In the program I am creating, I convert an array into a pointer and then use pointer arithmetic to index it. Example: a[3] *( a + 3 ) My question is how would I do this on a 2-dimensional array? Example: a[3][2] ?? Not sure how to implement this

quote:Original post by Sethius
In the program I am creating, I convert an array into a pointer and then use pointer arithmetic to index it.
...
My question is how would I do this on a 2-dimensional array?

My question is why? Array subscript is pointer notation. Try this in your compiler:

// x[3]3[x]; 

Anyway, *((*(a + 3)) + 2). Which is so ugly and convoluted that I can't understand why you'd bother.

[Edit: I never realized the board had a quick hyperlink tag]

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Thanks to Kylotan for the idea!

Edited by - Oluseyi on February 22, 2002 10:58:33 PM

Ok, if there is a way around pointer arithmetic I''ll use it, but I''m stuck at this one point. I have to return a 2d-array from an accessor function ( its a private member of a class ), and then send that array to another function. What would I return the array as? I converted it to a double pointer (**) and then sent that to the other function, but whenever I tried to index it, the program would crash. Is there a better way to return an array than to convert it to a pointer?

One more question. I have a function that looks similar to this

// Returns a pointer to the dimensions
int** CBlock::GetDimensions( void )
{
return dimensions;
}

Where dimensions is a [4][2] array. When compiled, I receive this error:

error C2440: ''return'' : cannot convert from ''int [4][2]'' to ''int ** ''

Isn''t a 2d-array([][]) the same as a double pointer(**)?

why not the normal method of accessing anything thats stored on a block. ie

ptr[x+y*width]
or
*(ptr+x*width) // same thing as above

EDIT: removed highly harsh comments that seem to offend some ppl due to a misuncderstanding of what consitutes a 2d array. maybe ppl should start calling the video buffer varity 2d linear arrays.

Edited by - a person on February 25, 2002 3:05:02 AM

calm down there sparky

quote:Original post by a person
i dont understand HOW you ppl are so ignorent of this.
...
i am amazed that you ppl (repliers and sethius) have not even bothered to learn the fundamentals of arrays and pointers before you started creating games.

Shove a sock in it.

There are different levels of indirection between 1- and 2-dimensional arrays. You can access pretty much anything as anything else with casting. Yes, it''s all linear memory (in protected mode) and yes, my last answer was a crock. Couldn''t be bothered to come back to it. operator[] adds the offset to the base and returns the dereferenced value, unless overloaded.

a[2][3] and a[2*3] (ie a[6]) are not the same. Why? Because a[0][2] and a[1][0] are not guaranteed to be next to each other as a[3] and a[4] are.

Does everybody feel better now?

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Thanks to Kylotan for the idea!

Thanks a person, I am using single indexing and it works fine now.

i may have been a bit rough, and was wrong about how the [][] notation works (i never use it). maybe because i have never seen a good use for using static arrays of arrays and seeing MANY questions and ppl refering to 2d arrays when speaking of video buffers. unlike most, i am self taught c/c++/asm coder from the help files. i learned most of my c stuff from the bordland turbo c help docs (gold ole f1). though later with internet access i was able to learn dx, c++, win32 api quite quickly.

i have erased my harsh statments, since they probally were a bit too harsh. (never post when frusterated/angry, it never bodes well). i also tend to see things differently then most ppl, since i tend to be able to visulize how the code should work to do certain tasks. i say no reason why he should be using an array of arrays when a simple array would suffice.

when using arrays of arrays i tend to allocate the memory myself and not deal with using [][] notation even for access (ussually only use char** or CSomeClass**).

i think next time the poster should make it clearer what is trying to be done vs ysing ambigous terms (yes 2d arrays is ambigous since both you and I saw it as two different meanings).

please tell me how to refer to a 2 dimensional block of memory accessed via a linear buffer? since calling 2d array is not correct. btw this is not being sarcastic (well slightly), since i truly am curious to what they should be called? a linear 2d array perhaps? if so, please add this to the faq/dictionary, i am sure many would appreciate knowing this so nobody has this misunderstanding again.