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Need an In-depth pointer explanation

Posted on 2002-07-26
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Last Modified: 2010-04-01
I have an array declared like this...

Float64 (* rawData[2]) [2];

It gets accessed like this...

Float64 packedData = rawData[0][0][0];
 

How is this accessed as a three dimensional array?  I am confused because, as I see it, rawData is an array of pointers to arrays of Float64's.  I just can't seem to get my head around this.  Why can't it be accessed like this...

Float64 packedData = rawData[0][0];

Can somebody give me a detailed explanation?  Perhaps a few visual aids?  I can't seem to visulize this as a 3D array.

Thanks a lot
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Question by:kirts
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4 Comments
 
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Accepted Solution

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jhance earned 100 total points
ID: 7180346
There is a rough equivalency between a pointer to an object and an array of objects.  So if you have something like:

int *p;

you can access the CONTENTS of p with:

int q = *p;  // q gets the CONTENTS of what p points to

but also with:

int q = p[0];

In this case, p[0] is equivalent to *p.  So just extend this to greater indirection:

int **p;
int q = **p;
int q = p[0][0];

In your example declaration, you are mixing the * and [] syntax which is OK...  

So you have the equivalent of:

int (* p[2])[2];

which is declaring an array of 2 int pointers 2 times.  (The parentheses are redundant here.)

So an item in this (confusing mess) can be accessed like:

q = p[0][0][0];

or

q = *p[0][0];
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Expert Comment

by:Sabnis
ID: 7180775
the thing is an array of pointers to an array
that is
Its an Array of Pointers
the rawData[2] which are pointing to a array of Float64

so u have array of pointers (that is a 2 d array) each of these pointers is pointing to an array of Float64 so there u have the 3 dimension

i cant make a diagram here but hope the above paragraph helps
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Author Comment

by:kirts
ID: 7180904
Wicked.  Thanks alot for the explanation.  Pointer notation can get so crazy.  
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LVL 32

Expert Comment

by:jhance
ID: 7181053
If you want to get a wizard at this, get this book:

http://www.softpro.com/0-201-60461-2.html

If you remember 10% of what's in it, you'll never miss a beat on this stuff again...
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