Re: "for E of Vector1 loop" equivalent to "for n in Vector1.First_Index .. Vector1.Last_Index loop ?

[reinkor <reinkor@gmail.com>]

On Monday, September 19, 2016 at 9:37:11 AM UTC+2, Stephen Leake wrote:
> On Monday, September 19, 2016 at 2:23:10 AM UTC-5, reinkor wrote:
> > Assume Vector1 is a vector (container) and I want to loop through it in the direction Vector1.First_Index .. Vector1.Last_Index.
> > 
> > Is the following equivalent:
> > 
> > for n in Vector1.First_Index .. Vector1-Last_Index loop
> >    <do domething with Vector1(n)>
> > end loop;
> > 
> > and 
> > 
> > for E of Vector1 loop
> >   <do something with E>
> > end loop;
> 
> Yes
> 
> > It is for me not directly intuitive that the latter construct
> > process the vector elements in the same order as the former. 
> > I feel I should find the answer in the definition of Ada - but din not :-)
> 
> See LRM 5.5.2 10:
> 
> For a generalized iterator, the loop parameter is created, the
> iterator_name is evaluated, and the denoted iterator object becomes the
> loop iterator.  In a forward generalized iterator, the operation First
> of the iterator type is called on the loop iterator, to produce the
> initial value for the loop parameter.  If the result of calling
> Has_Element on the initial value is False, then the execution of the
> loop_statement is complete.  Otherwise, the sequence_of_statements is
> executed and then the Next operation of the iterator type is called with
> the loop iterator and the current value of the loop parameter to produce
> the next value to be assigned to the loop parameter.  This repeats until
> the result of calling Has_Element on the loop parameter is False, or the
> loop is left as a consequence of a transfer of control.  For a reverse
> generalized iterator, the operations Last and Previous are called rather
> than First and Next.

Thanks. And a follow-up question:

I like the construct

for E of Vector1 loop
..
end loop;

but is there a possibility somehow to keep this style 
and execute opposite direction, equivalent to:

for i in reverse Vector1.First_Index .. Vector1.Last_Index loop
 ....
end loop

?

reinert