Question

Write the code for postfix expression in C++ using a linked stack that can take numbers bigger than 9 (any size the user gives) and pushes the final result onto the top of the stack

Answer 1

This is a updated code capable of taking integers greater than 9 (any size that user gives) for postfix expression evaluation

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

using namespace std;


// Function to verify whether a character is numeric digit.
bool IsNumericDigit(char C)
{
        if(C >= '0' && C <= '9') return true;
        return false;
}



// Function to verify whether a character is operator symbol or not.
bool IsOperator(char C)
{
        if(C == '+' || C == '-' || C == '*' || C == '/' || C == '^')
                return true;

        return false;
}



// Function to perform an operation and return output.
int PerformOperation(char operation, int operand1, int operand2)
{
        if(operation == '+') return (operand1 + operand2);
        else if(operation == '-') return (operand1 - operand2);
        else if(operation == '*') return (operand1 * operand2);
        else if(operation == '/') return (operand1 / operand2);

        // Using typecasting for integer result for pow() function
        // The pow() function takes ‘double’ as the arguments and returns a ‘double’ value.
        // This function does not always work for integers. One such example is pow(5, 2).
    // When assigned to an integer, it outputs 24 on some compilers and works fine for some other compilers.
    // But pow(5, 2) without any assignment to an integer outputs 25.
        // To overcome this and output the accurate answer in integer format,
        // we can add 0.5 to the result and typecast it to int e.g (int)(pow(5, 2)+0.5) will give
        // the correct answer(25, in above example), irrespective of the compiler.

        else if(operation == '^') return ( (int)(pow(operand1, operand2) + 0.5) );

        else cout<<"Unexpected Error \n";
        return -1;
}



// Function to evaluate Postfix expression and return output
int EvaluatePostfix(string expression)
{
        // Declaring a Stack from Standard template library in C++.
        stack<int> S;

        for(int i = 0;i< expression.length();i++) {

                // Scanning each character from left.
                // If character is a delimitter, move on.
                if(expression[i] == ' ' || expression[i] == ',') continue;

                // If character is operator, pop two elements from stack, perform operation and push the result back.
                else if(IsOperator(expression[i])) {
                        // Pop two operands.
                        int operand2 = S.top(); S.pop();
                        int operand1 = S.top(); S.pop();
                        // Perform operation
                        int result = PerformOperation(expression[i], operand1, operand2);
                        //Push back result of operation on stack.
                        S.push(result);
                }
                else if(IsNumericDigit(expression[i])){
                        // Extract the numeric operand from the string
                        // Keep incrementing i as long as you are getting a numeric digit.
                        int operand = 0;
                        while(i<expression.length() && IsNumericDigit(expression[i])) {
                                // For a number with more than one digits, as we are scanning from left to right.
                                // Everytime , we get a digit towards right, we can multiply current total in operand by 10
                                // and add the new digit.
                                operand = (operand*10) + (expression[i] - '0');
                                i++;
                        }
                        // Finally, you will come out of while loop with i set to a non-numeric character or end of string
                        // decrement i because it will be incremented in increment section of loop once again.
                        // We do not want to skip the non-numeric character by incrementing i twice.
                        i--;

                        // Push operand on stack.
                        S.push(operand);
                }
        }
        // If expression is in correct format, Stack will finally have one element. This will be the output.
        return S.top();
}


int main()
{
        string expression;
        cout<<"Important instruction to be followed\n\nEvaluation Of postfix Expression in C++\nInput Postfix expression must be in a desired format.\nOperands must be integers and there should be space in between two operands.\nOnly '+'  ,  '-'  , '*' , '/' and '^'  operators are expected.";
        cout<<"\n\n\n\nEnter Postfix Expression: ";
        getline(cin,expression);
        int result = EvaluatePostfix(expression);
        cout<<"\n\nOutput = "<<result<<"\n";
        return 0;
}

Output screenshot