Class 8 Exam  >  Class 8 Notes  >  Evaluation of Postfix Expression

Evaluation of Postfix Expression - Class 8 PDF Download

The Postfix notation is used to represent algebraic expressions. The expressions written in postfix form are evaluated faster compared to infix notation as parenthesis are not required in postfix. We have discussed infix to postfix conversion. In this post, evaluation of postfix expressions is discussed.
Following is an algorithm for evaluation postfix expressions.

1) Create a stack to store operands (or values).
2) Scan the given expression and do the following for every scanned element. …..

  • If the element is a number, push it into the stack …..
  • If the element is an operator, pop operands for the operator from the stack. Evaluate the operator and push the result back to the stack 

3) When the expression is ended, the number in the stack is the final answer

Example:

Let the given expression be “2 3 1 * + 9 -“. We scan all elements one by one.

  • Scan ‘2’, it’s a number, so push it to stack. Stack contains ‘2’ 
  • Scan ‘3’, again a number, push it to stack, stack now contains ‘2 3’ (from bottom to top) 
  • Scan ‘1’, again a number, push it to stack, stack now contains ‘2 3 1’ 
  • Scan ‘*’, it’s an operator, pop two operands from stack, apply the * operator on operands, we get 3*1 which results in 3. We push the result ‘3’ to stack. The stack now becomes ‘2 3’. 
  • Scan ‘+’, it’s an operator, pop two operands from stack, apply the + operator on operands, we get 3 + 2 which results in 5. We push the result ‘5’ to stack. The stack now becomes ‘5’. 
  • Scan ‘9’, it’s a number, we push it to the stack. The stack now becomes ‘5 9’. 
  • Scan ‘-‘, it’s an operator, pop two operands from stack, apply the – operator on operands, we get 5 – 9 which results in -4. We push the result ‘-4’ to the stack. The stack now becomes ‘-4’. 
  • There are no more elements to scan, we return the top element from the stack (which is the only element left in a stack).

Below is the implementation of the above algorithm.

C++

// C++ program to evaluate value of a postfix expression

#include <iostream>

#include <string.h>

 

using namespace std;

 

// Stack type

struct Stack

{

    int top;

    unsigned capacity;

    int* array;

};

 

// Stack Operations

struct Stack* createStack( unsigned capacity )

{

    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));

 

    if (!stack) return NULL;

 

    stack->top = -1;

    stack->capacity = capacity;

    stack->array = (int*) malloc(stack->capacity * sizeof(int));

 

    if (!stack->array) return NULL;

 

    return stack;

}

 

int isEmpty(struct Stack* stack)

{

    return stack->top == -1 ;

}

 

char peek(struct Stack* stack)

{

    return stack->array[stack->top];

}

 

char pop(struct Stack* stack)

{

    if (!isEmpty(stack))

        return stack->array[stack->top--] ;

    return '$';

}

 

void push(struct Stack* stack, char op)

{

    stack->array[++stack->top] = op;

}

 

 

// The main function that returns value of a given postfix expression

int evaluatePostfix(char* exp)

{

    // Create a stack of capacity equal to expression size

    struct Stack* stack = createStack(strlen(exp));

    int i;

 

    // See if stack was created successfully

    if (!stack) return -1;

 

    // Scan all characters one by one

    for (i = 0; exp[i]; ++i)

    {

        // If the scanned character is an operand (number here),

        // push it to the stack.

        if (isdigit(exp[i]))

            push(stack, exp[i] - '0');

 

        // If the scanned character is an operator, pop two

        // elements from stack apply the operator

        else

        {

            int val1 = pop(stack);

            int val2 = pop(stack);

            switch (exp[i])

            {

            case '+': push(stack, val2 + val1); break;

            case '-': push(stack, val2 - val1); break;

            case '*': push(stack, val2 * val1); break;

            case '/': push(stack, val2/val1); break;

            }

        }

    }

    return pop(stack);

}

 

// Driver program to test above functions

int main()

