C++中缀到后缀
编译时出现以下错误：
convert.cpp: In function 'std::string convert()':
convert.cpp:62: error: expected primary-expression before 'else'
convert.cpp:62: error: expected `;' before 'else'
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
代码：
#include<iostream>
#include<stack>
#include<string>
using namespace std;
string infix; // infix expression string
string operand;
string operate;
stack <string> mystack; // this stack is used to push the operator
string postfix; // postfix string where the operand is appended
//....................................................................
// this function read the infix expression from user
string input()
{
cout<<" Enter the damn infix expression: "<<endl;
getline(cin, infix);
return infix;
}
//......................................................................
// this function checks for operator precedence in the stack
int precedence(string e)
{
int f;
if(e == "*" || e== "/" || e =="%")
f = 2;
else
{
if(e == "+" || e == "-")
f = 1;
}
if(e=="."){
f=0;
}
return f;
}
//....................................................................
// This function converts infix to postfix
string convert()
{
for(int i=0; i<infix.length(); i++)
{
switch(infix[i]){
// operate case start
case '+': case'-': case'*': case'/': case'^': case '(': case ')': case'.':
operate=infix[i];
{
if(mystack.empty() || precedence(operate)>= precedence(mystack.top()))
{
mystack.push(operate);
else
{
while(precedence(operate)<= precedence(mystack.top()))
{
postfix.append(mystack.top());
mystack.pop();
}
mystack.push(operate);
}
}
}//operate case closed
default: //when operand string char is parsed
{
operand=infix[i];
postfix.append(operand);
break;
} // default case closed
}//switch closed
}
while(!mystack.empty())
{
postfix.append(mystack.top())
mystack.pop();
}
return postfix;
cout<<postfix;
}
//...................................................................
int main()
{
input();
convert();
cout<<"postfix is "<<postfix<<endl;
}
I get the following error when I compile:
convert.cpp: In function 'std::string convert()':
convert.cpp:62: error: expected primary-expression before 'else'
convert.cpp:62: error: expected `;' before 'else'
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
convert.cpp:108: error: expected `}' at end of input
Code:
#include<iostream>
#include<stack>
#include<string>
using namespace std;
string infix; // infix expression string
string operand;
string operate;
stack <string> mystack; // this stack is used to push the operator
string postfix; // postfix string where the operand is appended
//....................................................................
// this function read the infix expression from user
string input()
{
cout<<" Enter the damn infix expression: "<<endl;
getline(cin, infix);
return infix;
}
//......................................................................
// this function checks for operator precedence in the stack
int precedence(string e)
{
int f;
if(e == "*" || e== "/" || e =="%")
f = 2;
else
{
if(e == "+" || e == "-")
f = 1;
}
if(e=="."){
f=0;
}
return f;
}
//....................................................................
// This function converts infix to postfix
string convert()
{
for(int i=0; i<infix.length(); i++)
{
switch(infix[i]){
// operate case start
case '+': case'-': case'*': case'/': case'^': case '(': case ')': case'.':
operate=infix[i];
{
if(mystack.empty() || precedence(operate)>= precedence(mystack.top()))
{
mystack.push(operate);
else
{
while(precedence(operate)<= precedence(mystack.top()))
{
postfix.append(mystack.top());
mystack.pop();
}
mystack.push(operate);
}
}
}//operate case closed
default: //when operand string char is parsed
{
operand=infix[i];
postfix.append(operand);
break;
} // default case closed
}//switch closed
}
while(!mystack.empty())
{
postfix.append(mystack.top())
mystack.pop();
}
return postfix;
cout<<postfix;
}
//...................................................................
int main()
{
input();
convert();
cout<<"postfix is "<<postfix<<endl;
}
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评论（3）
看来您的代码只是缺少一些右大括号。就这样。
例如，看看这个：
else 语句之前的结束 } 在哪里。只需检查您的代码并修复这些愚蠢的错误即可。
如果你让间距和缩进更整齐一些，那么摆脱这些东西就会容易得多。
It appears that your code is just missing some closing braces. That's all.
For example, look at this:
Where is the closing } before the else statement. Just go through your code and fix these silly mistakes.
It would be a lot easier to rid yourself of these things if you made your spacing and indentation a little neater.
“else 之前的预期主表达式”诊断是编译器作者报告您粗鲁地强加给他一个“else”的方式，而没有前面的“if”或（相当于同一件事）“else if”。亚历山大·拉弗蒂（Alexander Rafferty）正确地指出，这是因为代码......
当也许你的意思是......
尽管也许你不小心删除了一大堆东西，并且很幸运删除导致了在无法解析的代码中，所以你会意识到出了问题。
The "expected primary expression before else" diagnostic is the compiler-author's way of reporting you rudely foisted on him an "else" without either a preceding "if" or (what amounts to the same thing) "else if". Alexander Rafferty correctly points out that this is because the code has ...
... when perhaps what you meant was ...
... though maybe you deleted a whole bunch of stuff by accident, and were lucky enough that the deletion resulted in unparseable code, so you will realize that something is wrong.
看看这些行：
Look at these lines: