What is the proper Syntax for this Pointer?

[Neil]

What I am doing wrong

This works

batPointer = adaptors[position].adaptor[channel]->batData;
adaptors[position].batteries[channel] = batPointer->battery;

where:
batData is a pointer to a struct
batPointer is a pointer to a different kind of struct

I want one line, but I get compiler errors, what is the correct syntax?

[santosh]

Re: What is the proper Syntax for this Pointer?

Neil wrote:

What I am doing wrong
>
This works
>
batPointer = adaptors[position].adaptor[channel]->batData;
adaptors[position].batteries[channel] = batPointer->battery;
>
where:
batData is a pointer to a struct
batPointer is a pointer to a different kind of struct
>
I want one line, but I get compiler errors, what is the correct
syntax?

Please post a complete, minimal program that exhibits the concerned
error. Without precise definitions
of 'batPointer', 'adaptors', 'position', 'channel', 'batData',
and 'battery', we cannot say what the problem could be.

[Chris Dollin]

Re: What is the proper Syntax for this Pointer?

Neil wrote:

What I am doing wrong
>
This works
>
batPointer = adaptors[position].adaptor[channel]->batData;
adaptors[position].batteries[channel] = batPointer->battery;
>
where:
batData is a pointer to a struct
batPointer is a pointer to a different kind of struct
>
I want one line, but I get compiler errors, what is the correct syntax?

What was your one line, what were the declarations of the variables, and
what were the compiler errors?

[I'd also be likely to introduce a name for `adaptors[position]`, since it's
duplicated. And without seeing the rest of your code, I can't tell if your
assignment above should obviously be extracted into a well-named function
of its own.]

--
Chris "in some contexts, context is everything" Dollin

Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN

[James Kuyper]

Re: What is the proper Syntax for this Pointer?

Neil wrote:
....

The structs ( minus the uneeded stuff)are
>
struct adaptorBatteryD ata
{
struct batteryData const *battery;
};
>
struct adaptorData
{
struct adaptorBatteryD ata const *batData;
};
>
struct batteryData
{
>
};
>
struct adaptorBatteryD ata const *batPointer;
>
>
adaptors[position].batteries[channel] =
adaptors[position].adaptor[channel].batData->battery;
Gives the error "structure required on left side of . or *."

What is missing from what you just gave us is the data type for
adaptors[position] and adaptors[position].adaptor[channel]. As written,
your code requires that they both be structures. The message that was
generated implies that at least one of those expressions is a pointer to
a structure. If that is the case, then the your problem is due to the
fact that you've used "." rather than "->".

[Keith Thompson]

Re: What is the proper Syntax for this Pointer?

Neil <NeilKurzm@worl dnet.att.netwri tes:

Tomás Ó hÉilidhe wrote:

>Neil:

>> batPointer = adaptors[position].adaptor[channel]->batData;
>> adaptors[position].batteries[channel] = batPointer->battery;
>>>
>>I want one line, but I get compiler errors, what is the correct syntax?

>>

[...]

>>
>adaptors[position].batteries[channel] = adaptors[position].adaptor
>[channel]->batData->battery;
>>

I thought that, but then though it wrong. Thanks for the answer. And
the tip.
>
I hated to leave the two line solution.

Why? Is there some virtue in putting as much stuff as possible on one
line?

--
Keith Thompson (The_Other_Keit h) <kst-u@mib.org>
Looking for software development work in the San Diego area.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Barry Schwarz]

Re: What is the proper Syntax for this Pointer?

On Thu, 29 Nov 2007 03:42:51 -0500, Neil <NeilKurzm@worl dnet.att.net>
wrote:

>What I am doing wrong

What is your end objective? What are really trying to accomplish?

>
>This works

How can it work if you get compiler errors?

>
batPointer = adaptors[position].adaptor[channel]->batData;
adaptors[position].batteries[channel] = batPointer->battery;
>
>where:
>batData is a pointer to a struct
>batPointer is a pointer to a different kind of struct

If batPointer is of type struct x* and batData is of type struct y*,
they are incompatible and there is no implicit conversion between
them. The only way to assign a value of one type to an object of
incompatible type is with a cast. However, if the converted pointer
value does not have the proper alignment you have invoked undefined
behavior.

Why do you want a pointer to some object to hold the address of an
object of different type?

>
>I want one line, but I get compiler errors, what is the correct syntax?

One line of what? You have two independent assignment statements. Do
you intend to eliminate one?



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[Barry Schwarz]

Re: What is the proper Syntax for this Pointer?

On Fri, 30 Nov 2007 02:53:53 -0500, Neil <NeilKurzm@worl dnet.att.net>
wrote:


snip

>Yes it works. The structures are elaborate ( Not mine ) but they do
>want the are supposed to. Same the adapter, and the battery from the
>linked list.
>
>The structs ( minus the uneeded stuff)are

Unfortunately, also minus some needed stuff.

>
>struct adaptorBatteryD ata
>{
> struct batteryData const *battery;
>};
>
>struct adaptorData
>{
struct adaptorBatteryD ata const *batData;
>};
>
>struct batteryData
>{
>
>};
>
>struct adaptorBatteryD ata const *batPointer;
>
>
>adaptors[position].batteries[channel] =

You have not told us what type of struct the object adaptors is a
pointer to or an array of. The underlying structure apparently has a
member named batteries which does not match any of the three
structures you listed.

>adaptors[position].adaptor[channel].batData->battery;

It apparently also has a member named adaptor which also doesn't match
any of the three structures you listed. But adaptor appears to be an
array of or a pointer to struct adaptorData since it has a member
named batData.

batData is a pointer to struct adaptorBatteryD ata which does have a
member named battery. This member is a pointer to struct
batteryData.

So the first question: Is the undefined member batteries an object of
a suitable type so batteries[i] can receive a value of type pointer to
struct batteryData?

>Gives the error "structure required on left side of . or *."

C doesn't have an operator "*.". It does have two operators "*" but
neither one is applied to structures.

So the second question: What is the correct text of the error message?
Use cut and paste; don't retype.

And the third question: Which of the objects adaptors[i]. or
adaptor[i] is not a structure?


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[Neil]

Re: What is the proper Syntax for this Pointer?

Barry Schwarz wrote:

On Thu, 29 Nov 2007 03:42:51 -0500, Neil <NeilKurzm@worl dnet.att.net>
wrote:
>

>What I am doing wrong

>
What is your end objective? What are really trying to accomplish?

The pointer was use to search a linked list, If it fails I need the
first item as a place holder.

>This works

>
How can it work if you get compiler errors?

The two lines worked fine, When I tried to combine them I got the errors.

>

> batPointer = adaptors[position].adaptor[channel]->batData;
> adaptors[position].batteries[channel] = batPointer->battery;
>>
>where:
>batData is a pointer to a struct
>batPointer is a pointer to a different kind of struct

>
If batPointer is of type struct x* and batData is of type struct y*,
they are incompatible and there is no implicit conversion between
them. The only way to assign a value of one type to an object of
incompatible type is with a cast. However, if the converted pointer
value does not have the proper alignment you have invoked undefined
behavior.

struct x* contains a pointer to a linked list of struct y* (struct x* is
also a linked list)
->a->b->c->d->NULL
| | | |
1 1 1 1
| | |
2 2 2
| |
3 3

>
Why do you want a pointer to some object to hold the address of an
object of different type?
>

>I want one line, but I get compiler errors, what is the correct syntax?

>
One line of what? You have two independent assignment statements. Do
you intend to eliminate one?

No combine

>
>
>
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The earlier suggestion worked fine. Thanks to all.
Besides learning the pointer, I learn that I need to include more in a post.