Postfix expression not returning the correct value

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This program is to evaluate a postfix expression using a class. The program runs but does not output the correct answer. What is wrong with the logic? I'm wondering if the input is used correctly with the CLASS
``````````````````````````````````

C++

#include <iostream>
#include <string>

using namespace std;

class mystack
{
	public:
		char stackcap[10];
		int top;
		
	public:
		mystack ()
		{
			top = -1;
			for (int i=0;i<10;i++)		
			{
				stackcap[i]	=0;
	     	}
		}
	
		bool isFull()
		{
			if (top==10)
			{
				return true;
			}
			else
			{
			return false;
			}	
			
				
		}
		
		void push(int val)	
		{
			if(isFull())
			{
				cout << "stack overflow"<<endl;   // METHOD TO PUSH ONTO STACK 
			}
			else
			{
				top++;
				stackcap[top]=val;	
	     	}
	 }
	    bool isEmpty()
		{
			
			 if(top==-1)
			 {
			 	return true;
			 }
			 else
			 {
			 	 return false;
			 }
			
			 
		}
		
		int pop()
		{
			
			if (isEmpty())
			{
				cout << "stack underflow"<<endl;
				return 0;
			}
			else
			{
				int popvalue = stackcap[top];
				stackcap[top]=0;
				top--;
				return popvalue;
			}
		}
		int count()
		{
			return (top+1);
		}
			};

int main() {

mystack stack;  // declare an object called stack
int i=0;
int x,y,z;
int charconver;


  cout << "==========WELCOME TO THE POSTFIX EXPRESSION EVALUATER=============\n ";
 
  cout << " ENTER A  POST FIX EXPRESSION" <<  endl;
  cin >> stack.stackcap[i];  
   
   
   while(stack.stackcap[i]!='\0')
   {
   		if(stack.stackcap[i]>='0'&& stack.stackcap[i]<='9')
   		{
   			charconver=stack.stackcap[i]-48;  //convert character to integer
   			stack.push(charconver);
		}
		else 
		if(stack.stackcap[i]= '+' || stack.stackcap[i]== '-' || stack.stackcap[i]== '*' || stack.stackcap[i]== '/')
		{
			x=stack.pop();
			y=stack.pop();
			switch (stack.stackcap[i])
			{
				case '+':
						z=x+y;
						stack.push(z);
						break;
				case '-':
						z=x-y;
						stack.push(z);
						break;
				case '*':
						z=x*y;
						stack.push(z);
						break;
				case '/':
						z=x/y;
						stack.push(z);
						break;
			}
		
   }i++;
   
}
cout << "The result of the Expression is:"<<stack.pop()<<endl;
  return 0;
}

What I have tried:

Ive tried to changing the input of the class properties

Posted 12-Dec-21 4:30am

Judel Dobbs

Updated 12-Dec-21 21:24pm

Richard MacCutchan

v2

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Comments

Richard MacCutchan 12-Dec-21 10:42am

"The program runs but does not output the correct answer."
Please explain; we do not know what data you are putting, what results you get, and why you think they are wrong.

Judel Dobbs 12-Dec-21 10:52am

Ok a postfix expression gets characters such as 11*. Postfix gets the values before the perform the calculation on the operand. The Expression adds the characters one by one to a stack and performs the required operation if it's a value or an operand. The operation is pop or push. If a character is a number add it to the stack. IF ITS A operator pop and perform calculation until the stack is empty.
11* would return the value 1

Richard MacCutchan 12-Dec-21 11:39am

Yes, I understand that. But you still have not explained what your problem is. We cannot guess what results your code produces when you run it.

Richard MacCutchan 12-Dec-21 11:45am

cin >> stack.stackcap[i];

That statement inputs a single character so you never have a complete expression. Change it to

cin >> stack.stackcap;

The code would probably work better if you removed that part altogether, and changed it to:

int value;
char operator;
cout << Enter the first number: ";
cin >> value;
stack.push(value);
cout << Enter the second number: ";
cin >> value;
stack.push(value);
cout << Enter the postfix operator: ";
cin >> operator;
stack.push(operator);

You can then operate on numbers larger than 9.

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Stefan_Lang · Answer 1 · 2021-12-12T21:24:00

Haven't checked your code in detail, but in this line:

C++

if(stack.stackcap[i]= '+' || stack.stackcap[i]== '-' || stack.stackcap[i]== '*' || stack.stackcap[i]== '/')

you should replace the first '=' with '=='

(Also, Richards comment looks like good advice. I'd second that.)

P.S.: your compiler should have warned you about that '='! If not, check your compiler settings and make sure that it does give you warnings of that type. If it did, then learn this lesson, and do not ignore compiler warnings: they typically indicate actual semantic errors and need to be fixed semantically.

OriginalGriff · Answer 2 · 2021-12-12T04:52:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Postfix expression not returning the correct value

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