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Postfix Evaluation using C++ Stack

Any equation in the form "5 + ((1 + 2) × 4) − 3" is called Infix expression. The postfix expression of this infix notation will be: "5 1 2 + 4 × + 3 −". It is also known as Reverse Polish notation.



Algorithm:

  • Scan input expression from left to right
    • If scanned input is an operand, push it into the stack
    • If scanned input is an operator, pop out two values from stack. Then, perform operation between popped values and then push back the result into the stack.
  • Repeat above two steps till all the characters are scanned.
  • After all characters are scanned, there will be only one element in the stack, and this is the result of given expression.

C++ Implementation:

The following program evaluates a given postfix string. The numbers in inputs must be space separated:

/*
 * Postfix Evaluation
 * Language: C++
 */

#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include <stack>
#include <string.h>

using namespace std;

bool isOperator(char ch)
{
    if (ch=='+' || ch=='-' || ch=='*' || ch=='/')
        return true;
    else
        return false;
}


int performOperation(int op1, int op2, char op)
{
    int ans;
    switch(op){
    case '+':
        ans = op2 + op1;
        break;
    case '-':
        ans = op2 - op1;
        break;
    case '*':
        ans = op2 * op1;
        break;
    case '/':
        ans = op2 / op1;
        break;
    }
    return ans;
}


int main()
{
    char exp[1000], buffer[15];
    int i,op1, op2, len, j, x;
    stack<int> s;
    printf("Enter a Postfix Expression: ( e.g. 23 34 * )\n");
    gets(exp);
    len = strlen(exp);
    j = 0;
    for(i=0; i<len;i++){

        if(exp[i]>='0' && exp[i]<='9'){
            buffer[j++] = exp[i];
        }
        else if(exp[i]==' '){
            if(j>0){
                buffer[j] = '\0';
                x = atoi(buffer);
                s.push(x);
                j = 0;
            }
        }

        else if(isOperator(exp[i])){
            op1 = s.top();
            s.pop();
            op2 = s.top();
            s.pop();
            s.push(performOperation(op1, op2, exp[i]));
        }
    }

    printf("Answer is %d\n", s.top());

    return 0;
}