feat: Added Postfix Evaluation (#1083)

* feat:Added Postfix evaluation

* updating DIRECTORY.md

* Fixed global variable name and stack def

* Fixed c from int to float

* Fixed clang-tidy

* Modified code for larger values

* Added string header

* Suggested changes

* Fixed clang-tidy

* Apply suggestions from code review

Co-authored-by: David Leal <halfpacho@gmail.com>

* Added description for class

* Fixed function calling

* Apply suggestions from code review

Co-authored-by: David Leal <halfpacho@gmail.com>

* Updated evaluate function documentation

* Apply suggestions from code review

Co-authored-by: David Leal <halfpacho@gmail.com>

* Update others/postfix_evaluation.cpp

Co-authored-by: David Leal <halfpacho@gmail.com>

* Update others/postfix_evaluation.cpp

Co-authored-by: Krishna Vedala <7001608+kvedala@users.noreply.github.com>

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>
Co-authored-by: Darshana-Sarma <lownish@hotmail.com>
Co-authored-by: Krishna Vedala <7001608+kvedala@users.noreply.github.com>

DIRECTORY.md

@@ -190,6 +190,7 @@
   * [Palindrome Of Number](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/palindrome_of_number.cpp)
   * [Paranthesis Matching](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/paranthesis_matching.cpp)
   * [Pascal Triangle](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/pascal_triangle.cpp)
+  * [Postfix Evaluation](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/postfix_evaluation.cpp)
   * [Primality Test](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/primality_test.cpp)
   * [Smallest Circle](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/smallest_circle.cpp)
   * [Sparse Matrix](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/sparse_matrix.cpp)

others/postfix_evaluation.cpp

@@ -0,0 +1,178 @@
+/**
+ * @file
+ * @brief Evaluation of [Postfix Expression](https://en.wikipedia.org/wiki/Reverse_Polish_notation)
+ * @author [Darshana Sarma](https://github.com/Darshana-Sarma)
+ * @details
+ * Create a stack to store operands (or values).
+ * Scan the given expression and do following for every scanned element.
+ * If the element is a number, push it into the stack
+ * If the element is a operator, pop operands for the operator from stack.
+ * Evaluate the operator and push the result back to the stack
+ * When the expression is ended, the number in the stack is the final answer
+ */
+#include <algorithm>  // for all_of
+#include <array>      // for std::array
+#include <cassert>    // for assert
+#include <iostream>   // for io operations
+#include <string>     // for stof
+
+/**
+ * @namespace others
+ * @brief Other algorithms
+ */
+namespace others {
+/**
+ * @namespace postfix_expression
+ * @brief Functions for Postfix Expression algorithm
+ */
+namespace postfix_expression {
+ /**
+ * @brief Creates an array to be used as stack for storing values
+ */
+class Stack {
+ public:
+    std::array<float, 20> stack{}; ///< Array which will be used to store numbers in the input
+    int stackTop = -1;            ///< Represents the index of the last value added to array. -1 means array is empty
+};
+
+/**
+ * @brief Pushing operand, also called the number in the array to the stack
+ * @param operand float value from the input array or evaluation
+ * @param stack stack containing numbers
+ * @returns none
+ */
+void push(float operand, Stack *stack) {
+    stack->stackTop++;
+    stack->stack[stack->stackTop] = operand;
+}
+
+/**
+ * @brief Popping operand, also called the number from the stack
+ * @param stack stack containing numbers
+ * @returns operand float on top of stack
+ */
+float pop(Stack *stack) {
+    float operand = stack->stack[stack->stackTop];
+    stack->stackTop--;
+    return operand;
+}
+
+/**
+ * @brief Checks if scanned string is a number
+ * @param s scanned string
+ * @returns bool boolean value if string is number
+ */
+bool is_number(const std::string &s) {
+    return !s.empty() && std::all_of(s.begin(), s.end(), ::isdigit);
+}
+
+/**
+ * @brief Evaluate answer using given last two operands from and operation
+ * @param a second last added operand which will be used for evaluation
+ * @param b last added operand which will be used for evaluation
+ * @param operation to be performed with respective floats
+ * @param stack containing numbers
+ * @returns none
+ */
+void evaluate(float a, float b, const std::string &operation, Stack *stack) {
+    float c = 0;
+    const char *op = operation.c_str();
+    switch (*op) {
+        case '+':
+            c = a + b; // Addition of numbers
+            others::postfix_expression::push(c, stack);
+            break;
+
+        case '-':
+            c = a - b; // Subtraction of numbers
+            others::postfix_expression::push(c, stack);
+            break;
+
+        case '*':
+            c = a * b; // Multiplication of numbers
+            others::postfix_expression::push(c, stack);
+            break;
+
+        case '/':
+            c = a / b; // Division of numbers
+            others::postfix_expression::push(c, stack);
+            break;
+
+        default:
+            std::cout << "Operator not defined\n";
+            break;
+    }
+}
+
+/**
+ * @brief Postfix Evaluation algorithm to compute the value from given input
+ * array
+ * @tparam N number of array size
+ * @param input Array of characters consisting of numbers and operations
+ * @returns stack[stackTop] returns the top value from the stack
+ */
+template <std::size_t N>
+float postfix_evaluation(std::array<std::string, N> input) {
+    Stack stack;
+    int j = 0;
+
+    while (j < N) {
+        std::string scan = input[j];
+        if (is_number(scan)) {
+            push(std::stof(scan), &stack);
+
+        } else {
+            float op2 = pop(&stack);
+            float op1 = pop(&stack);
+
+            evaluate(op1, op2, scan, &stack);
+        }
+        j++;
+    }
+
+    std::cout << stack.stack[stack.stackTop] << "\n";
+
+    return stack.stack[stack.stackTop];
+}
+}  // namespace postfix_expression
+}  // namespace others
+
+
+/**
+ * @brief Test function 1 with input array
+ * {'2', '3', '1', '*', '+', '9', '-'}
+ * @returns none
+ */
+static void test_function_1() {
+    std::array<std::string, 7> input = {"2", "3", "1", "*", "+", "9", "-"};
+
+    float answer = others::postfix_expression::postfix_evaluation(input);
+
+    assert(answer == -4);
+}
+
+/**
+ * @brief Test function 2 with input array
+ * {'1', '2', '+', '2', '/', '5', '*', '7', '+'}
+ * @returns none
+ */
+static void test_function_2() {
+    std::array<std::string, 9> input = {"100", "200", "+", "2", "/",
+                                        "5",   "*",   "7", "+"};
+    float answer = others::postfix_expression::postfix_evaluation(input);
+
+    assert(answer == 757);
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test_function_1();
+    test_function_2();
+
+    std::cout << "\nTest implementations passed!\n";
+
+    return 0;
+}