Inserting in Linked List - c

Question

I'm having a small difficulty inserting a full sentence containing all characters in my string when I'm building a Linked List.
I would like to be able to insert a string like: word_#_2003_#_definition
But when I'm running my code in my main() method it continues repeating the choices to do like it never stops asking me to enter an option. Hope it's clear.
Here's my struct:
struct node
{
char data[100];
struct node *previous; // Points to the previous node
struct node *next; // Points out to the next node
} *head, *last;
Here's my function to insert a node:
void insert_beginning(char words[99])
{
struct node *var, *temp;
var=(struct node *)malloc(sizeof(struct node)); //explination about the (node *)
strncpy(var->data, words,99);
if (head==NULL)
{
head=var;
head->previous=NULL;
head->next=NULL;
last=head;
}
else
{
temp=var;
temp->previous=NULL;
temp->next=head;
head->previous=temp;
head=temp;
}
}
And this is in my main() method:
int main()
{
char loc[99];
char words[99];
int i, dat;
head=NULL;
printf("Select the choice of operation on link list");
printf("\n1.) Insert At Begning\n2.) Insert At End\n3.) Insert At Middle");
printf("\n4.) Delete From End\n5.) Reverse The Link List\n6.) Display List\n7.)Exit");
while(1)
{
printf("\n\n Enter the choice of operation you want to do ");
scanf("%d",&i);
switch(i)
{
case 1:
{
printf("Enter a word you want to insert in the 1st node ");
scanf(" %s",words);
insert_beginning(words);
display();
break;
}
Any ideas on how to do it?

Answer 1

The code is highly questionable:
return codes are never checked. You must check return codes,
especially if you are using scanf
You need to empty the whole buffer with scanf otherwise you will continue
reading old contents for the next command
A better alternative is to use sscanf
You must initialize your variables, for instance last, but there are additional
cases of variables that do not get initialized
Your data types are not consistently defined, this will create security problems
sometimes 99 characters and sometimes 100.
insert_beginning should not return void, the memory allocation can fail

Answer 2

Your code in main() should probably look more like:
int main()
{
char loc[99];
char words[99];
int i, dat;
head = NULL;
printf("Select the choice of operation on link list");
printf("\n1.) Insert At Beginning\n2.) Insert At End\n3.) Insert At Middle");
printf("\n4.) Delete From End\n5.) Reverse The Link List\n6.) Display List\n7.) Exit\n");
while(1)
{
printf("\nEnter the choice of operation you want to do: ");
if (scanf("%d", &i) != 1)
{
fprintf(stderr, "Failed to read a number: exiting\n");
return 1;
}
switch(i)
{
case 1:
{
printf("Enter a word you want to insert in the 1st node: ");
if (scanf("%98s", words) != 1)
{
fprintf(stderr, "Failed to read words; exiting\n");
return 1;
}
insert_beginning(words);
display();
break;
}
...
}
...
}
...
}
return 0;
}
As discussed in the comments, you are not checking the return status from scanf(), so you don't know when it is failing, and when it fails to read a number, it leaves the argument (i) alone, so you go back to the same option again, and rinse and repeat.
Elementary debugging techniques (not shown):
Print the values you get from your inputs. Print the value of i after the scanf() error checking. Print the value of words after the scanf() error checking.
Step through with a debugger.
Create a function to dump (print) your key data structures:
static void dump_data_structure(FILE *fp, char const *tag, data_structure const *data)
{
...code to dump the structure to the specified file stream,
...identified by tag (so you can tell which call it is you are looking at)
}
Use the structure dumper extensively while debugging. Keep it for use later when modifying the code. If done right, it can be enormously helpful.