(原創) 如何用C++實作eval()? (C/C++)
C/C++都是靜態語言，所以都沒有eval()這個函數，C#也沒有，在.NET語言中，只有JScript.NET有eval()，連VB也沒有，事實上，eval()是很好用的，以前在寫VFP時，常常利用字串湊程式，然後用eval()去執行，來到C#，一直想用eval()，若真的想在C#用，可以偷用JScript.NET的，有時間會另外討論這個主題。
在此程式我們試著用C++寫一個eval()，不過這個eval()功能有限，只能處理四則運算加上括號而已，在很多資料結構講stack的地方，會用C語言利用stack來寫，事實上，本程式也是參考『看程式實例學資料結構 使用Turbo C』的範例加以修改成C++和STL，以及OOP方式。
本程式的演算法是，先將人類習慣的『中序運算式』表示法先改成『後序運算式』表示法，因為後序式不需考慮括號的處理，比較簡單，然後再加以運算。
1
/*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : eval.cpp
5Compiler : Visual C++ 8.0 / ISO C++
6Description : Demo how to implement eval() by C++
7Release : 01/06/2007 1.0
8
*/
9
10#include <iostream> // cout
11#include <string> // string
12#include <sstream> // stringstream
13#include <stack> // stack
14#include <vector> // vector
15#include <cctype> // isdigit()
16
17using namespace std;
18
19// define const variable for readability
20const int OPERATOR = 0;
21const int OPERAND = 1;
22
23class Expression {
24// constructor
25public:
26 Expression();
27 Expression(const char*);
28
29// public member function
30public:
31 double eval(); // get eval result
32
33// private data member
34private:
35 stack<double> operandStack; // stack to store operand
36 stack<char> operatorStack; // stack to store operator
37 string infix; // string to hold infix expression
38 vector<pair<int, string> > suffix; // vector to hold suffix expression
39
40// private member function
41private:
42 string char2str(const char &); // convert char to string
43 string dbl2str(const double &); // convert double to string
44 double str2dbl(const string &); // convert string to double
45 bool isoperator(const char &); // identify whether it is an operator
46 void parseOperand(const double &); // parse operand to operandStack
47 void parseOperator(const char &); // parse operator to operatorStack
48 int operatorPriority(const char&); // define operator priority
49 void toSuffix(void); // convert infix to suffix
50 double calculate(const string &, const double &, const double &); // calculate result by operator and operand
51};
52
53int main(void) {
54 Expression x1("123/4+123*4-3");
55 cout << "x1=" << x1.eval() << endl;
56
57 Expression x2("1+(6+8)*4/3");
58 cout << "x2=" << x2.eval() << endl;
59}
60
61// constructor
62Expression::Expression() {
63
64}
65
66// constructor
67Expression::Expression(const char *val) {
68 this->infix = string(val); // fill infix by constructor
69 this->toSuffix(); // convert infix to suffix
70}
71
72// convert char to string
73string Expression::char2str(const char &c) {
74 stringstream ss;
75 ss << c;
76
77 return ss.str();
78}
79
80// convert double to string
81string Expression::dbl2str(const double &d) {
82 stringstream ss;
83 ss << d;
84
85 return ss.str();
86}
87
88// convert string to double
89double Expression::str2dbl(const string &s) {
90 stringstream ss(s);
91 double d;
92 ss >> d;
93
94 return d;
95}
96
97// identify whether it is an operator
98bool Expression::isoperator(const char &c) {
99
switch(c) {
100 case '(' :
101 case ')' :
102 case '+' :
103 case '-' :
104 case '*' :
105 case '/' : return true;
106 default : return false;
107
}
108}
109
110// parse operand to operandStack
111void Expression::parseOperand(const double &dOperand) {
112 suffix.push_back(make_pair(OPERAND, dbl2str(dOperand)));
113}
114
115// parse operator to operatorStack
116void Expression::parseOperator(const char &cOperator) {
117 if (operatorStack.empty() || cOperator == '(') {
118 operatorStack.push(cOperator);
119 }
120 else {
121 if (cOperator == ')') {
122 while(operatorStack.top() != '(') {
123 suffix.push_back(make_pair(OPERATOR, char2str(operatorStack.top())));
124 operatorStack.pop();
125
126 if (operandStack.empty()) break;
127 }
128 // Remove '('
129 operatorStack.pop();
130 }
131 else { // not ')'
132 while(operatorPriority(cOperator) <= operatorPriority(operatorStack.top()) && !operatorStack.empty()) {
133 suffix.push_back(make_pair(OPERATOR, char2str(operatorStack.top())));
134 operatorStack.pop();
135
136 if (operatorStack.empty())
137 break;
138 }
139 operatorStack.push(cOperator);
140 }
141 }
142}
143
144// define operator priority
145int Expression::operatorPriority(const char &cOperator) {
146 switch(cOperator) {
147 case '*' :
148 case '/' : return 3;
149 case '+' :
150 case '-' : return 2;
151 case '(' : return 1;
152 default : return 0;
153 }
154}
155
156// Convert infix to suffix
157// Algorithm : Parse infix string one char by one char. If char
158// is operator, check if _operand is "", if not, let
159// _operand to operandStack, and make _operand string
160// clear, then let operator to operatorStack. If char
161// is digit, concatenate to _operand string.
