Can a pointer to a multidimensional array in C be written simply as:
double *array;
Where arrayis an n by n matrix?
And then can I access the element in row i, column j, by array[i][j]?
Or is there such thing as a double pointer?
4 Answers
Can a pointer to a multidimensional array in C be written simply as:
double *array;
Yes.
Say you have M x N array. You can use:
double* array = malloc(M*N*sizeof(*array));
Then, you can access the elements by using:
size_t getArrayIndex(size_t m, size_t n, size_t M)
{
return (m*M+n);
}
double getArrayElement(double* array, size_t m, size_t n, size_t M)
{
return array[getArrayIndex(m, n, M)];
}
1 Comment
Cyberguille
@user3746892 See this question stackoverflow.com/questions/14015556/…, although it is of c ++ the answer can help you better understand this answer
double * is a pointer to a double. It's also pointer to an element of an array of double.
To do what you want:
double bla[23][42]; // array of arrays, type is double [23][42]
double (*p)[23][42] = &bla; // pointer to an object of type double [23][42]
Comments
to make a pointer to a multidimensional array array[i][j] where i is number of rows and j is number columns you can do double (*ptr)[j] which says that ptr is a pointer to double array of j elements. then you can assign ptr=array;
Comments
Can a pointer to a multidimensional array in C be written simply as:
double *array;
That could be something that is used to represent an array, when handing it over to a function! A C array is then only represented by a pointer to its first element. How these elements are organized internally is up to you; making sure you don't use indices that are actually outside the array, too.
Or is there such thing as a double pointer?
Of course; double ***ndim_array is perfectly valid. However, you could also use multidimensional arrays like double [][][].
(Thanks to all the contributors in comments!)
4 Comments
Marcus Müller
@EOF: yep, you're right of course with the "a pointer is not an array"; I think I'm pretty correct on
double[][] actually having to be read as "an array of arrays", ie. a pointer to a pointer.
Thomas Padron-McCarthy
@MarcusMüller: No, you've got it wrong. An array is not a pointer, and an array of arrays is not a pointer to a pointer. Check out the FAQ: c-faq.com/aryptr/index.html
Marcus Müller
@ThomasPadron-McCarthy: that FAQ has a nice explanation! Note, though, that I've used the verb represent deliberately. Ok, you've got me on the multidimensional array thing, indeed, though, so I'm removing this from my answer. Thanks, guys! BTW; if you can, you're absolutely welcome to correct my answer here.
**, but it's not the same as a[][]at all.array[rows * columns](or allocate memory for) and index by[row * columns + col]