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Re: 2D arrays, pointers, and pointer arithmetic: question

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James Kuyper於 2012年8月25日星期六UTC+8上午12時01分58秒寫道：
> On 08/24/2012 11:28 AM, gdotone@gmail.com wrote:
> 
> > I'm reading "A Book on C", 4th edition. The author gives the following example when discussing two dimensional arrays:
> 
> > 
> 
> > a[3][5], so let a[][] be of type int.
> 
> > 
> 
> > 
> 
> >            col 1        col 2      col 3       col 4      col 5
> 
> > row 1     a[0][0]     a[0][1]     a[0][2]     a[0][3]    a[0][4]
> 
> > row 2     a[1][0]     a[1][1]     a[1][2]     a[1][3]    a[1][4]
> 
> > row 3     a[2][0]     a[2][1]     a[2][2]     a[2][3]    a[2][4]
> 
> > 
> 
> > The author then says that the following expression are equivalent
> 
> > 
> 
> > a[i][j]
> 
> > *(a[i] + j)
> 
> > (*(a + i))[j]
> 
> > *((*(a+i)) + j)
> 
> > *(&a[0][0] + 5*i + j)
> 
> 
> 
> <pedantic>
> 
> This last expression takes a pointer to the first element of an array
> 
> containing only 5 elements, and (for i > 0) attempts to dereference it
> 
> after adding an integer greater than or equal to 5. Such derefencing has
> 
> undefined behavior (6.5.6p8). This interpretation of the standard is
> 
> controversial, but IMNSHO correct. In practice, on many machines it will
> 
> work exactly as expected. It's unlikely to fail except on systems which
> 
> provide run-time array bounds checking, which is rare.
> 
> 
> 
> Otherwise, the only way it's likely to fail is if the optimizer takes
> 
> advantage of the fact that it has undefined behavior by performing
> 
> optimizations that depend upon such code not being executed. For
> 
> instance, an implementation that performs an optimization based upon the
> 
> assumption that a[0]+i never refers to the same object a[1]+j,
> 
> regardless of the values of i and j, is perfectly legal despite the fact
> 
> that such an optimization could break code like this.
> 
> </pedantic>
> 
> 
> 
> > I understand that the table is just a convenient representation of the 2D array.
> 
> > In memory the 2D array is actually contiguous. OK. 
> 
> > 
> 
> > My question has to do with saying or writing out in words what is being shown when using the *, dereferencing, and &, address operators.
> 
> > 
> 
> > Is this the correct way of saying:
> 
> > 
> 
> > *(a[i] + j) the base address is pointed to by a[i] and we move along the contiguous memory j (interval), so we add j, j of size int, to get the value at that location we use the dereference operator.
> 
> > 
> 
> > I believe I understand what's happening, but am I saying it right.
> 
> 
> 
> Not quite, a[i] doesn't point at the base address. a[i] is an expression
> 
> of array type, and in most contexts (this being one of them) gets
> 
> automatically converted to a pointer (address) to the first element of
> 
> the array.
> 
> 
> 
> > (*(a + i))[j] the base address is at a, i is added to the base address, ok, I understand that we are looking at the row location here, but I'm stuck at how to say this.
> 
> 
> 
> a is an expression of array type, which automatically gets converted
> 
> into a pointer to the first element of that array, a[0]. Adding 'i' to
> 
> that pointer gives the address of the 'i+1'th element of that array,
> 
> a[i]. Dereferencing that pointer gives an expression of array type,
> 
> referring to a[i]. Like all such expressions, in most contexts, it gets
> 
> automatically converted to a pointer to the first element of that array
> 
> a[i][0]. Whenever x[j] is a valid expression, it is equivalent to
> 
> *(x+j). Adding 'j' to that pointer gives a pointer to the 'j+1'th
> 
> element of a[i], which is a[i][j]. Dereferencing that pointer gives the
> 
> value of that element.
> 
> 
> 
> > let's say addresses are given in decimal, an integer in one byte and each address points to a byte. I hope I'm saying this right?
> 
> > at address 100, a[0][0]
> 
> > at address 109, a[1][0]
> 
> > (if this thought is wrong please clear this up)
> 
> 
> 
> a[1][0] is stored at a position that is exactly 5*sizeof a[0][0] bytes
> 
> after the position of a[0][0]. Therefore, assuming a linear address
> 
> space (which C does NOT require), the difference in the addresses must
> 
> be an exact multiple of 5. 109-100=9 doesn't qualify.
> 
> 
> 
> > let's also say we are starting at location 100 (decimal).
> 
> > consider a[1][0]. row 2, col 1 for the following discussion please.
> 
> > 
> 
> > (*(a + i ))[j] --- (*(a + 1))[0]
> 
> > so, a, the start of the array is pointing to location 100
> 
> > What does it mean to add 1 to a?
> 
> 
> 
> a + 1 => &a[0] + 1 => &a[1].

