Thread: compare backward ll ints

Originally Posted by laserlight

Tracing _Mike's code by inspection, it looks like it should work for digit linked lists that are not equal. Perhaps you can try this instead:

Code:

int compare(const struct integer* lhs, const struct integer* rhs)
{
    int result = 0;

    while (lhs && rhs)
    {
        if (lhs->digit < rhs->digit)
        {
            result = -1;
        }
        else if (lhs->digit > rhs->digit)
        {
            result = 1;
        }
        /* Ignore equal digits. */

        lhs = lhs->next;
        rhs = rhs->next;
    }

    if (lhs)
    {
        return 1;
    }
    else if (rhs)
    {
        return -1;
    }
    else
    {
        return result;
    }
}

Cool beans! this works perfectly.

Originally Posted by _Mike

Hmm that's weird.
Could you please give me an example of numbers which doesn't work so I can verify if I messed something up?

I didn't try anything specific. Take any two numbers and see the results for yourself.

Originally Posted by itCbitC

Take any two numbers and see the results for yourself.

I have, which is why I thought it was weird that it's not working for you
Here are the results I get from comparing 11345, 12345 & 107483 against each other.
(manually formatted to look better here on the forums, but otherwise identical to the original output)

Code:

 11345 ==  11345 :  0
 11345 ==  12345 :  1
 11345 == 107483 :  1
 12345 ==  11345 : -1
 12345 ==  12345 :  0
 12345 == 107483 :  1
107483 ==  11345 : -1
107483 ==  12345 : -1
107483 == 107483 :  0

Full source for reference:

Code:

#include <stdio.h>
#include <string.h>
#include <malloc.h>

struct integer
{
	int digit;
	struct integer* next;
};

struct integer* make_int(const char* strnum)
{
	int i, offset = 0;
	struct integer *ret, *current, *next;
	if(!strnum)
		return 0;
	while(strnum[offset] == '0')
		offset++;

	ret = malloc(sizeof(*ret));
	ret->digit = 0;
	ret->next = 0;
	next = ret;

	i = strlen(strnum + offset);
	while(i)
	{
		current = next;
		current->digit = strnum[--i + offset] - '0';
		if(i)
			next = malloc(sizeof(*next));
		else
			next = 0;
		current->next = next;
	}
	return ret;
}

void free_int(struct integer* num)
{
	struct integer* current = num;
	while(num)
	{
		current = num;
		num = num->next;
		free(current);
	}
}

void print_int(const struct integer* num)
{
	if(num)
	{
		print_int(num->next);
		putchar(num->digit + '0');
	}
}

int compare(const struct integer* l, const struct integer* r)
{
	/* comparison value */
	int comp = 0;

	/* test for NULL pointers */
	if(!l && !r) return 0;
	if(!l) return 1;
	if(!r) return -1;

	while(1)
	{
		/* if both numbers are of equal length,
		 * return the comparison value */
		if(!l && !r) return comp;

		/* test the length of the numbers */
		if(l->next && !r->next) return -1;
		if(!l->next && r->next) return 1;

		/* compare digits */
		if(l->digit < r->digit) comp = 1;
		else if(l->digit > r->digit) comp = -1;

		/* move to next digit */
		l = l->next;
		r = r->next;
	}
}

void compare_and_print(const struct integer* lhs, const struct integer* rhs)
{
	print_int(lhs); printf(" == "); print_int(rhs); printf(" : %d\n", compare(lhs, rhs));
}

int main()
{

	struct integer* num1 = make_int("11345");
	struct integer* num2 = make_int("12345");
	struct integer* num3 = make_int("107483");

	compare_and_print(num1, num1);
	compare_and_print(num1, num2);
	compare_and_print(num1, num3);
	compare_and_print(num2, num1);
	compare_and_print(num2, num2);
	compare_and_print(num2, num3);
	compare_and_print(num3, num1);
	compare_and_print(num3, num2);
	compare_and_print(num3, num3);

	free_int(num1);
	free_int(num2);
	free_int(num3);

	return 0;
}

haha, well when i posted this original question, it was just for one method, the compare. the actual program wanted to add the two linked lists and subtract them. the meaning for the compare method was that whne you subtract the two linked lists, you must subtract the smaller one from the bigger one

laserlight:
I agree that your code is easier to read and less error-prone than mine. I have never had any proper (classes, books, etc) programming training. I've basically just learned using the "monkey method", writing some code and prodding it with a stick until it moves. So while I might know quite a bit on the technical side of programming I have never really put much thought in to good code design.
But it is posts like yours that makes me learn more, so thank you