sort elements of 2D matrix by 1st column reference

Hi everyone,

I am writing a function that needs to sort the rows of a 2D matrix in ascending order.
To so, I first wrote the function to treat a 1D array and, by analogy, tried to build one that takes an array of pointers as argument, but I am not sure where I am doing that wrong, since I am not getting the sorted array in return.

I post my code below, I hope someone can help
All the best
cfd

In the following code, *array[] that is passed is a 2D matrix of "lenght" rows and 6 columns:

Code:

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void sortAscend(float *array[], int length)
{
  int i, j, temp, test;
 
        for(i = length - 1; i > 0; i--)
        {
    test=0;
            for(j = 0; j < i; j++)
            {
              if(*array[1] > *(array[1]+1)) /* compare neighboring elements */
              {
                temp = *array[1];            /* swap array[j] and array[j+1] */
                *array[1] = *(array[1]+1);
                *(array[1]+1) = temp;
                test=1;
              }
            } /*end for j*/
            if(test==0) break; /*will exit if the list is sorted!*/
        } /*end for i*/
 }

---

There are many sorting algrithms out there. If speed is not all important, try bubblesort. Else you might have a look at merge- or quicksort.

Hi,

Thanks for the hint; I will look into those then

Best
cfd

Quote:

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I'm fairly certain you should be using your i and j variables in places other than just the two lines related to the for loops

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hi, I was thinking the same, although I wanted to be using the pointer arrays instead of a double loop

Also, why am I not seeing this last reply inside the forum but only in my email? Is anything wrong with the forum?

thanks

Heh, heh... I deleted my post right before you posted yours. It's just bad timing.

Quote:

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Originally Posted by Brafil

There are many sorting algrithms out there. If speed is not all important, try bubblesort.

---

Technically this is a (modified*) bubblesort. However, there are some syntax issues. The most obvious is that you use [1] as the subscript everywhere, and constantly use * to dereference "array". In the context of your function:

array[x] refers to a row. No asterisk.
array[x][y] would refer to a row element.

You do not explain in your post what you actually intend to sort by: the total for the row, or the first element (or perhaps the second element, hence [1]?)

This:

Code:

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array[1]+1

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would be the location of array[1] plus a byte. You probably want:

Code:

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array[x+1]

---

However, this will be a problem since you start with the last element (length-1), meaning x+1 does not exist.

A very useful thing to do here would be to use some printf statements in the loop to check the value of things to see if they refer to what you think they refer to, eg:

Code:

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printf("temp = %f %f %f\n",temp[i][0],temp[i][1],temp[i][2]);fflush(stdout);

---

*by virtue of the fact that you use a flag, "test"

Quote:

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Originally Posted by MK27

You do not explain in your post what you actually intend to sort by: the total for the row, or the first element (or perhaps the second element, hence [1]?)

---

Hi MK27,

Your reply is very helpful. To answer to your commented quoted above, some details here:
Imagine that the first column of my 2D array (call it mat2d for now), contains values that must be sorted in an ascending order, and all the rows of every other columns must be movied accordingly, since each column is related to the first

ex.:

Code:

---

300  4 6 7
200 1 4 2
10  5 7 32
150 6 2 4

---

must become:

Code:

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10  5 7 32
150 6 2 4
200 1 4 2
300  4 6 7

---

meaning that the column that dictates is column 1, and all the others are moved accordingly. That is why I though of using an array of pointers instead of a 2D array

Thanks again

Quote:

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Originally Posted by cfdprogrammer

meaning that the column that dictates is column 1, and all the others are moved accordingly. That is why I though of using an array of pointers instead of a 2D array

Thanks again

---

There is nothing wrong with using pointers here. But three little notes:

• the first element (ie, "column 1") of an array is 0, not 1
• to sort by that element using your row pointers, use array[x][0], where x is the row number.
• again, do not dereference "array" inside the function by placing an asterisk in front of it (this only indicates a pointer in a variable declaration).

Hi again MK27,

Thanks for the hints; about the index 1 instead of zero it was done on purpose but I didn't realize that I didn't comment on that; I am allocating my arrays on a 1-index base instead of zero.

For everything else, I will follow your suggestion and post it again if I find some porblem

Thank you again for helping
All the best!
cfd

You can't assign rows of arrays by simply using the = operator on them:

Code:

---

*temp = array[j][1];    /* swap array[j] and array[j+1] */
                array[j][1] = array[j+1][1];
                array[j+1][1] = *temp;

---

And even if it did, what would that line up with what you did here:

Code:

---

temp = malloc((cols+1)*sizeof(float*));

---

Quzah.

Quote:

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Originally Posted by quzah

You can't assign rows of arrays by simply using the = operator on them:

Code:

---

*temp = array[j][1];    /* swap array[j] and array[j+1] */
                array[j][1] = array[j+1][1];
                array[j+1][1] = *temp;

---

And even if it did, what would that line up with what you did here:

Code:

---

temp = malloc((cols+1)*sizeof(float*));

---

Quzah.

---

Hi Quazah,
thanks for replying.

How am I supposed to assign rows of an array if I use a pointer in this way?

Best,
cfd

Code:

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for each element in the row...
    temp[ element ] = here[ element ]
    here[ element ] = there[ elemenet ]
    there[ element ] = temp[ element ]

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The same way you normally swap variables, but you need to swap each element in the row by hand.


Quzah.

Perfect, it's now solved!

thank you very much for helping

Best,
cfd