{

    char exp[] = "231*+9-";

    cout<<"postfix evaluation: "<< evaluatePostfix(exp);

    return 0;

}

C

// C program to evaluate value of a postfix expression

#include <stdio.h>

#include <string.h>

#include <ctype.h>

#include <stdlib.h>

 

// Stack type

struct Stack

{

    int top;

    unsigned capacity;

    int* array;

};

 

// Stack Operations

struct Stack* createStack( unsigned capacity )

{

    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));

 

    if (!stack) return NULL;

 

    stack->top = -1;

    stack->capacity = capacity;

    stack->array = (int*) malloc(stack->capacity * sizeof(int));

 

    if (!stack->array) return NULL;

 

    return stack;

}

 

int isEmpty(struct Stack* stack)

{

    return stack->top == -1 ;

}

 

char peek(struct Stack* stack)

{

    return stack->array[stack->top];

}

 

char pop(struct Stack* stack)

{

    if (!isEmpty(stack))

        return stack->array[stack->top--] ;

    return '$';

}

 

void push(struct Stack* stack, char op)

{

    stack->array[++stack->top] = op;

}

 

 

// The main function that returns value of a given postfix expression

int evaluatePostfix(char* exp)

{

    // Create a stack of capacity equal to expression size

    struct Stack* stack = createStack(strlen(exp));

    int i;

 

    // See if stack was created successfully

    if (!stack) return -1;

 

    // Scan all characters one by one

    for (i = 0; exp[i]; ++i)

    {

        // If the scanned character is an operand (number here),

        // push it to the stack.

        if (isdigit(exp[i]))

            push(stack, exp[i] - '0');

 

        // If the scanned character is an operator, pop two

        // elements from stack apply the operator

        else

        {

            int val1 = pop(stack);

            int val2 = pop(stack);

            switch (exp[i])

            {

            case '+': push(stack, val2 + val1); break;

            case '-': push(stack, val2 - val1); break;

            case '*': push(stack, val2 * val1); break;

            case '/': push(stack, val2/val1); break;

            }

        }

    }

    return pop(stack);

}

 

// Driver program to test above functions

int main()

{

    char exp[] = "231*+9-";

    printf ("postfix evaluation: %d", evaluatePostfix(exp));

    return 0;

}

Java

// Java program to evaluate value of a postfix expression

 

import java.util.Stack;

 

public class Test

{

    // Method to evaluate value of a postfix expression

    static int evaluatePostfix(String exp)

    {

        //create a stack

        Stack<Integer> stack=new Stack<>();

         

        // Scan all characters one by one

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

        {

            char c=exp.charAt(i);

             

            // If the scanned character is an operand (number here),

            // push it to the stack.

            if(Character.isDigit(c))

            stack.push(c - '0');

             

            //  If the scanned character is an operator, pop two

            // elements from stack apply the operator

            else

            {

                int val1 = stack.pop();

                int val2 = stack.pop();

                 

                switch(c)

                {

                    case '+':

                    stack.push(val2+val1);

                    break;

                     

                    case '-':

                    stack.push(val2- val1);

                    break;

                     

                    case '/':

                    stack.push(val2/val1);

                    break;

                     

                    case '*':

                    stack.push(val2*val1);

                    break;

              }

            }

        }

        return stack.pop();   

    }

     

    // Driver program to test above functions

    public static void main(String[] args)

    {

        String exp="231*+9-";

        System.out.println("postfix evaluation: "+evaluatePostfix(exp));

    }

}

// Contributed by Sumit Ghosh

Python3

# Python program to evaluate value of a postfix expression

 

# Class to convert the expression

class Evaluate:

     

    # Constructor to initialize the class variables

    def __init__(self, capacity):

        self.top = -1

        self.capacity = capacity

        # This array is used a stack

        self.array = []

     

    # check if the stack is empty

    def isEmpty(self):

        return True if self.top == -1 else False

     

    # Return the value of the top of the stack

    def peek(self):

        return self.array[-1]

     

