case '(':
case ')':
case '=':
return curr_tok = Token_value( ch );
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
case '.':
cin.putback( ch );
cin >> number_value;
return curr_tok = NUMBER;
default:
if( isalpha( ch ) )
{
string_value = ch;
while( cin.get( ch ) && isalnum( ch ) ) string_value.push_back( ch );
cin.putback( ch );
return curr_tok = NAME;
}
error( "bad token" );
return curr_tok = PRINT;
}
}
double prim( bool get )
{
if( get ) get_token();
switch( curr_tok )
{
case NUMBER:
{
double v = number_value;
get_token();
return v;
}
case NAME:
{
double& v = table[ string_value ];
if( get_token() == ASSIGN) v = expr( true );
return v;
}
case MINUS:
return -prim( true );
case LP:
{
double e = expr( true );
if( curr_tok != RP) return error( ") expected" );
get_token();
return e;
}
default:
return error( "primary expected" );
}
}
double term( bool get )
{
double left = prim( get );
for(;;)
switch( curr_tok )
{
case MUL:
left *= prim( true );
break;
case DIV:
if( double d = prim( true ) )
{
left /= d;
break;
}
return error( "divide by 0" );
default:
return left;
}
}
double expr( bool get )
{
double left = term( get );
for(;;)
switch( curr_tok )
{
case PLUS:
left+=term( true );
break;
case MINUS:
left-=term( true );
break;
default:
return left;
}
}
int main()
{
table["pi"] = 3.1415926535897932385;
table["e"] = 2.7182818284590452354;
while(cin)
{
get_token();
if( curr_tok == END ) break;
if( curr_tok != PRINT )
cout << expr( false ) << endl;
}
return no_of_errors;
}
