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

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Barry Schwarz <schwarzb@dqel.com> writes:
> On Mon, 27 Aug 2012 13:35:35 -0700 (PDT), gdotone@gmail.com wrote:
[...]
> * is not a function so return is not the best term to use.  The
> expression *(a+i) and its identical twin a[i] will, with a few
> exceptions that don't apply here, both EVALUATE to the address of the
> i-th row, not the starting row.  If you meant the start of row i, that
> would be correct.

The usual term is "yield": functions return values, and expressions
yield values.

[...]

>>The i term added to a is being added to the base address of the array
>>and itself, i, representing the size of a row. j then adds to that
>>address giving the column position.
>
> When a pointer and an integer are added, the value of the integer is
> scaled by the size of the object pointed to, or if you prefer, the
> size of the object type pointed to.

Another way to look at it is that the value is *not* "scaled" by the
size of the object pointed to; rather, the addition yields a pointer
that points N *objects* past the object that the original pointer
points to.  (There has to be an array for this to make sense, possibly
the 1-element array that's equivalent to a single object.)

The standard's description doesn't talk about scaling (N1570 6.5.6p8):

    When an expression that has integer type is added to or
    subtracted from a pointer, the result has the type of the
    pointer operand. If the pointer operand points to an element
    of an array object, and the array is large enough, the result
    points to an element offset from the original element such that
    the difference of the subscripts of the resulting and original
    array elements equals the integer expression.  In other words, if
    the expression P points to the i -th element of an array object,
    the expressions (P)+N(equivalently, N+(P)) and (P)-N(where N
    has the valuen ) point to, respectively, the i+n-th and i−n-th
    elements of the array object, provided they exist.

Referring to addition being "scaled" implies that pointers are
*really* pointers to bytes.  That's likely true on the machine code
level, but it's not true in the C abstract machine; rather pointers
*really* point to whatever type they're defined to point to.

>      Given a pointer p to type T and an integer k, the C expression
>           p + k
>      is mathematically equivalent to
>           p + k * sizeof(T) 
>      or the synonymous
>           p + k * sizeof *p

I understand what you mean, but the fact that you're using "+"
with two very different meaning could be confusing.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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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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