Need help in writing a code (assignment)

I need to write a code (in C99). The programe receives an input number (integer) 'n' and then a matrix sized n*n (matrix[n][n]), all numbers are integers. We're supposed to define the matrix's size with malloc.
Now the program needs to sort the lines of the matrix (the number in each line), in a single function (!) this way:
even line index (0,2,4,6...): from small to big.
odds line index (1,3,5...): from big to small.
and then print it.
*note: first line is indexed 0, second line is 1, etc.

I was thinking to sort it with bubblesort function with the following if:
if(i%2==1)
do odds sorting.
else
do even sorting.
when i is the index of the row.

my problem is defining the malloc and how do I send the matrix to sorting.
please help :(

If needed I will attach my current (not so good (completly awful)) code and functions as well as an example of what the prog. supposed to do.

*sorry for grammer.

Quote:

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Originally Posted by Akruze

If needed I will attach my current (not so good (completly awful)) code and functions as well as an example of what the prog. supposed to do.

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Do that, plz. Don't forget to use code tags.

Need help in writing a code (assignment) - Dev Shed

Quote:

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Originally Posted by GReaper

Do that, plz. Don't forget to use code tags.

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Attachment 13305

Code:

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void swap(int *p, int *q){
    int temp=*p;
    *p=*q;
    *q=temp;
}


int bubble(int a[][], int m)
{
    int swapped=0;
    for (int i=0; i<m; i++)
    {
        if ((i%2)==0)
        {
            for (int left=0; left<m-1; left++)
            {
                int right=left+1;
                if (a[i][left]>a[i][right])
                {
                    swap (a+left, a+right);
                    swapped++;
                }
            }
        }
        if ((i%2)==1)
        {
            for (int left=0; left<m-1; left++)
            {
                int right=left+1;
                if (a[i][left]<a[i][right])
                {
                    swap (a+left, a+right);
                    swapped++;
                }
            }
        }


    }
    return swapped;
}


void bubblesort (int a[][], int n)
{
    for (int last=n-1; last>0; last--)
    {
        if (bubble (a, last+1)==0)
        break;
    }
}








int main()
{
    int n=0;
    scanf(" %d", &n);
    int matrix[n][n];
    for (int i=0; i<n; i++)
    {
        for (int j=0; j<n; j++)
        {
            scanf(" %d", &matrix[i][j]);
        }
    }
    bubblesort(matrix, n);
    for (int i=0; i<n; i++)
    {
        for (int j=0; j<n; j++)
        {
            printf("%d ", matrix[i][j]);
        }
    }






  return 0;
}

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I think my problem is how I send the matrix to the func. I think I need to seperate it into one dimensinal arrays and sending each individualy to sorting by the row index.

The one dimensional arrays you need, are already there - waiting for you. In C, a two D array has 2 (maybe more) one dimensional arrays, inside it. One for each row.

With a[n][n], a[0], is one one dimension array (the first row). a[1] is another, etc.

This looks like a problem from SPOJ.

Quote:

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Originally Posted by Adak

The one dimensional arrays you need, are already there - waiting for you. In C, a two D array has 2 (maybe more) one dimensional arrays, inside it. One for each row.

With a[n][n], a[0], is one one dimension array (the first row). a[1] is another, etc.

This looks like a problem from SPOJ.

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I've modified my code:

Code:

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


void bubble(int m, int a[m], int i)
{
    int c=0;
    if(m<=1) return;
    bubble(m-1, a, i);
    if ((i%2)==0)
    {
        for (int left=0; left<m-1; left++)
        {
            int right=left+1;
            if (a[left]>a[right])
            {
                c=a[left];
                a[left]=a[right];
                a[right]=c;
            }
        }
    }
    else
    {
        for (int left=0; left<m-1; left++)
        {
            int right=left+1;
            if (a[left]<a[right])
            {
                c=a[left];
                a[left]=a[right];
                a[right]=c;
            }
        }
    }




}


int main()
{
    int n=0;
    scanf(" %d", &n);
    int matrix[n][n];
    for (int i=0; i<n; i++)
    {
        for (int j=0; j<n; j++)
        {
            scanf(" %d", &matrix[i][j]);
        }
    }
    for (int i=0;i<n;i++)
    {
    bubble(n, matrix+i*n,i);
    }
    for (int i=0; i<n; i++)
    {
        for (int j=0; j<n; j++)
        {
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }






  return 0;
}

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now it says:

Code:

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In function 'main':|53|error: passing argument 2 of 'bubble' from incompatible pointer type|
|4|note: expected 'int *' but argument is of type 'int (*)[(unsigned int)(n)]'|

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why?
I believe that how I send my array does send it as a one dimensional array to my func.
Or am I mistaken?

Nope. matrix is an array of arrays (which can decay to a pointer-to-array, as it has done here). A pointer-to-array is not the same as an array. matrix[0] is a one-D array. matrix[i] is a one-D array. matrix is not, and matrix+i*n is not only not an array, it probably doesn't point to valid memory.

Quote:

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Originally Posted by tabstop

Nope. matrix is an array of arrays (which can decay to a pointer-to-array, as it has done here). A pointer-to-array is not the same as an array. matrix[0] is a one-D array. matrix[i] is a one-D array. matrix is not, and matrix+i*n is not only not an array, it probably doesn't point to valid memory.

