Thread: return array from external function

This is my external function

Code:

double *splitme(double *array, size_t p, size_t q)
{
	int i, n, j;
	double sum1, sum2; 
	double *bfun_split;
	size_t array_size, centroid;

	//cast centroid as size_t
	//round centroid sum
	//q is centroid -1
	//p is length - centroid - 1
	//allocate new array double centroidedarray length same as before
	//from 0 to p allocate everything from the right of centroid
	//from length-q-1 till end allocate left of centroid 
	//free old
	//return new
	
	array_size = p;
	
	for (i=0; i<array_size; i++)
	{
		sum1 += (i * array[i]);
		sum2 += array[i];
	}
	centroid = (round)(sum1/sum2);
	printf("centroid is %d\n", centroid);
	
	q = centroid - 1;
	p = array_size - centroid - 1;
	printf("array size is %i\n", array_size);
	printf("p is %i\n", p);
	printf("q is %i\n", q);
	bfun_split = malloc((array_size)*sizeof(double));
	
	i=0;
	for (i=0; i<p; i++)
	{
		bfun_split[i] = array[i+centroid];//allocate the right of the centroid
	}
	
	i=0;
	//x=(array_size - q - 1);
	for (i=(array_size - q - 1); i<array_size; i++)
	{
		bfun_split[i] = array[i-centroid]; //allocate the left of centroid
	}
	
	i=0;
	for (i=0; i<array_size; i++)
	{
		printf("split array %i is %lf\n", i, bfun_split[i]);
	}
	
	return(*array);
	free(array);
}

and this is my call

Code:

*split_array= *splitme(bfun_array, bfun_lines, 0);

When compiling it wont let me put return(*array) only return(array) and I was under the impression that in order to return an array it had to be a pointer?

In order to return an array you need to use a different programming language.

But here you appear to be happy to return a pointer, so that's what you need to do. "array" is a pointer, so that's what you should return. "*array" follows the pointer to the thing it is pointing to (in this case, that's a double), which is not (in this case) a pointer.

You probably want to return bfun_split because that's where all your calculations were put. In your calling function you'd use

Code:

split_array = splitme(bfun_array, bfun_lines, 0);

assuming split_array is pointer.
Also I don't think it's a good idea to free array inside the function.

Code:

double *bfun_split;

Code:

double *array;

Code:

return (*bfun_split);

or

Code:

return (*array);

I thought this meant that they were all double pointers?

even returning *bfun_split doesn't work (which, yes, is what I actually want to do...)

Originally Posted by browser

Code:

double *bfun_split;

Code:

double *array;

So that means bfun_split is a pointer-to-double, yes. That is so incredibly different than *bfun_split, which is not a pointer-to-double, but is just a double.

but returning

return(bfun_split);

just returns the pointer to the first array location.

I want to return the entire array. How would I go about doing this?

Originally Posted by browser

but returning

return(bfun_split);

just returns the pointer to the first array location.

I want to return the entire array. How would I go about doing this?

In C, you can return exactly one thing. That's all you get.

On the other hand, that one thing that you are returning is almost certainly the only thing you need to return, since arrays are contiguous in memory (i.e., no skipping about) so once you know where the starting point is in memory, you know where all the rest of them are (right behind it).

I did it here...

Code:

double ReadFile(FILE *filename, int num_lines, double *array)
{
	int	i;
	
	rewind(filename);
	
	i=0;
	for(i=0; i<(num_lines); i++)
	{
		if((i & 1) == 0)
		{
			fscanf(filename, "%lf", &array[i]);
		}
		else
		{
			array[i]=0.0;
		}	
	}
	
	i=0;
	for(i=0; i<(num_lines); i++)
	{
		printf("array %i is %lf\n", i, array[i]);
	}
	
	
	rewind(filename);
	return(*array);
	free(array);
}

with this as the call

Code:

*bfun_array= ReadFile(blurring, bfun_lines, bfun_array);

Originally Posted by browser

I did it here...

If you mean you're happy with that, then we're all happy with that. The thing to keep in mind is that you didn't return the array -- it got passed in as the third argument. Arrays that are passed in are edit-able (meaning the calling function can see the changes) so everybody's happy.

I don't feel like tracing through your code at the moment to see whether you can do this "in-place" or not. If so, then dump bfun_split completely. If not, then do your work with bfun_split in your function, and then *still in the function* do something like

Code:

array[i] = bfun_split[i];

(in a loop, naturally) to put your split array back where it belongs.

If you need both the original and the split, then you need two arrays in your calling function anyway, so just pass them both in.

I did that and it is still not compiling.

Originally Posted by browser

I did that and it is still not compiling.

Then you'll have to show us what you did.