Using selection sort with array of pointers

Question

I am currently trying to sort an array of pointers using selection sort, but with no success. I need to sort an array of int values with only pointers. I have already stored values in the array, and only thing left to do is to sort these values.

Here is my code:

void fill_rand_ver2(int *pointer, int size)
{
int randNbr;
int *i;

for(i = pointer; i < (pointer + size); i++)
{
    randNbr = (rand()%10)+1;
    *i = randNbr;
}
}

// sort array with help of selection sort using pointer
void sort_ver2(int *pointer, int size)
{
int *i, *j, swap;

for(i = pointer; i < (pointer + size); i++)
{
    for(j = i + 1; j < (pointer + size); j++)
    {
        if(*j < *i)
        {
            swap = *i;
            *i = *j;
            *j = swap;
        }
    }
}
}

// print out both arrays using pointer
void print_values_ver2(char *string, int *pointer, int size)
{
int *i;

printf("%s", string);
printf("\n");

for(i = pointer; i < (pointer + size); i++)
{
    printf("%d,", *i);
}
printf("\n\n");
}

When I run the program and print data, the entered values are not sorted and remain at the same place. I would appreciate any help given.

Hm, I believe you want to sort by value, not address, so it should be if(*j < *i) instead of if(j < i) — KBart
– KBart, Commented Oct 22, 2013 at 10:29
@KBart post that as an answer and you have my up-vote. And note to the OP, the reason nothing is done is simple: j is always greater than i). Also (j < i+size) is not correct in the inner for-condition. It will send j out of range and into undefined behavior if the pointer sequence is indeed only size elements wide. — WhozCraig
– WhozCraig, Commented Oct 22, 2013 at 10:31
@WhozCraig Yes, thanks, I'll edit my answer to note about incorrect range too. — KBart
– KBart, Commented Oct 22, 2013 at 10:35

KBart · Accepted Answer · 2013-10-22 10:57:26Z

1

I believe you want to sort by value, not by address, so it should be:

void sort_ver2(int *pointer, int size)
{
    int *i, *j, swap;
    int *end = NULL;

    if(size < 2 || pointer == NULL)
        return;

    end = pointer + size - 1;

    for(i = pointer; i < end; i++)
    {
        for(j = i + 1; j <= end; j++)
        {
            if(*j < *i)
            {
                swap = *i;
                *i = *j;
                *j = swap;
            }
        }
    }
}

Even if you would like to sort by address, your if statement is incorrect, as j is always greater than i in your code.

EDIT: added end variable to address issue with incorrect range of the inner for loop. Credits to @WhozCraig.

edited Oct 22, 2013 at 10:57

answered Oct 22, 2013 at 10:34

KBart

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5 Comments

Wooolie Over a year ago

I want to sort by value, and i tried your method. But it still doesnt seem to sort. How do you suggest I change my if statement?

KBart Over a year ago

What you mean by "doesn't sort"? Do you get runtime error? If no, then show your main(), might be a problem with the rest of your code.

Wooolie Over a year ago

This almost worked. Except for one thing, the first output is not sorted, it writes something like -858993460. Run-Time Check Failure #2 - Stack around the variable 'array' was corrupted.

KBart Over a year ago

@Wooolie, it depends on size parameter you pass, I guess it's a number of elements you use, so check it now.

Wooolie Over a year ago

Yes, instead of j <= end, I used only j < end. And it worked fine. Thank you !

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