Writing a function to evaluate postfix expression

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I am not getting the evaluated result of the postfix expression. I have implemented it in c++. Here, I have added the function which I had done in order to evaluate the expression.
But it seems that the evaluated output is not being printed.Can someone say whats wrong with this code?

What I have tried:

void Evaluate_infix::postfix_eval()
{
    int i=0;
    char a,b,res;
   while(postfix[i]!='\0')
  { 
    
    if((int)postfix[i]>=49 && (int)postfix[i]<=57)          
        {
            operand.push(postfix[i]);
            i++;
        }   
    else if( postfix[i]=='+') 
        {
            a=operand.pop();;
            a=a-48;
            b=operand.pop();
            b=b-48;
            (char)res=a+b;
            operand.push(res); 
            i++;
        }
    else if( postfix[i]=='-') 
        {
            a=operand.pop();
            a=a-48;
            b=operand.pop();
            b=b-48;
            (char)res=a-b;
            operand.push(res);
            i++;
        }
    else if( postfix[i]=='*') 
        {
            a=operand.pop();
            a=a-48;
            b=operand.pop();
            b=b-48;
            (char)res=a*b;
            operand.push(res);
            i++;
        }
    else if( postfix[i]=='/') 
        {
            a=operand.pop();
            a=a-48;
            b=operand.pop();
            b=b-48;
            res=a/b;
            operand.push(res);
            i++;
        }
    else if( postfix[i]=='^') 
        {
            a=operand.pop();
            a=a-48;
            b=operand.pop();
            b=b-48;
            res=a^b;
            operand.push(res);
            i++;
        }
   }
  
   cout<<"\nPostfix Evaluation : ";
   cout<<static_cast<int>(operand.arr[operand.top])<

Posted 30-Oct-20 4:35am

Nandini Sathyan S

Updated 30-Oct-20 8:28am

v3

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Comments

Rick York 30-Oct-20 10:50am

I have no solution for you, only questions. This code is incomplete and it looks rather questionable. Why is 48 subtracted from every value? If you have a character and are subtracting '0' to get its binary value then why was it stored as an ASCII value in the first place? Also, why is all of this operating on a single character? This will not give correct results if you have a value greater than ten.

Lastly, a comment : a debugger should show you everything you need to solve this problem. If your development environment does not have one the get one that does. There are several alternatives available.

Nandini Sathyan S 30-Oct-20 10:53am

Can I upload the full code then? I hope u may understand@Rick York

Rick York 31-Oct-20 0:00am

No. You can't. You can edit your question to have the code but don't expect anyone to dig through it all. I know I am not going to.

Nandini Sathyan S 30-Oct-20 10:54am

#include <iostream>
using namespace std;
class Stack{
public:
char arr[10];
int top=-1;
int push(char);
char pop();
};
class Evaluate_infix{
char infix[20];
char postfix[20];
Stack operators;
Stack operand;
public:
void printPostfix();
void readInfix();
void Infix_Postfix();
void postfix_eval();
int precedence (char);
};
void Evaluate_infix::readInfix()
{

cout<<"\nEnter the infix expression : ";
cin>>infix;

}
void Evaluate_infix::Infix_Postfix()
{
int i=0,k=0;
while(infix[i]!='\0')
{
if((int)infix[i]>=49 && (int)infix[i]<=57)
{
postfix[k]=infix[i];
i++;
k++;
}
else if(infix[i]=='(' || infix[i]=='{' || infix[i]=='[')
{
operators.push(infix[i]);
i++;
}
else if(infix[i]==')' || infix[i]=='}' || infix[i]==']')
{
if(infix[i]==')')
{
while(operators.arr[operators.top]!='(')
{
postfix[k]=operators.pop();
k++;
}
operators.pop();
i++;

}
if(infix[i]==']')
{
while(operators.arr[operators.top]!='[')
{
postfix[k]=operators.pop();
k++;
}
operators.pop();
i++;
}

if(infix[i]=='}')
{
while(operators.arr[operators.top]!='{')
{
postfix[k]=operators.pop();
k++;
}
operators.pop();
i++;
}
}
else
{
if(operators.top==-1)
{
operators.push(infix[i]);
i++;
}

else if( precedence(infix[i]) <= precedence(operators.arr[operators.top])) {
postfix[k]=operators.pop();
k++;

while(precedence(operators.arr[operators.top]) == precedence(infix[i])){
postfix[k]=operators.pop();
if(operators.top < 0) {
break;
}
}
operators.push(infix[i]);
i++;
}
else if(precedence(infix[i]) > precedence(operators.arr[operators.top])) {
operators.push(infix[i]);
i++;
}
}
}
while(operators.top!=-1)
{
postfix[k]=operators.pop();
k++;
}

}

int Evaluate_infix::precedence ( char ch )
{
if(ch == '^')
{
return(3);
}
if(ch == '+' || ch =='-')
{
return(2);
}

if(ch == '*' || ch =='/')
{
return(1);
}

if(ch == '(')
{
return(0);
}

return 0;
}

int Stack::push(char item)
{
if(top==19)
{
cout<<"Overflow";
return -1;
}
else{
top=top+1;
arr[top]=item;
return 0;
}
}
char Stack::pop()
{
if(top==-1)
{
cout<<"Underflow";
return-1 ;
}
else{
int item=arr[top];
top=top-1;
return item;
}
}
void Evaluate_infix::printPostfix()
{

cout<<"The converted postfix string is : "<<postfix;

}
void="" evaluate_infix::postfix_eval()
{
="" int="" i="0;
" char="" a,b,res;
="" while(postfix[i]!="\0" )
="" {=""
="" if((int)postfix[i]="">=49 && (int)postfix[i]<=57)
{
operand.push(postfix[i]);
i++;
}
else if( postfix[i]=='+')
{
a=operand.pop();;
a=a-48;

Patrice T 30-Oct-20 11:04am

Use Improve question to update your question.
So that everyone can pay attention to this information.

Dave Kreskowiak 30-Oct-20 13:32pm

If you think someone is going to debug the code for you, and do your work for you, you would be sorely mistaken. Doing so would teach you nothing.

Debuggers are there to debug YOU, not the code. They exist as a tool for you to change your understanding of what the code is doing. You think, and assume, the code should work "this way", when in reality, it's doing something different from that. You have to use the debugger to change your understanding. Once you do that, you can then change the code.

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OriginalGriff · Answer 1 · 2020-10-30T05:18:00

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!

Patrice T · Answer 2 · 2020-10-30T08:29:00

Quote:
But it seems that the evaluated output is not being printed.Can someone say whats wrong with this code?

Don't guess, make sure with the debugger.
Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Writing a function to evaluate postfix expression

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