Infix to Postfix to Output (Postfix Calculator) using stacks

Question

Good day, everyone! I'm new in C++ (and here in stackoverflow as well) and I need help from you experts.

I have here a code that should ask the user for the infix expression then converts it to postfix and outputs the result (postfix calculator). However, I cannot convert postfix to output instantly so as soon as it displays the postfix expression, it asks for the postfix expression again (with spaces after eg, 1 2 + ) before outputting the real answer.

There is no error or warning but when I run the program, the computer says "file.exe has stopped working" after displaying the postfix expression. So the program is able to convert infix to postfix expression correctly but there is still some jinx when displaying the output.

#include <iostream>
#include <cstring>
#include <cstdlib>

using namespace std;

struct node {
    char data;
    node *next;
};

node *top=NULL;
node *bottom=NULL;
node *key;
node *last;
node *before_last;


void push (const char Symbol) {
    key = new node;
    key->data = Symbol;
    key->next = top;
    top = key;
}

void push_for_output (node* &stack, int key) {
    node* newNode = new node;
    newNode->data = key;
    newNode->next = stack;
    stack = newNode;
}

const char pop() {
    if (!top) {
        cout << "Stack underflow\n" << endl;
        return ' ';
    }
    node* key = top;
    top = top->next;
    char ch = key->data;
    delete key;
    return ch;
}

int pop_for_output (node* &stack) {
    int key = stack->data;
    node* nodeToDelete = stack;
    stack = stack->next;
    delete nodeToDelete;
    return key;
}

bool isOperator (char *token) {
    if (strcmp(token, "+") == 0) {
        return true;
    }
    else if (strcmp(token, "-") == 0) {
        return true;
    }
    else if (strcmp(token, "*") == 0) {
        return true;
    }
    else if (strcmp(token, "/") == 0) {
        return true;
    }
    else {
        return false;
    }
}



const bool is_empty() {
    return !top;
}

int postfix(const char *infix) {
    char infix_ch[100]={NULL};
    char postfix_ch[100]={NULL};
    node* stack = NULL;

    strcpy(infix_ch,"(");
    strcat(infix_ch, infix);
    strcat(infix_ch,")");

    char symbol[5]={NULL};
    char temp[5]={NULL};

    for(int i=0; i<strlen(infix_ch); i++) {
        symbol[0]=infix_ch[i];

        if(symbol[0]=='(')
            push(symbol[0]);

        else if(symbol[0]==')') {
            symbol[0]=pop( );
            while(symbol[0]!='(') {
                strcat(postfix_ch, symbol);
                symbol[0]=pop( );
            }
        }

         else if(symbol[0]=='^' || symbol[0]=='*' || symbol[0]=='/' || symbol[0]=='+' || symbol[0]=='-') {
            if(symbol[0]=='*' || symbol[0]=='/') {
                temp[0]=pop( );
                while(temp[0]=='^' || temp[0]=='*' || temp[0]=='/') {
                   strcat(postfix_ch, temp);
                   temp[0]=pop( );
                }
                push(temp[0]);
            }

            else if(symbol[0]=='+' || symbol[0]=='-') {
                temp[0]=pop( );
                while(temp[0]!='(') {
                    strcat(postfix_ch, temp);
                    temp[0]=pop( );
                }
                push(temp[0]);
            }
            push(symbol[0]);
        }

            else
                strcat(postfix_ch, symbol);
    }

       cout << "Postfix: " << postfix_ch;

       char postfix[80];
        cout << "\nEnter postfix expression (include spaces between each operand and/or operator): ";
       cin.getline(postfix, 80);
       char *tokens = strtok(postfix, " ");
       while (tokens != NULL) {
        if (isOperator (tokens)) {
            int operand2 = pop_for_output(stack);
            int operand1 = pop_for_output(stack);
            int result;

            if (strcmp(tokens, "+") == 0) {
                result = operand1 + operand2;
            }
            else if (strcmp(tokens, "-") == 0) {
                result = operand1 - operand2;
            }
            else if (strcmp(tokens, "*") == 0) {
                result = operand1 * operand2;
            }
            else if (strcmp(tokens, "/") == 0) {
                result = operand1 / operand2;
            }

            push_for_output (stack, result);
        }
        else {
            push_for_output (stack, atoi (tokens));
        }
        tokens = strtok(NULL, " ");
    }
    cout << pop_for_output(stack);
    system("pause");
    return 0;
}


 int main( ) {
    char infix_values[100]={NULL};
    cout << "Enter the infix equation: ";
    cin >> infix_values;
    postfix(infix_values);
}

I'm a newbie and I really need help from you experts. I will really appreciate it if you help me correct my program. Thank you very much and have a nice day!

Answer 1

One possible issue is that the pop_for_output() function never checks for an empty/NULL stack like you do in pop() . If an invalid postfix expression is entered, or if your parsing is incorrect, you could very easily get into the case of referencing a NULL pointer which could very well explain the crash.