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Author: skjha1

Stack/04 infix to postfix.cpp

@@ -0,0 +1,106 @@
+#include<stdio.h>
+#include<stdlib.h>
+#include<string.h>
+struct Node
+{
+	int data;
+	struct Node* next;
+}*top=NULL;
+
+void push(char x)
+{
+	struct Node *t;
+	t = (struct Node*)malloc(sizeof(struct Node));
+	if (t == NULL)
+	{
+		printf("Stack overflow");
+	}
+	else
+	{
+		t->data = x;
+		t->next = top;
+		top = t;
+	}
+
+}
+int pop()
+{
+	struct Node* p;
+	int x = -1;
+	if (top == NULL)
+	{
+		printf("Stack underflow");
+	}
+	else
+	{
+		p = top;
+		top = top->next;
+		x = p->data;
+		free(p);
+		
+	}
+	return x;
+
+}
+/********** Function to return precedence of the operators ********/
+int precedence(char x)
+{
+	if (x == '+' || x == '-')					//if character (ie operator) is + or - return precedence as 1
+		return 1;
+	else if (x == '*' || x == '/')				//if character is * or / return precedence as 2
+		return 2;
+	else										//else return precedence as 0
+		return 0;
+}
+
+/********** Function to check if a character in infix notation is an operand or not **********/
+int isOperand(char x)
+{
+	if (x == '+' || x == '-' || x == '*' || x == '/')		//if character is not an operand return false as 0
+		return 0;
+	else													//if character is an operand return true as 1
+		return 1;
+}
+
+/****** Infix to Postfix Function*******/
+char* InfixToPostfix(char* infix)								//function which takes pointer to a character array as parameter and returns pointer to a character array
+{
+	int i = 0, j = 0;												//i and j initialised with 0 which will help in accessing and scanning the character array
+	char* postfix;												//pointer to a character array //it will hold the postfix notation created by the function
+	int len = strlen(infix);									//len is assigned with the string length of the infix array that has been passed as an argument //it is done to allocate same size of memory for postfix array in the HEAP
+	postfix = (char*)malloc((len + 1) * sizeof(char));			//memory is allocated for the postfix array inside the HEAP of size equal to (len+1) where len is string length of the infix array	// +1 has been added to len to store '\0' to make it a string
+
+	while (infix[i] != '\0')										//Using while loop instead of for loop as i is not incrementing continuosly after every loop
+	{
+		if (isOperand(infix[i]))									//if the character in the infix array is an operand it must be copied directly into the postfix array
+			postfix[j++] = infix[i++];							//postfix copies character of infix //i and j both are incremented
+		else
+		{
+			if (precedence(infix[i]) > precedence(top->data))	//if precedence of the character of infix array is greater than the precedence of the character in the top of the stack		//CAUTION: as there is a chance of the top of stack getting NULL and to avoid accessing NULL->data (error), '#' is pushed in the stack before hand to avoid this problem
+				push(infix[i++]);								//push the character of the infix array inside the stack //i is incremented
+			else												//if precedence of the character of infix array is less or equal than the precedence of the character in the top of the stack
+			{
+				postfix[j++] = pop();							//pop the character from the stack and copy the popped character in the postfix array //j is incremented
+			}
+		}
+	}
+
+	//If any operators are left inside the stack, all must pe popped out and copied into the postfix array
+	while (top->data != '#')									//Terminating condition for popping //'#' has been used to avoid the error of NULL->data (error)
+		postfix[j++] = pop();								//characters are popped out of the stack one by one and copied in the postfix array
+
+	postfix[j] = '\0';										//last index of postfix array is assigned with '\0' to make it a complete string
+
+	return postfix;                                         //postfix array is returned as char *
+}
+
+int main()
+{
+	char infix [] = "a+b*c-d/e";   						     //infix expression
+	push('#');												//'#' is pushed inside stack before hand to avoid the error of accessing NULL->data (error)
+
+	char* postfix = InfixToPostfix(infix);					//calling the function for conversion
+
+	printf("%s", postfix);
+	return 0;
+}