scan_expr.cpp

斯坦福Energy211/CME211课《c++编程——地球科学科学家和工程师》的课件

#include "eval_expr.h"

char nextchar();
void retract( char c );

// This token uses a state transition diagram to read
// tokens from the input: numbers, arithmetic operators,
// parentheses, and newlines
int nexttoken( string& tok_str )
{
	int state = 0;
	char c;
	tok_str = "";
	while ( true ) {
		switch( state ) {
		case 0:
			c = nextchar();
			tok_str += c;
			switch( c ) {
			case '+':
				return TOK_ADD;
			case '-':
				// We don't know if this - is for
				// subtraction or negation; it will
				// be determined at the syntax 
				// analysis level
				return TOK_SUB;
			case '*':
				return TOK_MUL;
			case '/':
				return TOK_DIV;
			case '(':
				return TOK_LPAR;
			case ')':
				return TOK_RPAR;
			case '\n':
				return TOK_EOL;
			case '.':
				state = 2;
				break;
			case -1:
				return TOK_EOF;
			default:
				if ( isdigit( c ) )
					state = 1;
				else
					return TOK_ERR;
			}
			break;
		case 1:
			// This state is used to process a token
			// that begins with a sequence of digits
			c = nextchar();
			if ( isdigit( c ) ) {
				tok_str += c;
			}
			else if ( c == '.' ) {
				tok_str += c;
				state = 2;
			}
			else {
				retract( c );
				return TOK_NUM;
			}
			break;
		case 2:
			// This state is used to process digits after
			// the decimal point, if any
			c = nextchar();
			if ( isdigit( c ) )
				tok_str += c;
			else {
				retract( c );
				// A decimal point by itself is not
				// a valid number!
				if ( tok_str.size() > 1 )
					return TOK_NUM;
				else
					return TOK_ERR;
			}
		}
	}
}

// This function puts a previously read character
// back into the input stream, so that it will be
// the next character to be read.  It is useful
// when reading tokens that consist of a sequence
// of characters whose length is not known in
// advance, thus making it necessary to read one
// character past the end of the token
void retract( char c )
{
	cin.putback( c );
}

// This character reads and returns the next
// character in the input.  If the read operation
// fails, it is assumed that we have reached the
// end of the input, and the value -1 (used in C
// to signify end-of-file, or EOF) is returned.
// Spaces and tabs are skipped, but newlines are
// not, because expressions are intended to be
// terminated by a newline
char nextchar()
{
	char c;
	do {
		c = cin.get();
		if ( cin == false )
			return -1;
	} while ( c == ' ' || c == '\t' ) ;
	return c;
}