Hi,

I'm writing an application where I read a file with a complex structure.
For this, I use Ada.Streams.Stream_IO package.
To read the file, I need to position the file pointer to the correct
location. I do this using Set_Index().

In ARM A.12.1, it is stated :
For Read and Write with a Positive_Count parameter, the value of the
current index is set to the value of the Positive_Count parameter plus
the number of stream elements read or written.

I don't understand the "plus the number of stream elements read or written".

Does this mean one can't go back to beginning of file once data have
been read ?

Nicolas

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2022-01-14 20:30, DrPi wrote:

Hi,

I'm writing an application where I read a file with a complex structure.
For this, I use Ada.Streams.Stream_IO package.
To read the file, I need to position the file pointer to the correct location. I do this using Set_Index().

In ARM A.12.1, it is stated :
For Read and Write with a Positive_Count parameter, the value of the
current index is set to the value of the Positive_Count parameter plus
the number of stream elements read or written.

I don't understand the "plus the number of stream elements read or
written".

For Write, the ARM also says: "The Write procedure with a Positive_Count parameter starts writing at the specified index". And then, after the
data have been written, naturally the current index of the stream has
been increased, to point at the position after the written data. That is
the meaning of the "plus" phrase.

And Read analogously. The "plus" phrase explains what the value of the
current index is _after_ the Read or Write. But both the Read and the
Write _start_ reading/writing at the index given in the From/To parameters.

Does this mean one can't go back to beginning of file once data have
been read ?

No, Set_Index can do that, whether the last action was writing or
reading. And the Write/Read with an index parameter can also do that.

But if you have set the current index to the desired position with
Set_Index, you don't need to use the Read/Write that have a
Positive_Count (index) parameter.

This code:

Set_Index (F, I);
Write (F, Item);

is equivalent to:

Write (F, Item, I);

and ditto Read.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Le 14/01/2022 à 21:35, Niklas Holsti a écrit :

On 2022-01-14 20:30, DrPi wrote:

Hi,

I'm writing an application where I read a file with a complex structure.
For this, I use Ada.Streams.Stream_IO package.
To read the file, I need to position the file pointer to the correct
location. I do this using Set_Index().

In ARM A.12.1, it is stated :
For Read and Write with a Positive_Count parameter, the value of the
current index is set to the value of the Positive_Count parameter plus
the number of stream elements read or written.

I don't understand the "plus the number of stream elements read or
written".

For Write, the ARM also says: "The Write procedure with a Positive_Count parameter starts writing at the specified index". And then, after the
data have been written, naturally the current index of the stream has
been increased, to point at the position after the written data. That is
the meaning of the "plus" phrase.

And Read analogously. The "plus" phrase explains what the value of the current index is _after_ the Read or Write. But both the Read and the
Write _start_ reading/writing at the index given in the From/To parameters.

Does this mean one can't go back to beginning of file once data have
been read ?

No, Set_Index can do that, whether the last action was writing or
reading. And the Write/Read with an index parameter can also do that.

But if you have set the current index to the desired position with
Set_Index, you don't need to use the Read/Write that have a
Positive_Count (index) parameter.

This code:

   Set_Index (F, I);
   Write (F, Item);

is equivalent to:

   Write (F, Item, I);

and ditto Read.

Ok, so Set_Index() works as expected. Nothing special.

Reading the LRM made me understand things incorrectly because of the way
it is written.

It is written :
32 The Set_Index procedure sets the current index to the specified value.

Then, it is written :
32.1/1 ...
32.2/1 ...
...

Reading this, I thought 32.1/1, 32.2/1 etc lines apply to 32
(Set_Index() function).
So, I was completely lost to understand the meaning of these lines.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

I can suggest, by the way, to many students who are also looking to order a good and high-quality essay for themselves. It is necessary to get acquainted in advance with the companies that can help with such services. I already know for sure that it will
be better to turn here https://www.topwritersreview.com/reviews/get-essay/ to get acquainted with one of them in detail. It helped me to figure it out, I hope it will help you too.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)