    # Pop the element from the stack

    def pop(self):

        if not self.isEmpty():

            self.top -= 1

            return self.array.pop()

        else:

            return "$"

     

    # Push the element to the stack

    def push(self, op):

        self.top += 1

        self.array.append(op)

 

 

    # The main function that converts given infix expression

    # to postfix expression

    def evaluatePostfix(self, exp):

         

        # Iterate over the expression for conversion

        for i in exp:

             

            # If the scanned character is an operand

            # (number here) push it to the stack

            if i.isdigit():

                self.push(i)

 

            # If the scanned character is an operator,

            # pop two elements from stack and apply it.

            else:

                val1 = self.pop()

                val2 = self.pop()

                self.push(str(eval(val2 + i + val1)))

 

        return int(self.pop())

                 

 

             

# Driver program to test above function

exp = "231*+9-"

obj = Evaluate(len(exp))

print ("postfix evaluation: %d"%(obj.evaluatePostfix(exp)))

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

// C# program to evaluate value of a postfix expression

using System;

using System.Collections;

 

namespace GFG

{

    class Geek

    {

        //Main() method

        static void Main()

        {

            Geek e = new Geek();

            e.v = ("231*+9-");

            e.expression();

            Console.WriteLine("postfix evaluation: " + e.answer);

            Console.Read();

        }

 

        public string v;

        //'v' is variable to store the string value

 

        public string answer;

        Stack i = new Stack();

        //'Stack()' is inbuilt function for namespace 'System.Collections'

 

        public void expression()

        //evaluation method

        {

            int a, b, ans;

            for (int j = 0; j < v.Length; j++)

            //'v.Length' means length of the string

            {

                String c = v.Substring(j, 1);

                if (c.Equals("*"))

                {

                    String sa = (String)i.Pop();

                    String sb = (String)i.Pop();

                    a = Convert.ToInt32(sb);

                    b = Convert.ToInt32(sa);

                    ans = a * b;

                    i.Push(ans.ToString());

 

                }

                else if (c.Equals("/"))

                {

                    String sa = (String)i.Pop();

                    String sb = (String)i.Pop();

                    a = Convert.ToInt32(sb);

                    b = Convert.ToInt32(sa);

                    ans = a / b;

                    i.Push(ans.ToString());

                }

                else if (c.Equals("+"))

                {

                    String sa = (String)i.Pop();

                    String sb = (String)i.Pop();

                    a = Convert.ToInt32(sb);

                    b = Convert.ToInt32(sa);

                    ans = a + b;

                    i.Push(ans.ToString());

 

                }

                else if (c.Equals("-"))

                {

                    String sa = (String)i.Pop();

                    String sb = (String)i.Pop();

                    a = Convert.ToInt32(sb);

                    b = Convert.ToInt32(sa);

                    ans = a - b;

                    i.Push(ans.ToString());

 

                }

                else

                {

                    i.Push(v.Substring(j, 1));

                }

            }

            answer = (String)i.Pop();

        }

    }

}

Javascript

<script>

 

// Javascript program to evaluate value of a postfix expression

 

// Method to evaluate value of a postfix expression

function evaluatePostfix(exp)

{

    //create a stack

        let stack=[];

          

        // Scan all characters one by one

        for(let i=0;i<exp.length;i++)

        {

            let c=exp[i];

              

            // If the scanned character is an operand (number here),

            // push it to the stack.

            if(! isNaN( parseInt(c) ))

            stack.push(c.charCodeAt(0) - '0'.charCodeAt(0));

              

            //  If the scanned character is an operator, pop two

            // elements from stack apply the operator

            else

            {

                let val1 = stack.pop();

                let val2 = stack.pop();

                  

                switch(c)

                {

                    case '+':

                    stack.push(val2+val1);

                    break;

                      

                    case '-':

                    stack.push(val2- val1);

                    break;

                      

                    case '/':

                    stack.push(val2/val1);

                    break;

                      

                    case '*':

                    stack.push(val2*val1);

                    break;

              }

            }

        }

        return stack.pop();  

}

 

// Driver program to test above functions

let exp="231*+9-";

document.write("postfix evaluation: "+evaluatePostfix(exp));

 

// This code is contributed by avanitrachhadiya2155

</script>

Output

postfix evaluation: -4

Complexity:
Time complexity: O(N) as we need to traverse N times.
Auxiliary Space: O(N) as we are having n elements in stack data structure
There are the following limitations of the above implementation.