In summary one should view int ** a, and int *a2[4000], and int a3[4000][4000]
differently.

The last one has been very  easy to be used in the unix or linux
 systems long time ago.

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2D arrays, pointers, and pointer arithmetic: question gdotone@gmail.com - 2012-08-24 08:28 -0700
  Re: 2D arrays, pointers, and pointer arithmetic: question Adrian Ratnapala <adrian.ratnapala@gmail.com> - 2012-08-24 08:41 -0700
  Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-24 12:01 -0400
    Re: 2D arrays, pointers, and pointer arithmetic: question 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-24 11:10 -0700
  Re: 2D arrays, pointers, and pointer arithmetic: question gdotone@gmail.com - 2012-08-27 13:35 -0700
    Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-27 16:46 -0400
      Re: 2D arrays, pointers, and pointer arithmetic: question Adrian Ratnapala <raka@scooter.fritz.box> - 2012-08-28 19:06 +0000
        Re: 2D arrays, pointers, and pointer arithmetic: question Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-28 15:22 -0400
        Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-28 15:34 -0400
          Re: 2D arrays, pointers, and pointer arithmetic: question Keith Thompson <kst-u@mib.org> - 2012-08-28 14:11 -0700
            Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-28 17:26 -0400
            Re: 2D arrays, pointers, and pointer arithmetic: question "io_x" <a@b.c.invalid> - 2012-08-29 09:25 +0200
          Re: 2D arrays, pointers, and pointer arithmetic: question Adrian Ratnapala <raka@scooter.fritz.box> - 2012-08-29 07:20 +0000
            Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-29 07:17 -0400
              Re: 2D arrays, pointers, and pointer arithmetic: question Adrian Ratnapala <raka@scooter.fritz.box> - 2012-08-29 11:41 +0000
            Re: 2D arrays, pointers, and pointer arithmetic: question Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-29 08:21 -0400
              Re: 2D arrays, pointers, and pointer arithmetic: question Keith Thompson <kst-u@mib.org> - 2012-08-29 11:37 -0700
                Re: 2D arrays, pointers, and pointer arithmetic: question Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-08-29 20:05 +0000
                Re: 2D arrays, pointers, and pointer arithmetic: question James Kuyper <jameskuyper@verizon.net> - 2012-08-29 17:27 -0400
                Re: 2D arrays, pointers, and pointer arithmetic: question Keith Thompson <kst-u@mib.org> - 2012-08-29 18:05 -0700
                Re: 2D arrays, pointers, and pointer arithmetic: question Les Cargill <lcargill99@comcast.com> - 2012-08-29 20:14 -0500
                Re: 2D arrays, pointers, and pointer arithmetic: question Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-08-30 04:47 -0700
                Re: 2D arrays, pointers, and pointer arithmetic: question Richard Damon <news.x.richarddamon@xoxy.net> - 2012-09-01 23:47 -0400
                Re: 2D arrays, pointers, and pointer arithmetic: question 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-02 04:43 -0700
                Re: 2D arrays, pointers, and pointer arithmetic: question Richard Damon <news.x.richarddamon@xoxy.net> - 2012-09-02 23:12 -0400
                Re: 2D arrays, pointers, and pointer arithmetic: question ais523 <ais523@nethack4.org> - 2012-08-30 11:35 +0000
                Re: 2D arrays, pointers, and pointer arithmetic: question ralph <nt_consulting64@yahoo.com> - 2012-08-30 11:17 -0500
                Re: 2D arrays, pointers, and pointer arithmetic: question Keith Thompson <kst-u@mib.org> - 2012-08-30 13:52 -0700
                Re: 2D arrays, pointers, and pointer arithmetic: question Anders Wegge Keller <wegge@wegge.dk> - 2012-08-30 23:13 +0200
                Re: 2D arrays, pointers, and pointer arithmetic: question ralph <nt_consulting64@yahoo.com> - 2012-08-31 16:37 -0500
    Re: 2D arrays, pointers, and pointer arithmetic: question Barry Schwarz <schwarzb@dqel.com> - 2012-08-27 19:45 -0700
      Re: 2D arrays, pointers, and pointer arithmetic: question Keith Thompson <kst-u@mib.org> - 2012-08-27 20:40 -0700
        Re: 2D arrays, pointers, and pointer arithmetic: question Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 06:28 -0700

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