Infix to Postfix using stack returning error -1073741510-CodePudding

So, I was doing Data Structure and ALgorithm using C and STL, I was trying to implement Infix to postfix using stack. I am not sure what is this issue with the code? There is no compile error and when the code runs it returns -1073741510. I have rechecked the whole code, couldn't found any issues

#include<iostream>
#include<stack>
#include<string>

using namespace std;

int isOperator(char ch)
{
    if(ch==' ' || ch == '-' || ch == '*' || ch == '/')
        return 1;
    else
        return 0;
}

int precedence(char ch)
{
    //for limited input only
    if(ch == '*' || ch == '/')
        return 3;
    else if(ch==' ' || ch == '-' )
        return 2;
    else
        return 0;
}

string infixtopostfix(string infix)
{
    stack <char> st;
    int i=0;
    string postfix;
    while(infix[i]!='\0')
    {
        if(!isOperator(infix[i]))
        {
           postfix.push_back(infix[i]);
            i  ;
        }
        else
        {
            if(precedence(infix[i])>precedence(st.top()))
            {
                st.push(infix[i]);
                i  ;
            }
            else{
                postfix.push_back(st.top());
                st.pop();
            }
        }
    }

    while(!st.empty())
    {
        postfix.push_back(st.top());
        st.pop();
    }

    return postfix;
}


int main()
{
string infix= "a b";
cout<<"Postfix-->"<<infixtopostfix(infix)<<endl;
return 0;
}

CodePudding user response：

Your postfix string has a size of zero. This means that any attempt to access the characters of that string is an error. If you want to add characters to a string use push_back.

postfix[j] = infix[i];

should be

postfix.push_back(infix[i]);

and (twice)

postfix[j] = st.top();

should be

postfix.push_back(st.top());

and

postfix[j]='\0';

should be removed.

There is no need to nul terminate std::string. Once you have made all these changes you will also see that the j variable can be removed. A std::string knows it's own size, you don't need a separate variable to keep track. It seems that you are programming a std::string as if it's like a C string.

It seems to be a very common misunderstanding that std::string (or a std::vector) automatically grows when you subscript it. This is not true.

EDIT

You have another error here,

if(precedence(infix[i])>precedence(st.top()))

The stack maybe empty when executing this statement, leading to a crash. I'm guessing the code should read

if(st.empty() || precedence(infix[i])>precedence(st.top()))

With that change your code works for me.

CodePudding user response：

Thank You everyone for the help. Here is the final answer which is running without any error.

#include<iostream>
#include<stack>
#include<string>

using namespace std;

int isOperator(char ch)
{
    if(ch==' ' || ch == '-' || ch == '*' || ch == '/')
        return 1;
    else
        return 0;
}

int precedence(char ch)
{
    //for limited input only
    if(ch == '*' || ch == '/')
        return 3;
    else if(ch==' ' || ch == '-' )
        return 2;
    else
        return 0;
}

string infixtopostfix(string infix)
{
    stack <char> st;
    int i=0;
    string postfix;
    while(infix[i]!='\0')
    {
        if(!isOperator(infix[i]))
        {
           postfix.push_back(infix[i]);
            i  ;
        }
        else{
            if(st.empty() || precedence(infix[i])>precedence(st.top()))
            {
                st.push(infix[i]);
                i  ;
            }
            else{
                postfix.push_back(st.top());
                st.pop();
            }
        }
    }

    while(!st.empty())
    {
        postfix.push_back(st.top());
        st.pop();
    }

    return postfix;
}


int main()
{
string infix= "a b";
cout<<"Postfix-->"<<infixtopostfix(infix)<<endl;
return 0;
}