CODEWITHSUNDEEP
carrayssortingbubble-sort
Nir Tzezana
Nir Tzezana

Reputation: 2342

Bubble sorting an array with nulls in C

I'm trying to create a bubble sort with nulls in the middle in C.
The code works ok when the array is ordered in a way so the nulls are at the end of the array (hense the "continue" condition works).

My array looks like this: [John,David,NULL,Grace,NULL,NULL] on which I run this function:

void SortContacts(char * people[]) {
    int i,j;
    char *tempswap;
    for (i=0; i<storage-1; i++) {
        for (j=0; j<storage-i-1; j++) {
                if (people[j+1]==NULL) {
                    continue;
                }
            if (strcmp(people[j],people[j+1]) > 0) {
                tempswap = people[j];
                people[j] = people[j+1];
                people[j+1] = tempswap;
            }
        }
    }
}

When executing with the NULL in the middle of the array the exe crashes.

Upvotes: 0

Views: 1014

Answers (3)

abligh
abligh

Reputation: 25129

You cannot strcmp a NULL value. Although you are guarding against a strcmp of people[j+1] being NULL, you don't check people[j].

Try the following (untested), which simply provides a strcmp function which treats a NULL like "".

int
strcmpwithnull(const char *a, const char *b)
{
    return strcmp(a?a:"", b?b:"");
}

void SortContacts(char * people[]) {
    int i,j;
    char *tempswap;
    for (i=0; i<storage-1; i++) {
        for (j=0; j<storage-i-1; j++) {
        if (strcmpwithnull(people[j],people[j+1]) > 0) {
                tempswap = people[j];
                people[j] = people[j+1];
                people[j+1] = tempswap;
            }
        }
    }
}

If you want a NULL to be treated as greater than any other string, then try (again untested):

int
strcmpwithnull(const char *a, const char *b)
{
    if (a == b)
        return 0; /* handles 2 NULLs and two strings at the same location */
    else if (!a)
        return 1;
    else if (!b)
        return -1;
    else
        return strcmp(a, b);
}

If you want them to be less than any other string (including the empty string), swap the return 1 and return -1.

Upvotes: 6

mrk
mrk

Reputation: 3191

You want to end up with the nulls at the right of the array. This can be achieved by making nulls strictly greater than any string. You have to encode this into the comparison function.

int cmp(const char *x, const char *y)
{   if(x == 0) return 1;
    if(y == 0) return -1;
    return strcmp(x, y);
}

Upvotes: 0

Coldsteel48
Coldsteel48

Reputation: 3512

Here is the problem

            if (people[j+1]==NULL) 
            {
                continue;
            }

You need to check both j and j+1 Also consider What if you have a NULL in the head of the array ? or you have 2 NULLs one after another

So you need also to check the j not only the j+1

you should also check thejisn't null

Upvotes: 0

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