  • It supports only 4 binary operators ‘+’, ‘*’, ‘-‘ and ‘/’. It can be extended for more operators by adding more switch cases. 
  • The allowed operands are only single-digit operands. The program can be extended for multiple digits by adding a separator-like space between all elements (operators and operands) of the given expression.

Below given is the extended program which allows operands to have multiple digits.

C++ 14

// C++ program to evaluate value of a postfix

// expression having multiple digit operands

#include <bits/stdc++.h>

using namespace std;

 

// Stack type

class Stack

{

    public:

    int top;

    unsigned capacity;

    int* array;

};

 

// Stack Operations

Stack* createStack( unsigned capacity )

{

    Stack* stack = new Stack();

 

    if (!stack) return NULL;

 

    stack->top = -1;

    stack->capacity = capacity;

    stack->array = new int[(stack->capacity * sizeof(int))];

 

    if (!stack->array) return NULL;

 

    return stack;

}

 

int isEmpty(Stack* stack)

{

    return stack->top == -1 ;

}

 

int peek(Stack* stack)

{

    return stack->array[stack->top];

}

 

int pop(Stack* stack)

{

    if (!isEmpty(stack))

        return stack->array[stack->top--] ;

    return '$';

}

 

void push(Stack* stack,int op)

{

    stack->array[++stack->top] = op;

}

 

 

// The main function that returns value

// of a given postfix expression

int evaluatePostfix(char* exp)

{

    // Create a stack of capacity equal to expression size

    Stack* stack = createStack(strlen(exp));

    int i;

 

    // See if stack was created successfully

    if (!stack) return -1;

 

    // Scan all characters one by one

    for (i = 0; exp[i]; ++i)

    {

        //if the character is blank space then continue

        if(exp[i] == ' ')continue;

         

        // If the scanned character is an

        // operand (number here),extract the full number

        // Push it to the stack.

        else if (isdigit(exp[i]))

        {

            int num=0;

             

            //extract full number

            while(isdigit(exp[i]))

            {

            num = num * 10 + (int)(exp[i] - '0');

                i++;

            }

            i--;

             

            //push the element in the stack

            push(stack,num);

        }

         

        // If the scanned character is an operator, pop two

        // elements from stack apply the operator

        else

        {

            int val1 = pop(stack);

            int val2 = pop(stack);

             

            switch (exp[i])

            {

            case '+': push(stack, val2 + val1); break;

            case '-': push(stack, val2 - val1); break;

            case '*': push(stack, val2 * val1); break;

            case '/': push(stack, val2/val1); break;

             

            }

        }

    }

    return pop(stack);

}

 

// Driver code

int main()

{

    char exp[] = "100 200 + 2 / 5 * 7 +";

    cout << evaluatePostfix(exp);

    return 0;

}

 

// This code is contributed by rathbhupendra

C

// C program to evaluate value of a postfix

// expression having multiple digit operands

#include <stdio.h>

#include <string.h>

#include <ctype.h>

#include <stdlib.h>

 

// Stack type

struct Stack

{

    int top;

    unsigned capacity;

    int* array;

};

 

// Stack Operations

struct Stack* createStack( unsigned capacity )

{

    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));

 

    if (!stack) return NULL;

 

    stack->top = -1;

    stack->capacity = capacity;

    stack->array = (int*) malloc(stack->capacity * sizeof(int));

 

    if (!stack->array) return NULL;

 

    return stack;

}

 

int isEmpty(struct Stack* stack)

{

    return stack->top == -1 ;

}

 

int peek(struct Stack* stack)

{

    return stack->array[stack->top];

