Question

I am working on a single linked-list calculator in C (yes, it’s homework). I have the add functions “working” but for some reason I can only add two values that are the same length. I can’t really figure out how to add something like 12 + 128. Currently my code only accepts 120 + 128. What did I do wrong, how can I fix this code?

struct digit* add(struct digit *x, struct digit *y)
{
    int carry = 0;
    struct digit *xHead;
    struct digit *yHead;
    struct digit *totalHead;
    struct digit *current_Digit;

    xHead = x;
    yHead = y;
    totalHead = NULL;


    while(x != NULL && y != NULL)
    {
        current_Digit = (struct digit *)malloc(sizeof(struct digit));
        current_Digit->value = x->value + y->value + carry;

        //calculates the carry
        carry = 0;
        if(current_Digit->value > 9)
        {
            carry = 1;
            current_Digit->value = current_Digit->value % 10;
        }
        else
        {
            carry = 0;
        }
        current_Digit->next = totalHead;
        totalHead = current_Digit;

        x = x->next;
        y = y->next;
    }
    return totalHead;
}

Answer 1

Instead of going to x->next and y->next simultaneously, your function should do the following:

while (x != NULL || y != NULL) {
    // malloc

    current_Digit->value = (x ? x->value : 0)
                         + (y ? y->value : 0)
                         + carry;

    // compute

    if (x) x = x->next;
    if (y) y = y->next;
}

(It also looks as if you’re constructing your result list backwards…)