Thread: Pointers & dereferencing confusion

The reason result is pointing to the same place is because that's what you told it to do. I'm not sure what you expect to happen, so I can't really offer a solution. I can say, however, that there are some fundamental changes I think you should make to your code.

First, read this link. Global variables are evil. Declare them in the appropriate functions and pass them into other functions as needed. Your list variables should be declared in main, your carry variable should probably be declared in add, and you don't need current.

Second, you should work on separating your list implementation from your add/subtract algorithm and your input handling. I would write a generic function called list_prepend or something, that takes a list and a number, creates a new node with that number data, and sticks that node at the beginning of the list. You're close with storeNum, but it's not generic, it doesn't work with any list. I would work on list_prepend and list_print first. Here's a hint to get you started with list_prepend:

Code:

struct linked_list *list_prepend(struct linked_list *list, int num)
{
    allocate new node
    populate data with num
    set new node's next pointer to list
    return new node
}
...
// in main
struct linked_list *first_num = NULL;  // initialize to NULL for empty list
char c;
first_num = list_prepend(first_num, c - '0');

Note, it takes an int, not char. Do the conversion from char to int before calling list_prepend. This will allow result to use list_prepend as well since it uses the data elements from the linked_list nodes, not chars from user input.

I know restarting from scratch sounds scary, but believe me, your code will be much simpler this way, less error prone, and easier to debug if need be. A good design makes coding and testing go much more quickly.

You should also be testing frequently, and never move on to the next part until the part you are currently working on is error-free. Write 10-20 lines, then test, write 10-20 lines, then test, ad infinitum.

Hopefully that helps.

I think if there is a carry left over at the end, you have to allocate another new_item_ptr and insert that at the head of the list.

Your storeNum and printList look great. A few small things to clear up first:

Here's what happens when I compile your program:

Code:

$ gcc -Wall -g -std=c99  list.c   -o list
list.c: In function ‘storeNum’:
list.c:15: warning: assignment from incompatible pointer type
list.c:16: warning: assignment from incompatible pointer type
list.c: In function ‘main’:
list.c:69: warning: implicit declaration of function ‘isdigit’
list.c:71: warning: implicit declaration of function ‘strlen’
list.c:71: warning: incompatible implicit declaration of built-in function ‘strlen’
list.c:72: warning: passing argument 1 of ‘storeNum’ from incompatible pointer type
list.c:10: note: expected ‘struct list *’ but argument is of type ‘struct linked_list *’
list.c:72: warning: assignment from incompatible pointer type
list.c:74: warning: incompatible implicit declaration of built-in function ‘strlen’
list.c:75: warning: passing argument 1 of ‘storeNum’ from incompatible pointer type
list.c:10: note: expected ‘struct list *’ but argument is of type ‘struct linked_list *’
list.c:75: warning: assignment from incompatible pointer type
list.c:83: warning: implicit declaration of function ‘strncmp’

If you don't a list of warnings like this, then turn up the warnings on your compiler.

• Lines 10, 15, 16, 72, 75: All these are because you used 'struct list' on line 10 instead of 'struct linked_list'
• Line 69: You need to #include <ctype.h>
• Line 71, 74, 83: You need to #include <string.h>

I fixed that and ran your program with the given input. Here's what I got:

Code:

$ ./list
Enter a command: 67+89
First number:
{ 9 8 -5 7 6 }
Second number:
{ }

Your add doesn't work because you incorrectly build first_num and second_num. The problems are happening in main, not in add. You can tell because the lists are printed before add is called, so that's not where the problem is. Think about your parsing. Right now, you fill first_num with everything from the user input, from index 0 to length-1. You need to stop adding things to first_num when you get a '+' or '-'. Then, store the + or - for later, and read all the subsequent digits into second num. Again, there is no need for your 'current' variable. Something like:

Code:

i = 0
length = strlen(input)  // this just saves us from calling strlen a bunch of times
while i < length and input[i] is a digit
    add it to first_num
    i++
if i < length and input[i] is a '+' or a '-'
    op = input[i]
    i++
while i < length and input[i] is a digit
    add input[i] to second_num
    i++

if op is a '+'
    result = add(first_num, second_num)
else if op is a '-'
    result = subtract(first_num, second_num)
print result

result shouldn't be global. You can have the list built inside add, using storeNum (which is why I suggested you rewrite it), and simply return the pointer to the head of the list. This will also avoid any strange pointer issues. Lastly, you are making your computations overly complex. There's no need for sum_nc or your if (sum>9) bit. Try:

Code:

struct linked_list *add(struct linked_list *first_num, struct linked_list *second_num)
{
    struct linked_list result = NULL; // local list, initialized to NULL
    ...
    sum = temp1->data + temp2->data + carry;
    result = storeNum(result, sum % 10);
    carry = sum / 10;
    ...
    return result;
}

Originally Posted by anduril462

I fixed that and ran your program with the given input. Here's what I got:

Code:

$ ./list
Enter a command: 67+89
First number:
{ 9 8 -5 7 6 }
Second number:
{ }

Your add doesn't work because you incorrectly build first_num and second_num.

The assignment I was given instructed me to follow the following sample format:

Code:

$./list
Enter a command: 67

Enter a command: 89

Enter a command: +
67 + 89 = 156

Oh, okay. Well, I still recommend ditching your 'current' variable. Just ask for the input 3 times:

Code:

do {
    print "Enter a command: "
    read line
    add digits to first_num

    print "Enter a command: "
    read line
    add digits to second_num

    print "Enter a command: "
    read line
    if it's a "+"
        result = add(first_num, second_num)

    print result
} while (!done);

You'll have to figure out exactly what conditions to end your loop on, but that's the general idea. It should help with a few of the problems you're having.

Thanks anduril for all your help... I'm just about done I think but I have one last thing tripping me up.

Code:

void printList(struct linked_list* list)
{	struct linked_list *temp = list;
	int len = *(&temp->index);
	int arr[len];
	int i = 1;
	while(temp!=NULL){
		arr[len-i] = *(&temp->data);
		temp = temp->next_ptr;
		++i;
	}
	printf("{ ");
	for(int i=0; i<len; i++)
		printf("%d",arr[i]);
	printf("}\n");
}

When I step through printList in gdb, it seems as though my array is not being modified, despite my attempts to assign temp->data to it.. I also tried declaring arr using int* arr = malloc(len*sizeof(int)), but same result.

I don't know with out seeing your whole code. You've made some changes (notably adding an index field), so I'm not sure you're building your list with correct index values. Run through your code in a debugger and step through each line of printList one by one.

Note that the algorithm I gave you results in first_num and second_num being stored in reverse, but result will be stored forward. You should probably leave printList as it was, and write a second function, printListReverse to print it backwards. There is a very easy recursive solution to printListReverse if you want to try that way.