Re: Usage of Stream Set_Index() function

[DrPi <314@drpi.fr>]

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.