Sequence insert at arbitrary index implemented with Linked list - c

Question

I'm trying to implement sequence_insert_at using the add_to_front function here
Everything before
typedef struct sequence *Sequence;
is pasted from another c file.
void sequence_insert_at(Sequence s, int pos, int item)
{
struct node* temp = s->lst;
for(; pos > 0; --pos)
{
temp = temp->rest;
}
add_to_front(&temp, item);
++s->length;
if(!temp->rest)
{
s->end = temp;
}
//s->lst = temp;
}
I don't know why I keep getting a runtime error. if I clone s->lst and traverse the clone, I'm not modifying the pointer to the node in s, but if I change temp, s->lst should have the reflected changes since the nodes are all linked still. Any ideas as to how to fix this? I tried creating another node that is one before the temp after traversal, and then setting it->rest = temp, but that failed as well.

Answer 1

following mistakes a could spot but only so far to get the main function run
new_sequence does not initialize anything in Sequence it creates. lst is not initialized when you access it in sequence_insert_at
struct node* temp = s->lst;
here how it should look like
Sequence new_sequence()
{
Sequence s = malloc(sizeof(struct sequence));
if(!s)
{
printf("Out of memory. Can't allocate s\n");
exit(EXIT_FAILURE);
}
s->lst = malloc(sizeof(struct node));
if(! s->lst) {
printf("Out of memory. Can't allocate lst\n");
}
s->lst->rest = NULL;
s->length = 0;
return s;
}
also s->lst->rest has to be set to NULL, this is what tells that the list has no more elements an not end witch turns obsolete.
struct sequence
{
struct node* lst;
int length;
};
You should be passing the sequence itself to your functions not a pointer to some internal data in the sequence.
add_to_front(&temp, item);
Your sequence_insert_at function should be the one that can handle any position not add_to_front() so it is easier to call with the position 0 from add_to_front() and your having the the hole work done in one function, not a half here and a half there.
void sequence_insert_at(Sequence s, int pos, int item)
{
if(s && pos <= s->length) {
print_sequence(s);
struct node *newnode = malloc(sizeof(struct node));
if (newnode == NULL) {
printf("ERROR! add_to_front ran out of memory!\n");
exit(EXIT_FAILURE);
}
newnode->first = item;
struct node* temp = s->lst;
struct node* prv = NULL;
for(int i = 0; i < pos; i++) {
printf("skip %d\n", temp->first);
prv = temp;
temp = temp->rest;
}
newnode->rest = temp;
if(pos == 0) {
printf("insert as first\n");
s->lst = newnode;
} else {
printf("insert before %d\n", temp->first);
prv->rest = newnode;
}
++s->length;
}
}
and in add_to_front only one statement is needed
void add_to_front(Sequence s, int item) {
sequence_insert_at(s, 0, item);
}
as for inserting at the back of the list
void add_to_back(Sequence s, int item) {
sequence_insert_at(s, s->length, item);
}
A small test with the main function
void print_sequence(Sequence s)
{
struct node* temp = s->lst;
for(int i = 0; i < s->length; temp = temp->rest) {
printf("%d ", temp->first);
i++;
}
printf("\n");
}
int main()
{
Sequence derp = new_sequence();
sequence_insert_at(derp, 0, 14);
add_to_front(derp, 16);
sequence_insert_at(derp, 0, 17);
sequence_insert_at(derp, 2, 15);
add_to_back(derp, 13);
print_sequence(derp);
delete_sequence(derp);
return 0;
}
output is:
17 16 15 14 13
You'll have to go trough the other functions and fix them.
Finally i should note that variable names you have choosen are little bit confusing if not misleading, i would name them this way
typedef struct node {
int data; /* the data that a node holds */
struct node* next; /* the pointer to the next node */
} Node_t;
typedef struct sequence {
struct node* head; /* head or first element of the sequence/list */
int length; /* length is ok but size is better */
} Sequence_t;