Question

program in C - Starter code below

//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 <stdio.h>
#include <stdlib.h>
#include <assert.h>

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("\n");
}

//Do not change the code in main function

int main(int argc, char *argv[])
{
   if(argc != 2)
   {
       printf("Usage: %s n\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\n", 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\n", length);
       print_list(head);
       //remember we need to free all the memory allocated from heap
       free(p);
   }
   return 0;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

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 = (node *)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;
}
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
if(*head != NULL)
{
// store the old value of head pointer
node *temp = *head;
// make the head point to next node in the list
*head = temp->next;

(*length)--;   
// return the previous head node
return temp;
}
}

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

//Do not change the code in main function

int main(int argc, char *argv[])
{
if(argc != 2)
{
printf("Usage: %s n\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\n", 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\n", length);
print_list(head);
//remember we need to free all the memory allocated from heap
free(p);
}
return 0;
}

Output:

length=1                                                                                                                         

1                                                                                                                                

length=2                                                                                                                         

2 1                                                                                                                              

length=3                                                                                                                         

3 2 1                                                                                                                            

length=4                                                                                                                         

4 3 2 1                                                                                                                          

length=5                                                                                                                         

5 4 3 2 1                                                                                                                        

length=4                                                                                                                         

4 3 2 1                                                                                                                          

length=3                                                                                                                         

3 2 1                                                                                                                            

length=2                                                                                                                         

2 1                                                                                                                              

length=1                                                                                                                         

1                                                                                                                                

length=0