//In this assignment, we practice call by reference.

//Below description of call by reference is from the following link //https://www.tutorialspoint.com/cprogramming/c_function_call_by_reference.htm

//The call by reference method of passing arguments to a function copies //the address of an argument into the formal parameter. Inside the function, //the address is used to access the actual argument used in the call. //It means the changes made to the parameter affect the passed argument.

//We use an example of linked list for this purpose. //In the example, an implementation of how to add a node to the head of the list //is given. We need to implement how to remove a node from the head of the list. //Both of the functions should keep track of the length of the list. //If we successuflly added a node to the list, the length should be incremented by one. //Also, if we successfully rmoeved a node from the list, the length should be decrementd //by one.

#include #include #include

typedef struct node_tag { int v; // data struct node_tag * next; // A pointer to this type of struct } node; // Define a type. Easier to use.

node * create_node(int v) { node * p = malloc(sizeof(node)); // Allocate memory assert(p != NULL); // you can be nicer

// Set the value in the node. p->v = v; p->next = NULL; return p; // return }

//This function show us how to add a new node to the font of a linked list, //at the same time, how to update the length of the list. //Note the ** in front of head //Note the * in front of length //Think why this is call by reference for a point and an integer void add_first(node **head, node *newnode, int *length) { if(*head == NULL) { *head = newnode; newnode->next = NULL; } else { newnode->next = *head; *head = newnode; } (*length) ++; }

//Now we need to implement the remove_first function node * remove_first(node **head, int *length) { //Add your code below

}

void print_list(node *head) { while(head!=NULL) { printf("%d ", head->v); head = head->next; } printf(" "); }

//Do not change the code in main function

int main(int argc, char *argv[]) { if(argc != 2) { printf("Usage: %s n ", argv[0]); return -1; } int n = atoi(argv[1]); assert(n>=1 && n<=25); node *head, *p; int length = 0; //Do not forget the initilization of head head = NULL;

for(int i=1; i<=n; i++) { p = create_node(i); //Note how we call by reference for head and length //Note the & in front of head and length add_first(&head, p, &length); printf("length=%d ", length); print_list(head); } while(head!=NULL) { //Note how we call by reference for head and length //Note the & in front of head and length p = remove_first(&head, &length); printf("length=%d ", length); print_list(head); //remember we need to free all the memory allocated from heap free(p); } return 0; }