162void Expression::toSuffix(void) {
163 string _operand;
164 for(string::iterator p = infix.begin(); p != infix.end(); ++p) {
165 if (isoperator(*p)) {
166 if (_operand != "") {
167 parseOperand(str2dbl(_operand));
168 _operand.clear();
169 }
170 parseOperator(*p);
171 } else if (isdigit(*p))
172 _operand.push_back(*p);
173 }
174
175 // If _operand is not "", let _operand to operandStack.
176 if (_operand != "")
177 parseOperand(str2dbl(_operand));
178
179 // If operatorStack is not empty, push it to suffix vector until
180 // operatorStack is empty.
181 while(!operatorStack.empty()) {
182 suffix.push_back(make_pair(OPERATOR,char2str(operatorStack.top())));
183 operatorStack.pop();
184 }
185}
186
187// calculate result by operator and operand
188double Expression::calculate(const string &op, const double &operand1, const double &operand2) {
189 if (op == "+")
190 return operand2 + operand1;
191 else if (op == "-")
192 return operand2 - operand1;
193 else if (op == "*")
194 return operand2 * operand1;
195 else if (op == "/")
196 return operand2 / operand1;
197 else
198 return 0;
199}
200
201// get eval result
202double Expression::eval(void) {
203 // Clear OperandStack
204 while(!operandStack.empty())
205 operandStack.pop();
206
207 for(vector<pair<int, string> >::iterator iter = suffix.begin(); iter != suffix.end(); ++iter) {
208 if (iter->first == OPERATOR) {
209 double operand1 = operandStack.top();
210 operandStack.pop();
211 double operand2 = operandStack.top();
212 operandStack.pop();
213 operandStack.push(calculate(iter->second, operand1, operand2));
214 }
215 else if (iter->first == OPERAND) {
216 operandStack.push(str2dbl(iter->second));
217 }
218 }
219
220 return operandStack.top();
221}
/* 
2
(C) OOMusou 2007 http://oomusou.cnblogs.com3
4
Filename : eval.cpp5
Compiler : Visual C++ 8.0 / ISO C++6
Description : Demo how to implement eval() by C++7
Release : 01/06/2007 1.08
*/9
10
#include <iostream> // cout11
#include <string> // string12
#include <sstream> // stringstream13
#include <stack> // stack14
#include <vector> // vector15
#include <cctype> // isdigit()16
17
using namespace std;18
19
// define const variable for readability20
const int OPERATOR = 0; 21
const int OPERAND = 1;22
23
class Expression {
24
// constructor25
public: 26
Expression();27
Expression(const char*);28
29
// public member function30
public:31
double eval(); // get eval result32
33
// private data member34
private: 35
stack<double> operandStack; // stack to store operand36
stack<char> operatorStack; // stack to store operator37
string infix; // string to hold infix expression38
vector<pair<int, string> > suffix; // vector to hold suffix expression39
40
// private member function41
private:42
string char2str(const char &); // convert char to string43
string dbl2str(const double &); // convert double to string44
double str2dbl(const string &); // convert string to double45
bool isoperator(const char &); // identify whether it is an operator46
void parseOperand(const double &); // parse operand to operandStack47
void parseOperator(const char &); // parse operator to operatorStack48
int operatorPriority(const char&); // define operator priority49
void toSuffix(void); // convert infix to suffix50
double calculate(const string &, const double &, const double &); // calculate result by operator and operand51
};52
53
int main(void) {54
Expression x1("123/4+123*4-3");55
cout << "x1=" << x1.eval() << endl;56
57
Expression x2("1+(6+8)*4/3");58
cout << "x2=" << x2.eval() << endl;59
}60
61
// constructor62
Expression::Expression() {63
64
}65
66
// constructor67
Expression::Expression(const char *val) {68
this->infix = string(val); // fill infix by constructor69
this->toSuffix(); // convert infix to suffix70
}71
72
// convert char to string73
string Expression::char2str(const char &c) {74
stringstream ss;75