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So can I send different rows to my func as it is now with this?:

Code:

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for (int i=0;i<n;i++)
{
  bubble(n, matrix[i], i)
}

---

considering matrix is a 2D array?

Yes you can send rows to the bubble() function like that. Note that you pass matrix[i] which is an entire row. Therefore matrix[i] must be a pointer to an allocated area that holds a row's worth of integers.

You haven't defined the matrix with malloc yet - that's the first thing.

You don't need to use malloc, at all. This is a static array.

Your sort code is the first thing - it's a Substitution sort, not a Bubble sort. While these two sort algorithms are very close to each other, here's the difference: the Bubble sort ALWAYS compares adjacent values, as it sorts. The Substitution sort STARTS comparing just one adjacent value, and then compares only non-adjacent values, as it proceeds to sort.

Your call to Bubble sort should be:

Code:

---

    for (int i=0;i<n;i++)
    {
      bubble(n,i,matrix[i]);  //i goes first, before matrix[i]
    }

---

The function's first line should be:

Code:

---

void bubble(int n, int i, int a[i])

---

If int i, is not listed before a[i], you should receive an error or warning, from your compiler.

This is a correct Substitution sort:

Code:

---

        for (left=0; left<n-1; left++)  //Substitution sort
        {
           
            for(right=left+1;right<n;right++) //starts with 1 comparison to
            if (a[left]>a[right])            //the adjacent value
            {
                c=a[left];                    //then begins comparing non
                a[left]=a[right];            //adjacent values
                a[right]=c;
            }
        }

---

and this is a correct Bubble sort:

Code:

---

    do {
      swapped = 0;
      for(left =0; left < n-1; left++) {  //Bubble sort
        if(a[left] < a[left + 1 ]) {      //always compares adjacent values
            c=a[left];
            a[left]=a[left+1];
            a[left+1]=c;
         
            swapped = 1;
        }
      } //end for
      --n;                              //a reasonable optimization
    }while(swapped);              //for a small quantity sorter, only.

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Both of these sorting algorithms are good for small quantities, only. Don't even THINK about using them for a large quantity sort - they fall right on their faces.

Quote:

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Originally Posted by nonoob

Yes you can send rows to the bubble() function like that. Note that you pass matrix[i] which is an entire row. Therefore matrix[i] must be a pointer to an allocated area that holds a row's worth of integers.

You haven't defined the matrix with malloc yet - that's the first thing.

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Quote:

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Originally Posted by Adak

You don't need to use malloc, at all. This is a static array.

Your sort code is the first thing - it's a Substitution sort, not a Bubble sort. While these two sort algorithms are very close to each other, here's the difference: the Bubble sort ALWAYS compares adjacent values, as it sorts. The Substitution sort STARTS comparing just one adjacent value, and then compares only non-adjacent values, as it proceeds to sort.

Your call to Bubble sort should be:

Code:

---

    for (int i=0;i<n;i++)
    {
      bubble(n,i,matrix[i]);  //i goes first, before matrix[i]
    }

---

The function's first line should be:

Code:

---

void bubble(int n, int i, int a[i])

---

If int i, is not listed before a[i], you should receive an error or warning, from your compiler.

This is a correct Substitution sort:

Code:

---

        for (left=0; left<n-1; left++)  //Substitution sort
        {
           
            for(right=left+1;right<n;right++) //starts with 1 comparison to
            if (a[left]>a[right])            //the adjacent value
            {
                c=a[left];                    //then begins comparing non
                a[left]=a[right];            //adjacent values
                a[right]=c;
            }
        }

---

and this is a correct Bubble sort:

Code:

---

    do {
      swapped = 0;
      for(left =0; left < n-1; left++) {  //Bubble sort
        if(a[left] < a[left + 1 ]) {      //always compares adjacent values
            c=a[left];
            a[left]=a[left+1];
            a[left+1]=c;
         
            swapped = 1;
        }
      } //end for
      --n;                              //a reasonable optimization
    }while(swapped);              //for a small quantity sorter, only.

---

Both of these sorting algorithms are good for small quantities, only. Don't even THINK about using them for a large quantity sort - they fall right on their faces.

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Thank you both so much!!!! It seems I didn't really understood few things with func. and sorting, you cleared some of my misunderstandings, thank you!!!
BTW, so I won't have to bother this forum with stupid questions so much, can you recommand me on a site (or anything of the sort) that can help me improve in C? A site that has theoretical material (this part is not really critical as this->) and assigments (pref. with solutions)?

Quote:

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Originally Posted by Adak

You don't need to use malloc, at all. This is a static array.

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Read the requirement. Although in its current form the array is static, it will need to be changed to comply with what was asked for.

Hello mate, I am new too. Here, this site is actually the best place I have found go to their tutorials C Tutorial - Learn C - Cprogramming.com. beyond that, I recommend reading the Kernighan and Richie book "The C programming language" they wrote the language, so it's definitely worth a read. Then I discovered this site for things to program, simple iterative mathematical problems designed to be programed and they fit nicely with each other: Project Euler they are all mathematical problems which work well with C.

Regards

Quote:

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Originally Posted by nonoob

Read the requirement. Although in its current form the array is static, it will need to be changed to comply with what was asked for.

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I was helping fix the sorting function problem. You were answering about the malloc'ing problem. I saw no reason to add the malloc'ing code into the example of the sorting code - you were there.