}

 

int pop(struct Stack* stack)

{

    if (!isEmpty(stack))

        return stack->array[stack->top--] ;

    return '$';

}

 

void push(struct Stack* stack,int op)

{

    stack->array[++stack->top] = op;

}

 

 

// The main function that returns value

// of a given postfix expression

int evaluatePostfix(char* exp)

{

    // Create a stack of capacity equal to expression size

    struct Stack* stack = createStack(strlen(exp));

    int i;

 

    // See if stack was created successfully

    if (!stack) return -1;

 

    // Scan all characters one by one

    for (i = 0; exp[i]; ++i)

    {

        //if the character is blank space then continue

        if(exp[i]==' ')continue;

         

        // If the scanned character is an

        // operand (number here),extract the full number

        // Push it to the stack.

        else if (isdigit(exp[i]))

        {

            int num=0;

             

            //extract full number

            while(isdigit(exp[i]))

            {

            num=num*10 + (int)(exp[i]-'0');

                i++;

            }

            i--;

             

            //push the element in the stack

            push(stack,num);

        }

         

        // If the scanned character is an operator, pop two

        // elements from stack apply the operator

        else

        {

            int val1 = pop(stack);

            int val2 = pop(stack);

             

            switch (exp[i])

            {

            case '+': push(stack, val2 + val1); break;

            case '-': push(stack, val2 - val1); break;

            case '*': push(stack, val2 * val1); break;

            case '/': push(stack, val2/val1); break;

             

            }

        }

    }

    return pop(stack);

}

 

// Driver program to test above functions

int main()

{

    char exp[] = "100 200 + 2 / 5 * 7 +";

    printf ("%d", evaluatePostfix(exp));

    return 0;

}

 

// This code is contributed by Arnab Kundu

Java

// Java program to evaluate value of a postfix

// expression having multiple digit operands

import java.util.Stack;

 

class Test1

{

    // Method to evaluate value of a postfix expression

    static int evaluatePostfix(String exp)

    {

        //create a stack

        Stack<Integer> stack = new Stack<>();

         

        // Scan all characters one by one

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

        {

            char c = exp.charAt(i);

             

            if(c == ' ')

            continue;

             

            // If the scanned character is an operand

            // (number here),extract the number

            // Push it to the stack.

            else if(Character.isDigit(c))

            {

                int n = 0;

                 

                //extract the characters and store it in num

                while(Character.isDigit(c))

                {

                    n = n*10 + (int)(c-'0');

                    i++;

                    c = exp.charAt(i);

                }

                i--;

 

                //push the number in stack

                stack.push(n);

            }

             

            // If the scanned character is an operator, pop two

            // elements from stack apply the operator

            else

            {

                int val1 = stack.pop();

                int val2 = stack.pop();

                 

                switch(c)

                {

                    case '+':

                    stack.push(val2+val1);

                    break;

                     

                    case '-':

                    stack.push(val2- val1);

                    break;

                     

                    case '/':

                    stack.push(val2/val1);

                    break;

                     

                    case '*':

                    stack.push(val2*val1);

                    break;

            }

            }

        }

        return stack.pop();

    }

     

    // Driver program to test above functions

    public static void main(String[] args)

    {

        String exp = "100 200 + 2 / 5 * 7 +";

        System.out.println(evaluatePostfix(exp));

    }

}

 

// This code is contributed by Arnab Kundu

Python3

# Python program to evaluate value of a postfix

# expression with integers containing multiple digits

 

class evalpostfix:

    def __init__(self):

        self.stack =[]

        self.top =-1

    def pop(self):

        if self.top ==-1:

            return

        else:

            self.top-= 1

            return self.stack.pop()

    def push(self, i):

        self.top+= 1

        self.stack.append(i)

 

    def centralfunc(self, ab):

        for i in ab:

 

            # if the component of the list is an integer

            try:

                self.push(int(i))

