这是完整实现的中缀到后缀转换和后期表达式的评估。 ＆＃34;中缀到后缀转换＆＃34;是成功的，但评估＆＃34;给出一个段错误。
代码是：
#include<iostream>
//#include<stack>
#include<string>
#include<cmath>
using namespace std;
template<class T1>
struct node
{
T1 data;
node<T1> *next;
};
template<class T1>
class stack
{
node<T1> *head;
public:
stack()
{
head=NULL;
}
void push(T1);
int empty()
{
if(head==NULL)
return 1;
return 0;
}
T1 top()
{
T1 temp;
temp=head->data;
return (temp);
}
T1 pop();
void show();
};
template<class T1>
void stack<T1>::push(T1 input)
{
node<T1> *ptr;
ptr=new node<T1>;
ptr->data=input;
ptr->next=NULL;
if(head!=NULL)
{
ptr->next=head;
}
head=ptr;
}
template<class T1>
T1 stack<T1>::pop()
{
node<T1> *temp;
T1 output;
temp=head;
output=temp->data;
head=head->next;
delete temp;
return output;
}
template<class T1>
void stack<T1>::show()
{
node<T1> *temp;
temp=head;
while(temp!=NULL)
{
cout<<temp->data;
temp=temp->next;
}
}
void evaluate(string postfix)
{
stack<float>s;
float a;
int i;
for(i=0; i < postfix.length(); i++)
{
if(isalpha(postfix[i]))
{
cout<<"enter the value of\t";
cout<<postfix[i];
cin>>a;
s.push(a);
}
else
{
float x,y;
y=s.pop(); //s.pop();
x=s.pop(); //s.pop();
char token=postfix[i];
if(token=='+')
{
x=x+y;
s.push(x);
}
if(token=='-')
{
x=x-y;
s.push(x);
}
if(token=='*')
{
x=x*y;
s.push(x);
}
if(token=='/')
{
x=x/y;
s.push(x);
}
if(token=='^')
{
x=pow(x,y);
s.push(x);
}
}
}
cout<<"\t Evaluated result is \t";
cout<<s.top();
s.pop();
}
// Function to convert Infix expression to postfix
string InfixToPostfix(string expression);
// Function to verify whether an operator has higher precedence over other
int HasHigherPrecedence(char operator1, char operator2);
// Function to verify whether a character is operator symbol or not.
bool IsOperator(char C);
// Function to verify whether a character is alphanumeric chanaracter (letter or numeric digit) or not.
bool IsOperand(char C);
int main()
{
string expression;
cout<<"Enter Infix Expression \n";
getline(cin,expression);
string postfix = InfixToPostfix(expression);
cout<<"Postfix is : = "<<postfix<<"\n";
evaluate(expression);
}
// Function to evaluate Postfix expression and return output
string InfixToPostfix(string expression)
{
// Declaring a Stack from Standard template library in C++.
stack<char> S;
string postfix = ""; // Initialize postfix as empty string.
for(int i = 0;i< expression.length();i++) {
// Scanning each character from left.
// If character is a delimitter, move on.
if(expression[i] == ' ' || expression[i] == ',') continue;
// If character is operator, pop two elements from stack, perform operation and push the result back.
else if(IsOperator(expression[i]))
{
while(!S.empty() && S.top() != '(' && HasHigherPrecedence(S.top(),expression[i]))
{
postfix+= S.top();
S.pop();
}
S.push(expression[i]);
}
// Else if character is an operand
else if(IsOperand(expression[i]))
{
postfix +=expression[i];
}
else if (expression[i] == '(')
{
S.push(expression[i]);
}
else if(expression[i] == ')')
{
while(!S.empty() && S.top() != '(') {
postfix += S.top();
S.pop();
}
S.pop();
}
}
while(!S.empty()) {
postfix += S.top();
S.pop();
}
return postfix;
}
// Function to verify whether a character is english letter or numeric digit.
