feat: add postfix evaluation algorithm

DIRECTORY.md

@@ -271,6 +271,7 @@
   * [Palindrome](https://github.com/TheAlgorithms/C/blob/master/misc/palindrome.c)
   * [Pid](https://github.com/TheAlgorithms/C/blob/master/misc/pid.c)
   * [Poly Add](https://github.com/TheAlgorithms/C/blob/master/misc/poly_add.c)
+  * [Postfix Evaluation](https://github.com/TheAlgorithms/C/blob/master/misc/postfix_evaluation.c)
   * [Prime](https://github.com/TheAlgorithms/C/blob/master/misc/prime.c)
   * [Prime Factoriziation](https://github.com/TheAlgorithms/C/blob/master/misc/prime_factoriziation.c)
   * [Prime Seive](https://github.com/TheAlgorithms/C/blob/master/misc/prime_seive.c)

misc/postfix_evaluation.c

@@ -0,0 +1,128 @@
+/**
+ * @file
+ * @brief [Postfix evaluation algorithm](https://www.includehelp.com/c/evaluation-of-postfix-expressions-using-stack-with-c-program.aspx) implementation
+ * @details
+ * The input postfix expression is of type string upto 49 characters (including space delimiters).
+ * Supported operations- '+', '-', '/', '*', '%'
+ * @author [Kumar Yash](https://github.com/kumaryash18)
+ */
+
+#include <stdio.h>	/// for IO operations
+#include <string.h>	/// for strlen()
+#include <ctype.h>	/// for isdigit()
+#include <stdlib.h>	/// for exit()
+#include <stdint.h>	/// for int8_t
+#include <assert.h>	/// for assert
+
+/**
+ * @brief array implementation of stack using structure
+ */
+struct Stack {
+	int8_t stack[20];		///< array stack
+	int top;		///< stores index of the top element
+};
+struct Stack st;		///< global declaration of stack st
+
+/**
+ * @brief Function to push on the stack
+ * @param opd number to be pushed in the stack
+ * @returns void
+ */
+void push(int8_t opd) {
+	if(st.top == 19)	{		// overflow condition
+		printf("Stack overflow...");
+		exit(1);
+	}
+	st.top++;
+	st.stack[st.top] = opd;
+}
+
+/**
+ * @brief Function to pop from the stack
+ * @returns popped number
+ */
+int8_t pop() {
+	int8_t item;				///< to store the popped value to be returned
+	if(st.top == -1) {		// underflow condition
+		printf("Stack underflow...");
+		exit(1);
+	}
+	item = st.stack[st.top];
+	st.top--;
+	return item;
+}
+
+/**
+ * @brief Function to evaluate postfix expression
+ * @param post the input postfix expression
+ * @returns evaluated answer
+ */
+int8_t evaluate(char post[]) {
+	int8_t it1;
+	int8_t it2;
+	int8_t temp;
+	int8_t number;
+    int i;
+    for(i = 0; i < strlen(post); i++) {
+		if(post[i] == ' ') {
+			continue;			// ignore delimiter
+		}
+		else if(isdigit(post[i])) {
+			number = 0;
+			do {
+				number = number * 10 + (post[i]-'0');
+				i++;
+			} while(i < strlen(post) && isdigit(post[i]));
+			push(number);
+		}
+		else {
+			it2 = pop();
+			it1 = pop();
+			switch(post[i]) {
+				case '+':
+				temp = it1 + it2; break;
+				case '-':
+				temp = it1 - it2; break;
+				case '*':
+				temp = it1 * it2; break;
+				case '/':
+				temp = it1 / it2; break;
+				case '%':
+				temp = it1 % it2; break;
+				default:
+				printf("Invalid operator"); exit(1);
+			}
+			push(temp);
+		}
+	}
+	return pop();
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    /* check sample test case
+	   input: "2 10 + 9 6 - /"
+	   expected output: 4
+	 */
+	char temp1[50] = "2 10 + 9 6 - /";
+	assert(evaluate(temp1) == 4); 			/// this ensures that the algorithm works as expected
+	/* input: "4 2 + 3 5 1 - * +"
+	   expected output: 18
+	 */
+	char temp2[50] = "4 2 + 3 5 1 - * +";
+	assert(evaluate(temp2) == 18); 			/// this ensures that the algorithm works as expected
+	printf("All tests have successfully passed!\n");
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+	st.top = -1;			/// initialize
+	test();				/// run self-test implementations
+	return 0;
+}