Evaluation of Postfix Expressions

In the previous screen, we learned about converting infix expressions to postfix notation. Now, let's move on to evaluating postfix expressions using stacks.

In postfix notation, the operands appear before their operators. For example, the postfix expression 23+4* is equivalent to the infix expression (2 + 3) * 4.

To evaluate a postfix expression, we can use a stack to keep track of the operands. When we encounter an operand, we push it onto the stack. When we encounter an operator, we pop the top two operands from the stack, perform the operation, and push the result back onto the stack.

Here is an example code implementation in C++:

1#include <iostream>
2#include <stack>
3using namespace std;
4
5int evaluatePostfix(string postfix) {
6  stack<int> s;
7  for (char c : postfix) {
8    if (isdigit(c)) {
9      s.push(c - '0');
10    }
11    else {
12      int operand2 = s.top();
13      s.pop();
14      int operand1 = s.top();
15      s.pop();
16      int result;
17      switch (c) {
18        case '+': result = operand1 + operand2; break;
19        case '-': result = operand1 - operand2; break;
20        case '*': result = operand1 * operand2; break;
21        case '/': result = operand1 / operand2; break;
22      }
23      s.push(result);
24    }
25  }
26  return s.top();
27}
28
29int main() {
30  string postfix;
31  cout << "Enter a postfix expression: ";
32  cin >> postfix;
33  int result = evaluatePostfix(postfix);
34  cout << "Result: " << result << endl;
35  return 0;
36}

In this code, we use a stack to store the operands. We iterate over each character in the postfix expression. If the character is a digit, we convert it to an integer and push it onto the stack. If the character is an operator, we pop the top two operands from the stack, perform the operation, and push the result back onto the stack. Finally, we return the top of the stack, which contains the result of the expression.

Now that we have learned about evaluating postfix expressions, let's move on to exploring the applications of stacks and queues in real-world scenarios.

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}

#include <iostream>

#include <stack>

using namespace std;

​

int evaluatePostfix(string postfix) {

  stack<int> s;

  for (char c : postfix) {

    if (isdigit(c)) {

      s.push(c - '0');

    }

    else {

      int operand2 = s.top();

      s.pop();

      int operand1 = s.top();

      s.pop();

      int result;

      switch (c) {

        case '+': result = operand1 + operand2; break;

        case '-': result = operand1 - operand2; break;

        case '*': result = operand1 * operand2; break;

        case '/': result = operand1 / operand2; break;

      }

      s.push(result);

    }

  }

  return s.top();

}

​

int main() {