Evaluate Postfix Expression in C, C++, Java & Python – Code with Explanation & Examples in Short and Simple

   

C Program

#include <stdio.h>
#include <ctype.h>
int s[100],t=-1;
void push(int v){s[++t]=v;} int pop(){return s[t--];}
int main(){
    char e[]="23*54*+9-";
    for(int i=0;e[i];i++){
        if(isdigit(e[i])) push(e[i]-'0');
        else{int b=pop(),a=pop();
            switch(e[i]){case '+':push(a+b);break;case '-':push(a-b);break;
            case '*':push(a*b);break;case '/':push(a/b);}
        }
    }
    printf("%d",pop());
}

C Output

17

C++ Program

#include <bits/stdc++.h>
using namespace std;
int main(){
    string e="23*54*+9-"; stack<int> st;
    for(char c:e){
        if(isdigit(c)) st.push(c-'0');
        else{int b=st.top();st.pop();int a=st.top();st.pop();
            if(c=='+') st.push(a+b);
            else if(c=='-') st.push(a-b);
            else if(c=='*') st.push(a*b);
            else st.push(a/b);
        }
    }
    cout<<st.top();
}

C++ Output

17

JAVA Program

import java.util.*;
class Main{
    public static void main(String[] a){
        String e="23*54*+9-"; Stack<Integer> st=new Stack<>();
        for(char c:e.toCharArray()){
            if(Character.isDigit(c)) st.push(c-'0');
            else{
                int b=st.pop(),x=st.pop();
                st.push(c=='+'?x+b:c=='-'?x-b:c=='*'?x*b:x/b);
            }
        }
        System.out.println(st.pop());
    }
}

JAVA Output

17

Python Program

e="23*54*+9-"
st=[]
for c in e:
    if c.isdigit(): st.append(int(c))
    else:
        b,a=st.pop(),st.pop()
        st.append(a+b if c=='+' else a-b if c=='-' else a*b if c=='*' else a//b)
print(st.pop())

Python Output

17

Detailed Explanation

Example

Consider the postfix expression 23*54*+9-. "Postfix" (also referred to as Reverse Polish Notation) is the term used when operators follow their operands. If we rewrite in infix notation, it would be (2*3) + (5*4) - 9. Postfix is easy to evaluate because you don't use parentheses or rules of operator precedence — you just process from left to right with a stack.

Real-Life Analogy

Imagine a stack as a stack of plates in a cafeteria. When you notice a number (operand), you place it on the top of the plate stack. When you notice an operator, you remove the top two plates, do the operation, and return the result to the top. In postfix, the sequence is always fixed, so the "plate waiter" never has to wonder if multiplication or addition comes first — the sequence already knows that.

How It Works Step-by-Step

Read 2 → push it. Stack: [2]

Read 3 → push it. Stack: [2, 3]

Read * → pop 3 and 2, multiply → push 6. Stack: [6]

Read 5 → push. Stack: [6, 5]

Read 4 → push. Stack: [6, 5, 4]

Read * → pop 4 and 5, multiply → push 20. Stack: [6, 20]

Read + → pop 20 and 6, add → push 26. Stack: [26]

Read 9 → push. Stack: [26, 9]

Read - → pop 9 and 26, subtract → push 17. Stack: [17]

The remaining value in the stack is the solution.

Why It Matters

It's important to understand postfix evaluation because it's the basis for how compilers and interpreters evaluate arithmetic with no confusion. Postfix does away with the use of operator precedence tables while evaluating. This translates into quicker and easier execution in low-level environments, calculators, and virtual machines.

Learning Insights

You will learn how to implement a stack, one of the most significant data structures used in programming. The cognitive model here assists in analyzing expressions, handling undo operations, solving depth-first problems, and dealing with recursive calls. It also presents a way of thinking regarding problems as a sequence of instructions that operate on a storage structure.

Real-World Applications

Postfix evaluation occurs within calculators, expression parsers, formula engines in spreadsheets, and even some database query engines. It's also the way many bytecode-based languages (such as Java, Python, and PostScript) perform arithmetic internally.

Interview Use

Interviewers adore this problem because it checks several concepts simultaneously — string processing, stack operations, control flow, and edge case handling. They may modify it by adding multi-digit numbers, negative numbers, or user-defined operators such as exponentiation. Having knowledge of postfix evaluation makes you more flexible in such situations.

SEO-Friendly Closing

Evaluating a postfix expression using a stack is one of the most fundamental problems in data structures and algorithms. It teaches how to handle operators, operands, and precedence without parentheses, making it essential for learning compiler design and expression parsing. Whether you’re preparing for coding interviews, working on a calculator app, or exploring how programming languages handle math internally, mastering postfix evaluation will strengthen your problem-solving skills and understanding of stack-based computation.