// We are assuming in this solution that operand will be a single character
bool IsOperand(char C)
{
if(C >= '0' && C <= '9') return true;
if(C >= 'a' && C <= 'z') return true;
if(C >= 'A' && C <= 'Z') return true;
return false;
}
// Function to verify whether a character is operator symbol or not.
bool IsOperator(char C)
{
if(C == '+' || C == '-' || C == '*' || C == '/' || C== '^')
return true;
return false;
}
// Function to verify whether an operator is right associative or not.
int IsRightAssociative(char op)
{
if(op == '^') return true;
return false;
}
// Function to get weight of an operator. An operator with higher weight will have higher precedence.
int GetOperatorWeight(char op)
{
int weight = -1;
switch(op)
{
case '+':
case '-':
weight = 1;
break;
case '*':
case '/':
weight = 2;
break;
case '^':
weight = 3;
break;
}
return weight;
}
// Function to perform an operation and return output.
int HasHigherPrecedence(char op1, char op2)
{
int op1Weight = GetOperatorWeight(op1);
int op2Weight = GetOperatorWeight(op2);
// If operators have equal precedence, return true if they are left associative.
// return false, if right associative.
// if operator is left-associative, left one should be given priority.
if(op1Weight == op2Weight)
{
if(IsRightAssociative(op1)) return false;
else return true;
}
return op1Weight > op2Weight ? true: false;
}
在调试时我得到：
Enter Infix Expression
(5^2)+5
Breakpoint 1, main () at stack.cpp:155
warning: Source file is more recent than executable.
155 string postfix = InfixToPostfix(expression);
Missing separate debuginfos, use: dnf debuginfo-install libgcc-5.3.1-2.fc23.x86_64 libstdc++-5.3.1-2.fc23.x86_64
(gdb) n
156 cout<<"Postfix is : = "<<postfix<<"\n";
(gdb) n
Postfix is : = 52^5+
157 evaluate(expression);
(gdb) s
evaluate (postfix="(5^2)+5") at stack.cpp:81
81 stack<float>s;
(gdb) s
stack<float>::stack (this=0x7fffffffddd0) at stack.cpp:23
23 head=NULL;
(gdb) s
24 }
(gdb) s
evaluate (postfix="(5^2)+5") at stack.cpp:84
84 for(i=0; i < postfix.length(); i++)
(gdb) s
86 if(isalpha(postfix[i]))
(gdb) s
96 y=s.pop(); //s.pop();
(gdb) s
stack<float>::pop (this=0x7fffffffddd0) at stack.cpp:60
60 temp=head;
(gdb) s
61 output=temp->data;
(gdb) s
Program received signal SIGSEGV, Segmentation fault.
0x0000000000401a05 in stack<float>::pop (this=0x7fffffffddd0) at stack.cpp:61
61 output=temp->data;
(gdb) s
s
Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
问题在于：
float x,y;
y=s.pop(); //s.pop();
x=s.pop(); //s.pop();
of evaluation（）函数。
我再次明确地使用了堆栈实现，以便我可以看到问题所在。
在pop函数中，seg。错误来自：output=temp->data;
答案 0 :(得分：1)
问题出在您的pop方法中。您需要检查head == nullptr。像：
template<class T1>
T1 stack<T1>::pop()
{
node<T1> *temp;
T1 output;
if (head == nullptr)
{
// Error handling.... Perhaps:
throw ....;
}
temp=head;
output=temp->data;
head=head->next;
delete temp;
return output;
}
如果您的evaluate函数在第一个或第二个循环中采用了错误路径，那么您将取消引用nullptr并获得seg错误。
像这样：
void evaluate(string postfix)
{
stack<float> s; // s is empty and head is nullptr
float a;
int i;
for(i=0; i < postfix.length(); i++)
{
if(isalpha(postfix[i])) // Assume this evalutes to false in first loop
{
cout<<"enter the value of\t";
cout<<postfix[i];
cin>>a;
s.push(a);
}
else
{
float x,y;
y=s.pop(); // then you pop from empty stack and dereference a nullptr
答案 1 :(得分：1)
您的代码中有2个问题：
expression时，您已将原始evaluate传递给postfix函数：评估中的循环内容应该是（或多或少）：
if(isalpha(postfix[i]))
{
cout<<"enter the value of\t";
cout<<postfix[i];
cin>>a;
s.push(a);
}
else if (isdigit(postfix[i])) { // do not forget to populate stack!
s.push(0.f + postfix[i] - '0'); // raw conversion of digit to float
}
else
...
在这个简单的情况下，这足以摆脱seg错误。但是您的算法目前无法处理超过1位的数字......