ss << c;76
77
return ss.str();78
}79
80
// convert double to string81
string Expression::dbl2str(const double &d) {82
stringstream ss;83
ss << d;84
85
return ss.str();86
}87
88
// convert string to double89
double Expression::str2dbl(const string &s) {90
stringstream ss(s);91
double d;92
ss >> d;93
94
return d;95
}96
97
// identify whether it is an operator98
bool Expression::isoperator(const char &c) {99
switch(c) {
100
case '(' :101
case ')' :102
case '+' :103
case '-' :104
case '*' :105
case '/' : return true;106
default : return false;107
}108
}109
110
// parse operand to operandStack111
void Expression::parseOperand(const double &dOperand) {112
suffix.push_back(make_pair(OPERAND, dbl2str(dOperand)));113
}114
115
// parse operator to operatorStack116
void Expression::parseOperator(const char &cOperator) {117
if (operatorStack.empty() || cOperator == '(') {118
operatorStack.push(cOperator);119
}120
else {121
if (cOperator == ')') {122
while(operatorStack.top() != '(') {123
suffix.push_back(make_pair(OPERATOR, char2str(operatorStack.top())));124
operatorStack.pop();125
126
if (operandStack.empty()) break;127
}128
// Remove '('129
operatorStack.pop();130
}131
else { // not ')'132
while(operatorPriority(cOperator) <= operatorPriority(operatorStack.top()) && !operatorStack.empty()) {133
suffix.push_back(make_pair(OPERATOR, char2str(operatorStack.top())));134
operatorStack.pop();135
136
if (operatorStack.empty()) 137
break;138
}139
operatorStack.push(cOperator);140
}141
}142
}143
144
// define operator priority145
int Expression::operatorPriority(const char &cOperator) {146
switch(cOperator) {147
case '*' :148
case '/' : return 3;149
case '+' :150
case '-' : return 2;151
case '(' : return 1;152
default : return 0;153
}154
}155
156
// Convert infix to suffix157
// Algorithm : Parse infix string one char by one char. If char 158
// is operator, check if _operand is "", if not, let 159
// _operand to operandStack, and make _operand string 160
// clear, then let operator to operatorStack. If char 161
// is digit, concatenate to _operand string.162
void Expression::toSuffix(void) {163
string _operand;164
for(string::iterator p = infix.begin(); p != infix.end(); ++p) {165
if (isoperator(*p)) {166
if (_operand != "") {167
parseOperand(str2dbl(_operand));168
_operand.clear();169
}170
parseOperator(*p);171
} else if (isdigit(*p)) 172
_operand.push_back(*p);173
}174
175
// If _operand is not "", let _operand to operandStack.176
if (_operand != "")177
parseOperand(str2dbl(_operand));178
179
// If operatorStack is not empty, push it to suffix vector until180
// operatorStack is empty.181
while(!operatorStack.empty()) {182
suffix.push_back(make_pair(OPERATOR,char2str(operatorStack.top())));183
operatorStack.pop();184
}185
}186
187
// calculate result by operator and operand188
double Expression::calculate(const string &op, const double &operand1, const double &operand2) {189
if (op == "+") 190
return operand2 + operand1;191
else if (op == "-") 192
return operand2 - operand1;193
else if (op == "*")194
return operand2 * operand1;195
else if (op == "/")196
return operand2 / operand1;197
else198
return 0;199
}200
201
// get eval result202
double Expression::eval(void) {203
// Clear OperandStack204
while(!operandStack.empty())205
operandStack.pop();206
207
for(vector<pair<int, string> >::iterator iter = suffix.begin(); iter != suffix.end(); ++iter) {208
if (iter->first == OPERATOR) {209
double operand1 = operandStack.top();210
operandStack.pop();211
double operand2 = operandStack.top();212
operandStack.pop();213
operandStack.push(calculate(iter->second, operand1, operand2));214
}215
else if (iter->first == OPERAND) {216
operandStack.push(str2dbl(iter->second));217
}218
}219
220
return operandStack.top();221
}
執行結果
x1=519.75x2=19.6667請按任意鍵繼續 . . .Reference
看程式實例學資料結構 使用Turbo C P.5-25 ~ P.5-54, 洪錦魁, 文魁出版社
