Sorting problem?

Code:

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#include <stdio.h>
#include <stdlib.h>

typedef struct {
  int id;
  const char *data;
} Test;

int main() {
  Test **array = malloc(3 * sizeof(Test *));

  (*(array)) = malloc(sizeof(Test));
  (*(array))->id = 3;
  (*(array))->data = "3";

  (*(array + 1)) = malloc(sizeof(Test));
  (*(array + 1))->id = 2;
  (*(array + 1))->data = "2";
 
  (*(array + 2)) = malloc(sizeof(Test));
  (*(array + 2))->id = 1;
  (*(array + 2))->data = "1";

  int i;
  for(i=0; i<3; i++) {
    printf("%s ", (*(array + i))->data);
  }
 
  for(i=1; i<3; i++) {
    int j;
    for(j=i; j>-1; j--) {
      if(((*(array + i))->id) < ((*(array + j))->id)) {
        Test *tmp = *(array + i);
        *(array + i) = *(array + j);
        *(array + j) = tmp;
      }
    }
  }

  printf("\n");

  for(i=0; i<3; i++) {
    printf("%s ", (*(array + i))->data);
  }

  return 0;
}

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Can anyone guess?
Is there something wrong with its algorithm or code?

The output should be: 1 2 3
But it is: 2 1 3

?

You seem to malloc more than needed.

First, what's wrong with:

Code:

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Test array[3];

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If, for some reason you need to allocate the array dynamically, why not

Code:

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Test* array = malloc( 3 * sizeof(Test) );

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The output also seems to indicate that the array is not sorted correctly. Look up bubble sort and don't try to be too clever (bubble-sort compares items that are next to each other and runs over the array (or part of it) as many times as needed.

In your sorting loop, it looks like you're starting at element 1, not zero, so it doesn't get sorted.

I'd recommend using this for your loops:

Code:

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for(i = 0; i < 3 - 1; i++)  {
  for(j = i + 1; j < 3; j++)  {
      if(...
      //do all your comparisons and swaps as needed

  }
}

//this is selection sort (very close to a bubble sort), and guaranteed to work right
//if your comparison and swaps are right.

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Quote:

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Originally Posted by anon

You seem to malloc more than needed.

First, what's wrong with:

Code:

---

Test array[3];

---

If, for some reason you need to allocate the array dynamically, why not

Code:

---

Test* array = malloc( 3 * sizeof(Test) );

---

The output also seems to indicate that the array is not sorted correctly. Look up bubble sort and don't try to be too clever (bubble-sort compares items that are next to each other and runs over the array (or part of it) as many times as needed.

---

Quote:

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Originally Posted by Adak

In your sorting loop, it looks like you're starting at element 1, not zero, so it doesn't get sorted.

I'd recommend using this for your loops:

Code:

---

for(i = 0; i < 3 - 1; i++)  {
  for(j = i + 1; j < 3; j++)  {
      if(...
      //do all your comparisons and swaps as needed

  }
}

//this is selection sort (very close to a bubble sort), and guaranteed to work right
//if your comparison and swaps are right.

---

---

So, the problem was its algorithm :D thanks Adak.

anon: I'm practicing my pointer logic. It seems that people in sf.net love using pointers so that I can't read their source code :D Thanks for reply anyway.

Quote:

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Originally Posted by audinue

Ugly C Lover

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Confirmed:

Quote:

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Originally Posted by audinue

Code:

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  (*(array + 1)) = malloc(sizeof(Test));

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Most people prefer "array[1] = ...";

Quote:

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Originally Posted by iMalc

Confirmed:

Most people prefer "array[1] = ...";

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Yay, thats too C :D