Problems with pointers: sorting algorithms in C

Question

I come from python and am trying to learn C, but I've only yesterday started with this pointer stuff. I wrote an inverted sorting algorithm that's supposed to take an unsorted array, then in each pass pick the highest and lowest elements, put them in each end then recursively do the same ignoring already sorted elements. The problem is I always get some error (different errors depending on what I try to change) which I can't fix regarding pointer types. Can you guys help?


#include <stdio.h>


void inv_cocksort(int *arr[], int first, int last);

int main(void)
{
    int unsorted[11] = {3, 1, 4, 5, 4, 2, 6, 9, 2, 8, 7};
    int unsorted_length = 11;

    for (int i = 0; i < unsorted_length; i++)
    {
        printf("%i", unsorted[i]);
    }

    inv_cocksort(&unsorted, 0, unsorted_length-1);

    for (int i = 0; i < unsorted_length; i++)
    {
        printf("%i", unsorted[i]);
    }


    void inv_cocksort(int *arr[], int first, int last)
    {
        if (first > last)
        {
            return;
        }
        else
        {
            for (int i = 0; i < last-1; i++)
            {
                if (arr[i] < arr[last])
                {
                    int temp = *arr[last];
                    *arr[i] = *arr[last];
                    *arr[last] = temp;
                }
                if (arr[i] > arr[first])
                {
                    int temp2 = *arr[first];
                    *arr[i] = *arr[first];
                    *arr[last] = temp2;
                }
            }
            inv_cocksort(&arr[], first+1, last-1)
        }

}

I think you need to change for (int i = 0; i < last-1; i++) to for (int i = first; i < last-1; i++) or similar — rtx13
– rtx13, Commented Apr 6, 2020 at 5:39
I change how I define the function; I try to define the first argument as int *unsorted[] to tell it to accept and address and pass &unsorted to it. I get errors like invalid pointer type and changed int into pointer without casting. Can't get it to work. — Lui Martinez Laskowski
– Lui Martinez Laskowski, Commented Apr 6, 2020 at 5:40

rtx13 · Accepted Answer · 2020-04-06 05:41:37Z

1

I think you need to change

for (int i = 0; i < last-1; i++)

to

for (int i = first+1; i < last-1; i++)

or similar.

answered Apr 6, 2020 at 5:41

rtx13

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1 Comment

Yunnosch Over a year ago

What makes you think so?

Jabberwocky · Accepted Answer · 2020-04-06 05:42:05Z

1

There are many issues:

You want this:

#include <stdio.h>


void inv_cocksort(int arr[], int first, int last)
{
  if (first > last)
  {
    return;
  }
  else
  {
    for (int i = 0; i < last - 1; i++)
    {
      if (arr[i] < arr[last])
      {
        int temp = arr[last];
        arr[i] = arr[last];
        arr[last] = temp;
      }
      if (arr[i] > arr[first])
      {
        int temp2 = arr[first];
        arr[i] = arr[first];
        arr[last] = temp2;
      }
    }
    inv_cocksort(arr, first + 1, last - 1);
  }
}

int main(void)
{
  int unsorted[11] = { 3, 1, 4, 5, 4, 2, 6, 9, 2, 8, 7 };
  int unsorted_length = 11;

  for (int i = 0; i < unsorted_length; i++)
  {
    printf("%i ", unsorted[i]);
  }

  inv_cocksort(unsorted, 0, unsorted_length - 1);

  for (int i = 0; i < unsorted_length; i++)
  {
    printf("%i ", unsorted[i]);
  }
}

This compiles at least without warnings, but it doesn't work correctly.

answered Apr 6, 2020 at 5:42

Jabberwocky

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1 Comment

Lui Martinez Laskowski Over a year ago

I can fix the logic later, really, just need to get it to compile in the first place

Lui Martinez Laskowski · Accepted Answer · 2020-04-06 06:18:23Z

0

Fixed it.

I had firstly added a whole bunch of starts and &s because I was afraid the function would only receive a copy of the passed values, order that copy and lose it as soon as the stack memory was no longer allocated to it. It turns out array names actually work as pointers to the first value even if the arrays are not strings, so I didn't really have to manipulate any memory at all.

Fixed the logic also, I'm sorry for uploading such a buggy code at first.

And cocksort is because it's cocktail shaker, not quite but maybe? idk.


#include <stdio.h>


void inv_cocksort(int arr[], int first, int last);

int main(void)
{
    int unsorted[11] = {3, 1, 4, 5, 4, 2, 6, 9, 2, 8, 7};
    int unsorted_length = 11;

    for (int i = 0; i < unsorted_length; i++)
    {
        printf("%i", unsorted[i]);
    }
    printf("\n");

    inv_cocksort(unsorted, 0, unsorted_length-1);

    for (int j = 0; j < unsorted_length; j++)
    {
        printf("%i", unsorted[j]);
    }
}


    void inv_cocksort(int arr[], int first, int last)
    {
        if (first > last)
        {
            return;
        }
        else
        {
            for (int i = first; i <= last; i++)
            {
                if (arr[i] < arr[last])
                {
                    int temp = arr[last];
                    arr[last] = arr[i];
                    arr[i] = temp;
                }
                if (arr[i] > arr[first])
                {
                    int temp2 = arr[first];
                    arr[first] = arr[i];
                    arr[i] = temp2;
                }
            }
            inv_cocksort(arr, first+1, last-1);
        }
    }

answered Apr 6, 2020 at 6:18

Lui Martinez Laskowski

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2 Comments

RobertS supports Monica Cellio Over a year ago

If you´re at your goal accept an answer. By the way I would not name it "cocksort" - if you want to abbreviate "cocktail shaker" then rather name it "cockt_sort". ;-)

Lui Martinez Laskowski Over a year ago

thanks for the feedback. I did upvote the answers, I didn't accept any specific one because none of them were what I was looking for, which was further clarification on how pointers and memory addresses work in C when in the context of arrays and being called as function arguments.

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