            # if the component of the list is not an integer,

            # it must be an operator. Using ValueError, we can

            # evaluate components of the list other than type int

            except ValueError:

                val1 = self.pop()

                val2 = self.pop()

                if i == '/':

                  self.push(val2 / val1)

                else:       

                # switch statement to perform operation

                  switcher ={'+':val2 + val1, '-':val2-val1, '*':val2 * val1, '^':val2**val1}

                  self.push(switcher.get(i))

        return int(self.pop())

 

str ='100 200 + 2 / 5 * 7 +'

 

# splitting the given string to obtain

# integers and operators into a list

strconv = str.split(' ')

obj = evalpostfix()

print(obj.centralfunc(strconv))

 

# This code is contributed by Amarnath Reddy

C#

// C# program to evaluate value of a postfix

// expression having multiple digit operands

using System;

using System.Collections.Generic;

 

class GFG

{

// Method to evaluate value of

// a postfix expression

public static int evaluatePostfix(string exp)

{

    // create a stack

    Stack<int> stack = new Stack<int>();

 

    // Scan all characters one by one

    for (int i = 0; i < exp.Length; i++)

    {

        char c = exp[i];

 

        if (c == ' ')

        {

            continue;

        }

 

        // If the scanned character is an 

        // operand (number here),extract

        // the number. Push it to the stack.

        else if (char.IsDigit(c))

        {

            int n = 0;

 

            // extract the characters and

            // store it in num

            while (char.IsDigit(c))

            {

                n = n * 10 + (int)(c - '0');

                i++;

                c = exp[i];

            }

            i--;

 

            // push the number in stack

            stack.Push(n);

        }

 

        // If the scanned character is

        // an operator, pop two elements

        // from stack apply the operator

        else

        {

            int val1 = stack.Pop();

            int val2 = stack.Pop();

 

            switch (c)

            {

                case '+':

                stack.Push(val2 + val1);

                break;

 

                case '-':

                stack.Push(val2 - val1);

                break;

 

                case '/':

                stack.Push(val2 / val1);

                break;

 

                case '*':

                stack.Push(val2 * val1);

                break;

            }

        }

    }

    return stack.Pop();

}

 

// Driver Code

public static void Main(string[] args)

{

    string exp = "100 200 + 2 / 5 * 7 +";

    Console.WriteLine(evaluatePostfix(exp));

}

}

 

// This code is contributed by Shrikant13

Javascript

<script>

    // Javascript program to evaluate value of a postfix

    // expression having multiple digit operands

     

    // Method to evaluate value of

    // a postfix expression

    function evaluatePostfix(exp)

    {

        // create a stack

        let stack = [];

 

        // Scan all characters one by one

        for (let i = 0; i < exp.length; i++)

        {

            let c = exp[i];

 

            if (c == ' ')

            {

                continue;

            }

 

            // If the scanned character is an

            // operand (number here),extract

            // the number. Push it to the stack.

            else if (c >= '0' && c <= '9')

            {

                let n = 0;

 

                // extract the characters and

                // store it in num

                while (c >= '0' && c <= '9')

                {

                    n = n * 10 + (c - '0');

                    i++;

                    c = exp[i];

                }

                i--;

 

                // push the number in stack

                stack.push(n);

            }

 

            // If the scanned character is

            // an operator, pop two elements

            // from stack apply the operator

            else

            {

                let val1 = stack.pop();

                let val2 = stack.pop();

 

                switch (c)

                {

                    case '+':

                    stack.push(val2 + val1);

                    break;

 

                    case '-':

                    stack.push(val2 - val1);

                    break;

 

                    case '/':

                    stack.push(parseInt(val2 / val1, 10));

                    break;

 

                    case '*':

                    stack.push(val2 * val1);

                    break;

                }

            }

        }

        return stack.pop();

    }

     

    let exp = "100 200 + 2 / 5 * 7 +";

    document.write(evaluatePostfix(exp));

 

// This code is contributed by decode2207.

</script>

Output

757
Complexity :
Time complexity : O(N) as we need to traverse N times.
Auxiliary Space : O(N) as we are having n elements in stack data structure

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