added prefix evaluation

Author: Divyansh-jain2

others/prefix_evaluation.cpp

@@ -0,0 +1,68 @@
+#include <iostream>
+#include <stack>
+#include <string>
+#include <cmath>
+#include <vector>
+#include <algorithm>
+#include <cctype>
+
+bool isOperator(const std::string &s) {
+    return s == "+" || s == "-" || s == "*" || s == "/";
+}
+
+bool isNumber(const std::string &s) {
+    return !s.empty() && std::all_of(s.begin(), s.end(), [](char c) { return std::isdigit(c) || c == '.'; });
+}
+
+float evaluate(float a, float b, const std::string &op) {
+    if (op == "+") return a + b;
+    if (op == "-") return a - b;
+    if (op == "*") return a * b;
+    if (op == "/") return a / b;
+    throw std::invalid_argument("Unknown operator");
+}
+
+float prefixEvaluation(const std::vector<std::string> &expression) {
+    std::stack<float> stack;
+
+    // Traverse the expression from right to left
+    for (auto it = expression.rbegin(); it != expression.rend(); ++it) {
+        const std::string &token = *it;
+
+        if (isNumber(token)) {
+            stack.push(std::stof(token));
+        } else if (isOperator(token)) {
+            if (stack.size() < 2) {
+                throw std::invalid_argument("Invalid prefix expression");
+            }
+            float op1 = stack.top(); stack.pop();
+            float op2 = stack.top(); stack.pop();
+            float result = evaluate(op1, op2, token);
+            stack.push(result);
+        }
+    }
+
+    if (stack.size() != 1) {
+        throw std::invalid_argument("Invalid prefix expression");
+    }
+
+    return stack.top();
+}
+
+int main() {
+    // Example expressions
+    std::vector<std::string> expression1 = {"+", "*", "2", "3", "1"}; // Equivalent to (2 * 3) + 1
+    std::vector<std::string> expression2 = {"-", "+", "100", "200", "/", "5", "2"}; // Equivalent to (100 + 200) - (5 / 2)
+
+    try {
+        float result1 = prefixEvaluation(expression1);
+        std::cout << "Result of expression1: " << result1 << std::endl; // Expected output: 7
+
+        float result2 = prefixEvaluation(expression2);
+        std::cout << "Result of expression2: " << result2 << std::endl; // Expected output: 297.5
+    } catch (const std::invalid_argument &e) {
+        std::cerr << e.what() << std::endl;
+    }
+
+    return 0;
+}