Refactored the code into multiple files and added comments

Makefile

@@ -1,10 +1,27 @@
+# CC=gcc
+# CFLAGS=-Wall
+# EXECUTABLE=list
+# list: linkedlist.o main.c
+# 	$(CC) $(CFLAGS) linkedlist.o main.c -o $(EXECUTABLE)
+# linkedlist.o: linkedlist.h 
+# clean:
+# 	rm -rf *.o *.gch $(EXECUTABLE)
+# leaks:
+# 	valgrind --leak-check=yes ./$(EXECUTABLE)
+
 CC=gcc
 CFLAGS=-Wall
 EXECUTABLE=list
-list: linkedlist.o main.c
-	$(CC) $(CFLAGS) linkedlist.o main.c -o $(EXECUTABLE)
-linkedlist.o: linkedlist.h 
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $<
+
+list: linkedlist.o main.o
+	$(CC) $(CFLAGS) linkedlist.o main.o -o $@
+
+# linkedlist.o: linkedlist.h 
+
 clean:
-	rm -rf *.o *.gch $(EXECUTABLE)
+	rm -rf *.o $(EXECUTABLE)
 leaks:
-	valgrind --leak-check=yes ./$(EXECUTABLE)
+	valgrind --leak-check=yes ./$(EXECUTABLE)

linkedlist.c

@@ -1,17 +1,16 @@
 #include <stdio.h>
 #include <stdlib.h>
-#include "linkedlist.h"
 
-struct node {
-  int data;
-  struct node * next;
-};
+#ifndef LINKED_STRUCT_HEADER
+    #define LINKED_STRUCT_HEADER
+    #include "linkedlist_struct.h"
+#endif
 
-struct list {
-  Node * head; 
-};
+#ifndef LINKED_HEADER
+    #define LINKED_HEADER
+    #include "linkedlist.h"
+#endif
 
-Node * createnode(int data);
 
 Node * createnode(int data){
   Node * newNode = malloc(sizeof(Node));
@@ -32,22 +31,32 @@ List * makelist(){
   return list;
 }
 
+/**
+ * This function displays the elements of the linked list in order from head to tail.
+ * Example: If a linked list contains the elements 1,2,3 in this order with 1 at the head then
+ * the display function will display the linked list as: 1 -> 2 -> 3 ->
+ * 
+ * The -> after 3 points to nothing to indicate that it is pointing to NULL
+ * 
+ * The function does not print anything in case the linked list is empty.
+ * */
 void display(List * list) {
-  Node * current = list->head;
-  if(list->head == NULL) 
-    return;
-
-  for(; current != NULL; current = current->next) {
-    printf("%d\n", current->data);
+  for(Node * current = list->head; current != NULL; current = current->next) {
+    printf("%d -> ", current->data);
   }
 }
 
+
+/**
+ * This functions adds an element at the end of the linked list.
+ * */ 
 void add(int data, List * list){
-  Node * current = NULL;
+  // If the linked list is empty
   if(list->head == NULL){
     list->head = createnode(data);
   }
-  else {
+  else { // If there are elements present in the linked list
+    Node * current = NULL;
     current = list->head; 
     while (current->next!=NULL){
       current = current->next;
@@ -56,42 +65,80 @@ void add(int data, List * list){
   }
 }
 
+
+// void delete(int data, List * list){
+//   Node * current = list->head;            
+//   Node * previous = current;           
+//   while(current != NULL){           
+//     if(current->data == data){      
+//       previous->next = current->next;
+//       if(current == list->head)
+//         list->head = current->next;
+//       free(current);
+//       return;
+//     }                               
+//     previous = current;             
+//     current = current->next;        
+//   }                                 
+// }  
+
+
+/**
+ * This function deletes the first occurrence of the node containing the data value in it.
+ * NOTE: This function only uses a single pointer for the deletion.
+ * */
 void delete(int data, List * list){
-  Node * current = list->head;            
-  Node * previous = current;           
-  while(current != NULL){           
-    if(current->data == data){      
-      previous->next = current->next;
-      if(current == list->head)
-        list->head = current->next;
-      free(current);
-      return;
-    }                               
-    previous = current;             
-    current = current->next;        
-  }                                 
-}                                   
+  Node * finder = list->head;  // Finder will be used to find the first occurrence of the node containing the value data
+
+  while(finder != NULL){
+    if(finder->data == data){
+      Node * temp = finder->next;  // temp points to the next node
 
+      if(temp != NULL){  // If the node to delete is the first node or somewhere in-between in the linked list
+        finder->data = temp->data;  // Copying the data from the next node to the current node
+        finder->next = temp->next;  // Setting the next of the current node to the next value of the next node
+        free(temp);
+        temp = NULL;  // To avoid dangling pointers
+      } else {  // If the node to delete is the last node in the linked list
+        free(finder);
+        finder = NULL;  // To avoid dangling pointers
+      }
+    }
+    finder = finder->next;
+  }
+}
+
+
+/**
+ * This function reverses the given linked list.
+ * The function does not do anything if the linked list passed in as the argument is empty.
+ * */
 void reverse(List * list){
-  Node * reversed = NULL;
+  Node * previous = NULL;
   Node * current = list->head;
   Node * temp = NULL;
   while(current != NULL){
-    temp = current;
-    current = current->next;
-    temp->next = reversed;
-    reversed = temp;
+    temp = current->next;
+    current->next = previous;
+    previous = current;
+    current = temp;
   }
-  list->head = reversed;
+  list->head = previous;
 }
 
+
+/**
+ * This function destroys the linked list passed in as the argument.
+ * */
 void destroy(List * list){
   Node * current = list->head;
-  Node * next = current;
+  Node * next = NULL;
+
   while(current != NULL){
     next = current->next;
     free(current);
     current = next;
   }
   free(list);
+  list = NULL;  // To avoid dangling pointers
 }

linkedlist.h

@@ -1,11 +1,13 @@
 #ifndef LINKEDLIST_HEADER
 #define LINKEDLIST_HEADER
 
-typedef struct node Node;
-
-typedef struct list List;
+#ifndef LINKED_STRUCT_HEADER
+    #define LINKED_STRUCT_HEADER
+    #include "linkedlist_struct.h"
+#endif
 
 List * makelist();
+Node * createnode(int data);
 void add(int data, List * list);
 void delete(int data, List * list);
 void display(List * list);

linkedlist_struct.h

@@ -0,0 +1,8 @@
+typedef struct node {
+  int data;
+  struct node * next;
+} Node;
+
+typedef struct list {
+  Node * head; 
+} List;

main.c

@@ -1,6 +1,11 @@
 #include <stdio.h>
 #include <stdlib.h>
-#include "linkedlist.h"
+#include <stddef.h>
+
+#ifndef LINKED_HEADER
+    #define LINKED_HEADER
+    #include "linkedlist.h"
+#endif
 
 int main(void){
   List * list = makelist();