🚀 go-pugleaf

In article <4762q8$1gaq@news.gate.net>, bhutto@gate.net (William Hutto)
writes: 
<snip>
|> Is the following not true?
|> 
|> Assuming:
|> 
|> [type] array[x][y]

One cannot `assume' this if x and y are not constants. There is _no_ variable
dimension array type in standard C.

|> 
|> 	array[i1][i2] ==  *(array + (x*i2 + i1) * (sizeof)[type])
|> 	AND
|> 	array[i1][i2] ==  *(array + (y*i1 + i2) * (sizeof)[type])

Not quite. The problem is that the meaning of pointer addition depends on the
type of the pointer. Assuming x and y are constants and the above declaration
of array, the value of `array' is &array[0] which has the type `[type] (*)
[y]' i.e. a pointer to an array [y] of [type]. Adding 1 to it skips over not
one `byte', but over an entire array [y] of [type], i.e. over sizeof
([type][y]) bytes, i.e. over y*sizeof([type]) bytes.

You also probably did not mean to write (sizeof)[type]. And if you think a
bit, you will see that both your statements cannot be true simultaneously.
(Easy way to see this, let x != y, where is array[1][1] stored? Your first
line says at x+1 offset whereas the second one says y+1!)

I am going to assume that you meant

* ([type]*) ( (char*) array + (y*i1+i2) * sizeof ([type]) )

Note that this is correct _only if_ the declaration of array is 

[type] array[x][y]

|> 
|> and if the compiler calculates 2 dim arrays using one of these methods a 
|> dynamic allocation in C would consist of:
|> 
|> [type] *array;
|> 
|> 	array = ([type] *)malloc(x*y);
|> 
|> and references to an element in the array be in the form:
|> 
|> 	array[i1][i2];	

But your declaration is different!!! With 

[type] *array

the meaning of array[i1][i2] is completely different, and is most often a
syntax error!!!

Cheers
Tanmoy
-- 
tanmoy@qcd.lanl.gov(128.165.23.46) DECNET: BETA::"tanmoy@lanl.gov"(1.218=1242)
Tanmoy Bhattacharya O:T-8(MS B285)LANL,NM87545 H:#9,3000,Trinity Drive,NM87544
Others see <gopher://yaleinfo.yale.edu:7700/00/Internet-People/internet-mail>,
<http://alpha.acast.nova.edu/cgi-bin/inmgq.pl>or<ftp://csd4.csd.uwm.edu/pub/
internetwork-mail-guide>. -- <http://nqcd.lanl.gov/people/tanmoy/tanmoy.html>
fax: 1 (505) 665 3003   voice: 1 (505) 665 4733    [ Home: 1 (505) 662 5596 ]