Postfix to Infix conversion in C++. "Process returned -1073740940 (0xC0000374) execution time : 5.538 s". No clue why

Question

The question is to convert a postfix expression to an infix expression using stacks and without stack library.

Im getting a statement after i run it and enter my postfix expression saying : "Process returned -1073740940 (0xC0000374) execution time : 5.538 s"

The second i enter my postfix expression, the computer freezes and takes up a few seconds and then gives the statement. It does not even return the infix expression.

I have absolutely no clue why. It also somestimes can take 30-40s just for this error to pop up.

Edit#1: Okay so as @churill said, the misplaced parenthesis was an issue. i corrected that. now im getting this error immediately: "terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc"

My code: (Edited once)

#include <iostream>
#include <ctype.h>
const int MAX = 20;

using namespace std;

class Stack {
private:
    int top = -1;
    string arr[MAX];
public:
    bool isEmpty(){
        if (top == -1){
            return true;
        }
        else{
            return false;
        }
    }

    bool isFull(){
        if(top == MAX-1){
            return true;
        }
        else{
            return false;
        }
    }

    void push(string c){
        if(isFull()){ //Edited line 
            cout<<"Stack Overflow"<<endl;
        }
        else{
            top++;
            arr[top] = c;
        }
    }

    string pop(){
        string val = arr[top];
        arr[top] = '0';
        top--;
        return val;
    }

    string topele(){
        string topelement = arr[top];
        return topelement;
    }

    string display(){
        int i;
        for(i=top;i>=0;i--){
            cout<<arr[i]<<endl;
        }
    }

    bool isOperand(char c){
        if(c >= 'a' && c <= 'z'){
            return true;
        }
        else{
            return false;
        }
    }

    string posfixtoinfix(string postfix){
        Stack s;
        int i;
        int len = postfix.size();
        for(i=0;i<=len;i++){
            if(s.isOperand(postfix[i])){ //Edited line
                string op(1, postfix[i]);
                s.push(op);
            }
            else{
                string op1 = s.topele();
                s.pop();
                string op2 = s.topele();
                s.pop();
                string res = ("("+ (op2 + postfix[i] + op1) + ")");
                s.push(res);
            }
        }
        return s.topele();
    }

};

int main(){
    Stack s1;
    string postfix, infix;
    cout<<"Enter postfix expression: "<<endl;
    cin>>postfix;
    infix = s1.posfixtoinfix(postfix);
    cout<<"The infix expression is: "<<infix<<endl;

    return 0;

}

Answer 1

For one, You have undefined behavior in your program. This is because you're going out of bounds when you wrote string op1 = s.topele(); at:

else{
        string op1 = s.topele(); //this will call topele
//...
}

Now the member funciton topele is called and you have

string topelement = arr[top]; //this will result in(when top = -1) undefined behavior and may crash the program

But note since top holds the value -1 by default, in the above snippet, you're essentially writing

string topelement = arr[-1];

which leads to undefined behavior and may result in a crash.

Remember that for a std::string the valid range of indexes is 0 to myString.length() - 1 (inclusive). When you use -1 as index, you get the character before the string, which is out of bounds and leads to undefined behavior.

Undefined behavior means anything¹ can happen including but not limited to the program giving your expected output. But never rely(or make conclusions based) on the output of a program that has undefined behavior.

---

^{1For a more technically accurate definition of undefined behavior see this where it is mentioned that: there are no restrictions on the behavior of the program.}