prefix using stack

To implement prefix notation using a stack in C++, you can follow these steps:

1. Start by including the necessary header files. In this case, you will need the <iostream> and <stack> headers.

2. Define a function to evaluate the prefix expression. This function should take the prefix expression as a string parameter and return the result of the evaluation.

3. Inside the evaluation function, create an empty stack to store the operands.

4. Traverse the prefix expression from right to left. Start from the last character and move towards the first character.

5. For each character encountered, check if it is an operand or an operator.

6. If the character is an operand, convert it to an integer using the appropriate conversion function like std::stoi() and push it onto the stack.

7. If the character is an operator, pop two operands from the stack and perform the corresponding operation on them. Then, push the result back onto the stack.

8. Repeat steps 5 to 7 until all the characters in the prefix expression have been processed.

9. Finally, the result of the evaluation will be the only element left on the stack. Pop it and return the result.

Here's an example implementation of the prefix evaluation using a stack in C++:

#include <iostream>
#include <stack>

int evaluatePrefix(std::string expression) {
    std::stack<int> operandStack;

    for (int i = expression.length() - 1; i >= 0; i--) {
        if (isdigit(expression[i])) {
            operandStack.push(expression[i] - '0');
        } else {
            int operand1 = operandStack.top();
            operandStack.pop();
            int operand2 = operandStack.top();
            operandStack.pop();

            switch (expression[i]) {
                case '+':
                    operandStack.push(operand1 + operand2);
                    break;
                case '-':
                    operandStack.push(operand1 - operand2);
                    break;
                case '*':
                    operandStack.push(operand1 * operand2);
                    break;
                case '/':
                    operandStack.push(operand1 / operand2);
                    break;
            }
        }
    }

    return operandStack.top();
}

int main() {
    std::string prefixExpression = "*+243";
    int result = evaluatePrefix(prefixExpression);

    std::cout << "Result: " << result << std::endl;

    return 0;
}

This implementation evaluates the prefix expression "*+243" and prints the result, which is 14.