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allocation of memory in 2d array

Discussion in 'C' started by reuby_tuesday, Dec 8, 2007.

  1. reuby_tuesday

    reuby_tuesday New Member

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    Ello all

    I am currently attempting to write a program that dynamically allocates memeory according to what is in a text file.

    The text file contains rows of numbers with the first row describing the size of the matrix
    eg
    2 3
    1 2 3
    2 1 3
    4 5 5


    after much hair pulling and fustration I am sure that it is in the memory allocation/deallocation that I am having difficulties with.

    I have attached my code for these two functions which are part of a larger program. The first function opens the file and calls the memory allocation function
    the next function then populates the 2d array
    the next function then calcculates the required calculations on the array
    the next function the displays the results
    the next functuion then frees the memory

    Code:
    /*--------------------------------------------------------*
     * Function to free the memory*/
    
    int FreeArray2d(int rowSize, int colSize)
    
    {
        int **array2d;
    
        /*free the memory row space */
        for (int index=0; index <rowSize; index++)
            {
                free (array2d[index]);
                array2d[index] = NULL;
    	}
    
        /*free the memory column space */
        for (int index=0; index <colSize; index++)
    	{
    	    free (array2d[index]);
    	    array2d[index] = NULL;
    	}
    
        /*free allocated memory*/
        free (array2d);
        array2d = NULL;
    
        return 0;
    }
    /*--------------------------------------------------------*/
    /*Function to allocte memory*/
    
    int AllocateMemory(int **array2d, int rowSize, int colSize)
    { 
    
        {    /*Allocate the memory for the rows of the array*/
    	 /*Used malloc, as calloc wouldn't work in the same way*/
             array2d = malloc (rowSize *  sizeof(int *));
    	
            /*Check that memory was allocated*/
            if (rowSize !=0 && array2d != NULL)
                {
                    for (int index = 0; index < rowSize; index++)
    	            {
    		        array2d[index] = malloc (colSize *  sizeof(int));
    		    }
    	    }
    
            else
                {
    	        fprintf(stderr, "Cannot allocate memory of size %d by %d.\n", rowSize, colSize);
                }
    
            return  **array2d;
    	 
        }
    }
    Can someone have a look at the two functions above. I know that i should be using calloc instead of malloc cause it zeros the memory first.
     
  2. Salem

    Salem New Member

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    Here are two different methods of allocating and freeing a dynamically allocated 2D array.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    int **make2D ( size_t rows, size_t cols ) {
        size_t  r;
        int   **result = malloc( rows * sizeof *result );
        for ( r = 0 ; r < rows ; r++ ) {
            result[r] = malloc( cols * sizeof *result[r] );
        }
        return result;
    }
    void free2D ( int **arr, size_t rows ) {
        size_t  r;
        for ( r = 0 ; r < rows ; r++ ) {
            free( arr[r] );
        }
        free( arr );
    }
    
    int **makeAnother2D ( size_t rows, size_t cols ) {
        size_t  r;
        int    *rp;
        int   **result = malloc( rows * sizeof *result );
        result[0] = malloc( rows * cols * sizeof *result[0] );
        for ( r = 0, rp = result[0] ; r < rows ; r++, rp += cols ) {
            result[r] = rp;
        }
        return result;
    }
    void freeAnother2D ( int **arr ) {
        free( arr[0] );
        free( arr );
    }
    
    int main ( ) {
        int **one = make2D( 10, 64 );
        int **two = makeAnother2D ( 10, 64 );
        int r;
    
        printf( "Number of bytes per row = %ld\n", 64 * sizeof(int) );
        printf( "Start of each row of one\n" );
        for ( r = 0 ; r < 10 ; r++ ) {
            printf( "%p\n", (void*)one[r] );
        }
        printf( "Start of each row of two (these are contiguous addresses)\n" );
        for ( r = 0 ; r < 10 ; r++ ) {
            printf( "%p\n", (void*)two[r] );
        }
    
        free2D( one, 10 );
        freeAnother2D( two );
    
        return 0;
    }
    
    Advantages of the 2nd method include:
    - it takes only 2 mallocs to allocate the whole thing.
    - the actual data is contiguous, as it would be for a true 2D array.
    - you don't need the number of rows in order to free it at the end.
     
  3. reuby_tuesday

    reuby_tuesday New Member

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    Thanks heaps for that. Ill implement that and will get back if I have any furether questions

    Thanks again
     
  4. reuby_tuesday

    reuby_tuesday New Member

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    Can someone explain what size_t does in this case. I have looked it up on the net and it doesent make any sense, so can you make your explaination preeschool level. thanks
     
  5. reuby_tuesday

    reuby_tuesday New Member

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    perhaps i should just submit my entire program and see if somebody wants to try and debug it. I have the code compliing, but it dumps out at the memory allocation.
     
  6. Salem

    Salem New Member

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    size_t is the type which sizeof() returns, that's all. In simplistic terms it is not that much different from an int.

    Did you try to write the whole program before trying to run it?
     
  7. reuby_tuesday

    reuby_tuesday New Member

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    No I was doing it slowly and in increments ,and got stuck on the momery allocation

    I can read in the file, and the processing part is doable, but with out the memory allocation, and the transfer of data into the 2d array its nothing
     
  8. Salem

    Salem New Member

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    Sorry, I've still no idea what it is you've tried and how you could possibly be going wrong.
    Post some code.
     
  9. reuby_tuesday

    reuby_tuesday New Member

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    Sorry, Im not bieng terribly helpful am I. Its easy to forget that you dont have a crystal ball.

    Ill PM you...
     
  10. shabbir

    shabbir Administrator Staff Member

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    You can PM but you can also post them clearly here as well so that we all can have a look.
     
  11. Salem

    Salem New Member

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    Please don't PM me for 1:1 support.
    Definitely a better idea to post here, because I don't do 1:1 support for forum questions.
     

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