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Re: Simple argument question

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"Jens Thoms Toerring" <jt@toerring.de> wrote in message 
news:9l76heFm6gU1@mid.uni-berlin.de...
> "Paul" <pchrist <nospam>or@yahoo.co.uk> wrote:
>
>> "Paul or@yahoo.co.uk>" <pchrist<nospam> wrote in message
>> news:jcio0k$1tni$1@adenine.netfront.net...
>> >
>> > "Bint" <bint@ign.com> wrote in message 
>> > news:CB123632.B739%bint@ign.com...
>> >>
>> >>> "Bint" <bint@ign.com> wrote in message
>> >>> news:CB114269.B5F7%bint@ign.com...
>> >>>> Hi can someone answer a simple question involving arguments to
>> >>>> functions?
>> >>>>
>> >>>> I need to call a library function which expects an argument like 
>> >>>> this
>> >>>> Function(double pointlist[][3]);
>> >>>>
>> >>>> My question is how do I make one of those if I am just wanting to
>> >>>> malloc
>> >>>> the
>> >>>> memory?  Can I use a double pointer?
>> >>>>
>> >>>> Can I have something like this:
>> >>>> double *points = (double *)malloc(10*3*sizeof(double));
>> >>>>
>> >>>> for int (j = 0; j < 10; j++) {
>> >>>>     *(points + j*3 + 0) = x;
>> >>>>     *(points + j*3 + 1) = y;
>> >>>>     *(points + j*3 + 2) = z;
>> >>>> }
>> >>>>
>> >>>> And then call the function with points as the argument?  It doesn't
>> >>>> compile
>> >>>> but I was wondering if I could cast it?  Or do I need something else
>> >>>> instead
>> >>>> of a double pointer for my points?
>> >>>>
>> >>>> Does that make sense? I am confused about how to call the function 
>> >>>> with
>> >>>> anything but something like:
>> >>>> double array[500][3];
>> >>>>
>> >>>> I don't want that because I don't know the number of points, see?
>> >>>>
>> >>> Hi.
>> >>> When you create 2d arrays using new you may use pointer to pointer 
>> >>> types
>> >>> ,
>> >>> but this is a different type from the built in array type.
>> >>> In your function argument your 2d array will decay into a pointer of
>> >>> type:
>> >>> double (*)[3]
>> >>>
>> >>> Each dimensional array is pointed to by a pointer of type one 
>> >>> dimension
>> >>> less. For example a one dimensional array of doubles is pointed to by 
>> >>> a
>> >>> double*( the same as a pointer to a single double ). A 2-d array is
>> >>> pointed
>> >>> to by a type T(*)[S] . a 3d array is pointed to by a type 
>> >>> T(*)[S1][S2]
>> >>> etc
>> >>> etc.
>> >>>
>> >>> When you create your pointer with malloc you  are creating a pointer 
>> >>> of
>> >>> type
>> >>> double* which is a pointer to a 1d array, or a pointer to a single
>> >>> double.
>> >>> But the function argument needs you to pass a 2d array.
>> >>>
>> >>
>> >> So how would I create and pass something valid to this function, say 
>> >> for
>> >> an
>> >> array of [10][3], but specifying that 10 at run-time?  Do I use new
>> >> instead
>> >> of malloc?
>> >>
>> > To solve your problem you need to cast the return value from the
>> > allocation function to a suitable pointer type, which is compatable 
>> > with
>> > the function argument.
>> > IIRC malloc returns a void pointer so intead of casting it to a 
>> > double*,
>> > as you have done in your example, you can cast it to a double(*)[3]. 
>> > This
>> > pointer type would then be suitable to pass to the function:
>> >
>> > double (*points)[3] = (double (*)[3])malloc(10*3*sizeof(double));
>> >
>> > Function( points);
>> >
>> >
>> > HTH.
>> >
>> Note: your for loop will need to be modified as each increment on this 
>> type
>> of pointer will increment it by a complete row of 3 elelments.
>
> I guess you mean that he should remove the multiplication of
> 'j' by three. But then, as an added bonus due to getting the
> types right, he now could rewrite the initialization loop more
> naturally as
>
> for ( size_t j = 0; j < 10; ++j ) {
>    points[ j ][ 0 ] = x;
> points[ j ][ 1 ] = y;
> points[ j ][ 2 ] = z;
> }
Yes I was thinking of suggesting this to the OP. It's much nicer this way.

>
> What I am still wondering about though is how the function re-
> ceiving the 2-d array is supposed to know how many "points"
> it will get - all it receives is a pointer to (the first three
> coordinates of) the first point, but no information about how
> many more there are...

This was also what I  thought. I guess the OP has simplified the function 
and the 'actual' lib function will also have a size parameter. Or maybe the 
function has access to the runtime size parameter by some other channel, 
IDK. 



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Simple argument question Bint <bint@ign.com> - 2011-12-16 18:46 -0600
  Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 01:13 +0000
    Re: Simple argument question Bint <bint@ign.com> - 2011-12-17 12:06 -0600
      Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 18:45 +0000
        Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 23:16 +0000
          Re: Simple argument question jt@toerring.de (Jens Thoms Toerring) - 2011-12-18 22:07 +0000
            Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-18 22:34 +0000
  Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 00:13 -0800
    Re: Simple argument question Juha Nieminen <nospam@thanks.invalid> - 2011-12-17 08:16 +0000
      Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 00:51 -0800
        Re: Simple argument question Juha Nieminen <nospam@thanks.invalid> - 2011-12-17 10:23 +0000
  Re: Simple argument question Goran <goran.pusic@gmail.com> - 2011-12-17 23:08 -0800
    Re: Simple argument question Heck <heckja_in@ordertostymieharvestersverizon.net> - 2012-01-05 09:59 -0500
  Re: Simple argument question Krice <paulkp@mbnet.fi> - 2011-12-18 00:46 -0800
    Re: Simple argument question jt@toerring.de (Jens Thoms Toerring) - 2011-12-18 22:14 +0000
      Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-19 12:01 +0000
        Re: Simple argument question Tobias Müller <troplin@bluewin.ch> - 2011-12-19 17:17 +0000
          Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-20 00:33 +0000
  Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-18 17:03 +0000
    Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 10:04 -0800

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