Allocating a double pointer as an array

This is a discussion on Allocating a double pointer as an array within the C Programming forums, part of the General Programming Boards category; I'm trying to allocate a double pointer within a structure as an array. This is the function so far: Code: ...

  1. #1
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    Allocating a double pointer as an array

    I'm trying to allocate a double pointer within a structure as an array. This is the function so far:

    Code:
    struct set
    	{
    	int filled;
    	int size;
    	char **data;
    	};
    
    struct set *createSet(int maxElts)
    	{
    	struct set *sp;
    	if ((sp=malloc(sizeof(struct set)))==NULL)
    		{
    		printf("Failed to allocate memory!\n");
    		abort();
    		}
    	sp->filled=0;
    	sp->size=maxElts;
    	if ((sp->data=malloc(sizeof(char *)*maxElts))==NULL)
    		{
    		printf("Failed to allocate memory!\n");
    		abort();
    		}
    		if (sp->data[0]==NULL) //I wanted this to be allocated in such a way that I could use it as an array at this point. However, this is simply coming up as NULL.
    			printf("it isnt working\n");
    	return sp;
    	}
    Anyone know how to do this? I know it is possible, since my instructor gave me the line that mallocs the double pointer with the intent to use it in this fashion.

  2. #2
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    You've dynamically allocated an array of pointers (maxElts of them) using malloc(). If you want to have a 2D array, each of those pointers also needs to point at a valid array of char - one way of obtaining those valid arrays is to also dynamically allocate them using malloc().
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Sunshine, and read this, this, and this before posting again.

  3. #3
    Registered User ssharish2005's Avatar
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    Cambridge, UK
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    Something like this?

    Code:
    /* Assuning we are allocating a[12][12] */
    
    int i;
    
    if( ( sp->data = malloc( 12 * sizeof( char * ) ) ) != NULL )
        for( i = 0; i < 12; i++ )
             if( ( sp->data[i] = malloc( 12 * sizeof( char ) ) ) != NULL )
             { }
    And of-course the free of allocation should be done as follow:

    Code:
    for( i=0; i < 12; i++)
         free( sp->data[i] );
    
    free( sp->data );
    Do some proper error checking and your code indendation is poor!

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

  4. #4
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    Thanks guys, that was my issue.

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