eval_expr.cpp

来自「斯坦福Energy211/CME211课《c++编程——地球科学科学家和工程师》」· C++ 代码 · 共 47 行

// This program reads arithmetic expressions from standard
// input, one per line, and evaluates them.  The program
// demonstrates stacks, scanning input using state transition
// diagrams implemented by switch statements, and predictive
// parsing of input according to a context-free grammar

// Declarations needed across the application are placed
// here, to ensure uniformity
#include "eval_expr.h"

int main()
{
	string tok_str;
	// Read the first token from the input
	int token = nexttoken( tok_str );
	// Keep reading and evaluating expressions until the
	// user inputs a blank line
	while ( token != TOK_EOL )
	{
		try
		{
			// Evaluate a single expression
			value_stack s;
			token = Expr( s, token, tok_str );
			// Every expresion must be followed
			// by an end-of-line
			if ( token != TOK_EOL )
				throw exception();
			// The top value on the stack is the
			// result of the evaluation
			double y = s.top();
			cout << y << endl;
			// The next token is the first of the
			// next expression, or an end-of-line
			token = nexttoken( tok_str );
		}
		catch (...) 
		{
			cout << "Syntax error" << endl;
			string garbage;
			getline( cin, garbage );
			token = nexttoken( tok_str );
		}
	}
}