feat: add infix to postfix converter algorithm (#869)

* add infix to postfix converter algorithm

* docs: documentation changes

* docs: documentation changes

* updating DIRECTORY.md

* docs: documentation changes

* fix: continuous integration

* [test, docs]: add test case, documentation changes

* docs: documentation changes

* fix: continuous integration

* docs: documentation changes

* docs: documentation changes

* test: add new test

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>

DIRECTORY.md

@@ -27,6 +27,7 @@
   * [Hexadecimal To Octal](https://github.com/TheAlgorithms/C/blob/master/conversions/hexadecimal_to_octal.c)
   * [Hexadecimal To Octal2](https://github.com/TheAlgorithms/C/blob/master/conversions/hexadecimal_to_octal2.c)
   * [Infix To Postfix](https://github.com/TheAlgorithms/C/blob/master/conversions/infix_to_postfix.c)
+  * [Infix To Postfix2](https://github.com/TheAlgorithms/C/blob/master/conversions/infix_to_postfix2.c)
   * [Int To String](https://github.com/TheAlgorithms/C/blob/master/conversions/int_to_string.c)
   * [Octal To Binary](https://github.com/TheAlgorithms/C/blob/master/conversions/octal_to_binary.c)
   * [Octal To Decimal](https://github.com/TheAlgorithms/C/blob/master/conversions/octal_to_decimal.c)

conversions/infix_to_postfix2.c

@@ -0,0 +1,164 @@
+/**
+ * @file
+ * @brief [Infix to Postfix converter](https://www.includehelp.com/c/infix-to-postfix-conversion-using-stack-with-c-program.aspx) implementation
+ * @details
+ * The input infix expression is of type string upto 24 characters.
+ * Supported operations- '+', '-', '/', '*', '%'
+ * @author [Kumar Yash](https://github.com/kumaryash18)
+ * @see infix_to_postfix.c
+ */
+ 
+#include <stdio.h>	/// for IO operations
+#include <string.h>	/// for strlen(), strcmp()
+#include <ctype.h>	/// for isalnum()
+#include <stdlib.h>	/// for exit()
+#include <stdint.h>	/// for uint16_t, int16_t
+#include <assert.h>	/// for assert
+
+/**
+ * @brief array implementation of stack using structure
+ */
+struct Stack {
+	char stack[10];		///< 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 character to be pushed in the stack
+ * @returns void
+ */
+void push(char opd) {
+	if(st.top == 9)	{		// overflow condition
+		printf("Stack overflow...");
+		exit(1);
+	}
+	st.top++;
+	st.stack[st.top] = opd;
+}
+
+/**
+ * @brief Function to pop from the stack
+ * @returns popped character
+ */
+char pop() {
+	char 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 check whether the stack is empty or not
+ * @returns 1 if the stack IS empty
+ * @returns 0 if the stack is NOT empty
+ */
+uint16_t isEmpty() {
+	if(st.top == -1) {
+		return 1;
+	}
+	return 0;
+}
+
+/**
+ * @brief Function to get top of the stack
+ * @returns top of stack
+ */
+char Top() {
+	return st.stack[st.top];
+}
+
+/**
+ * @brief Function to check priority of operators
+ * @param opr operator whose priority is to be checked
+ * @returns 0 if operator is '+' or '-'
+ * @returns 1 if operator is '/' or '*' or '%'
+ * @returns -1 otherwise
+ */
+int16_t priority(char opr) {
+	if(opr == '+' || opr == '-') {
+		return 0;
+	}
+	else if(opr == '/' || opr == '*' || opr == '%') {
+		return 1;
+	}
+	else {
+		return -1;
+	}
+}
+
+/**
+ * @brief Function to convert infix expression to postfix expression
+ * @param inf the input infix expression
+ * @returns output postfix expression
+ */
+char *convert(char inf[]) {
+	static char post[25];				///< to store the postfix expression
+	int i;								///< loop iterator
+	int j = 0;							///< keeps track of end of postfix string
+	for(i = 0; i < strlen(inf); i++) {
+		if(isalnum(inf[i]))	{			// if scanned element is an alphabet or number
+			post[j] = inf[i];			// append in postfix expression
+			j++;
+		}
+		else if(inf[i] == '(') {		// if scanned element is opening parentheses
+			push(inf[i]);				// push on stack.
+		}
+		else if(inf[i] == ')') {		// if scanned element is closing parentheses,
+			while(Top() != '(') {		// pop elements from stack and append in postfix expression
+				post[j] = pop();		// until opening parentheses becomes top.
+				j++;
+			}
+			pop();						// pop opening parentheses
+		}
+		else {							// if scanned element is an operator
+			while( (!isEmpty()) && (priority(inf[i]) <= priority(Top())) ) {	// pop and append until stack becomes
+				post[j] = pop();												// empty or priority of top operator
+				j++;															// becomes smaller than scanned operator
+			}																	// '(' has priority -1
+			push(inf[i]);				// push the scanned operator
+		}
+	}
+	while(!isEmpty()) {					// pop and append residual operators from stack
+		post[j] = pop();
+		j++;
+	}
+	post[j] = '\0';						// end postfix string with null character
+	return post;
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    /* check sample test case
+	   input- "(A/(B-C)*D+E)"
+	   expected output- "ABC-/D*E+"
+	 */
+	assert(strcmp(convert("(A/(B-C)*D+E)"), "ABC-/D*E+") == 0); 			/// this ensures that the algorithm works as expected
+	/* input- "7-(2*3+5)*(8-4/2)"
+	   expected output- "723*5+842/-*-"
+	 */
+	assert(strcmp(convert("7-(2*3+5)*(8-4/2)"), "723*5+842/-*-") == 0); 			/// 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
+	char inf[25];			///< to store input infix expression
+	printf("Enter infix: ");
+	scanf("%s", inf);
+	printf("Postfix: %s", convert(inf));
+	return 0;
+}