运行calc，进入下面的操作：
>>9 9 >>3+3 6 >>3*6 18 >>3*5/2 7.5 >>2/3 0.666667 >>quit
>>是提示符。
限制：
计算器源代码包含在词法分析器和语法分析器的压缩包里，HTML格式的源码 calc.cpp.html。因为Parser不接受表达式，所以我人为地给输入缓存加上a=，从而与输入的表达式成为一个赋值语句。
1 /**************************************************
2 * file: calc.cpp
3 * date: 2006-06-30
4 * author: ideawu
5 * describe: a really small and simple caculator.
6 * with known bugs.
7 **************************************************/
8
9 #include <stdio.h>
10 #include <stdlib.h>
11 #include <string.h>
12
13 #include "Parser.h"
14
15 float calc(SyntaxTree *tree);
16
17 int main(int argc, char* argv[]){
18 char buf[1024] = "a=";
19 SyntaxTree *tree = NULL;
20 Lexer *lexer = new Lexer();
21 Parser *parser = new Parser();
22
23 lexer->setSrc(buf, 1024);
24 parser->setLexer(lexer);
25
26 printf(">>");
27 scanf("%s", &buf[2]);
28 while(strcmp(buf, "a=quit") != 0){
29 tree = parser->parse();
30
31 if(tree!=NULL){
32 //tree->display();
33 printf("%g\n", calc(tree->getRight()));
34 }else{
35 parser->printError();
36 }
37 printf(">>");
38 scanf("%s", &buf[2]);
39 parser->reset();
40 }
41
42 printf("\n");
43
44 return 0;
45 }
46
47 float calc(SyntaxTree *tree){
48 float a=0, b=0, val=0;
49
50 if(tree != NULL){
51 if(tree->getRootNode()->getType() == NUM){
52 return tree->getRootNode()->getValue();
53 }
54 a = calc(tree->getLeft());
55 b = calc(tree->getRight());
56
57 switch(tree->getRootNode()->getType()){
58 case PLUS:
59 val = a + b;
60 break;
61 case MINUS:
62 val = a - b;
63 break;
64 case MUL:
65 val = a * b;
66 break;
67 case DIV:
68 if(b != 0){
69 val = a / b;
70 }
71 break;
72 default:
73 break;
74 }
75 }
76
77 return val;
78
79 }
80
81