multidimensional arrays

[adam2016]

I'm just wondering about two dimensional arrays I'm on chapter 14 (Dynamic Memory Allocation) of Alex Allains book and we are learning about multidimensional arrays and pointer arithmetic

one thing I don't get is why do you have to ONLY include the second size of the array and not the first for example

  
   void PrintArray(int array[][4){

    // do something 

}

and if that's possible how come you just can't declare an array in the first place with only the second size mentioned for example

  int ray[][4];

if anyone could explain this in layman's terms(if possible) that would be great

thanks,

[TheIdeasMan]

When you declare a 2d array, the compiler needs to know both dimensions*. 2d arrays are actually stored as 1d arrays using the formula for a position: pos = row * NumCols + col, where row and col are the 2d position.

When an array is sent to a function, the compiler knows the size of it, so only needs the NumCols in order to implement the formula above.

* Unless you initialise it directly, in which case the compiler can count them.

[Naughty Albatross]

Hi,
arr[] == *arr
.
arr[][5] == *arr[5]
.
arr[][][8] == **arr[8]
.
arr[][][][9] == ***arr[9]

etc

[Naughty Albatross]

Does that help you? :)

[adam2016]

thanks guys,it's strating to make sense still a pretty hard subject to get the head around

@closedaccount how come you use it with a pointer? or why do you use the asterix when declaring an array?

[48T7M4Gy]

.

[Thomas1965]

Why don't you skip this chapter in the book. Pointers and dynamic arrays are a thing of the past. In modern C++ we have the STL which makes things much easier.

[Duthomhas]

closed account 5a8Ym39o6 wrote:

arr[][][8] == **arr[8]
.
arr[][][][9] == ***arr[9]

etc

Er, that's wrong. arr[][]... is invalid.

[Naughty Albatross]

I already knew that long ago :)

[Duthomhas]

Heh, yeah. It's unusual for me to be the one pointing out the error instead of having made the error...