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digitalmars.D.learn - myrange.at(i) for myrange.dropExactly(i).front

reply Timothee Cour via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
Is there a function for doing this?
myrange.at(i)
(with meaning of myrange.dropExactly(i).front)
it's a common enough operation (analog to myrange[i]; the naming is from
C++'s std::vector<T>::at)
Jul 25 2014
parent reply "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Friday, 25 July 2014 at 21:33:23 UTC, Timothee Cour via 
Digitalmars-d-learn wrote:

 Is there a function for doing this?
 myrange.at(i)
 (with meaning of myrange.dropExactly(i).front)
 it's a common enough operation (analog to myrange[i]; the 
 naming is from
 C++'s std::vector<T>::at)


That would require a random access range, in which case you can 
just index directly. For a non-random access range, which you're 
doing would be the most direct way of doing it.

- Jonathan M Davis
Jul 25 2014
parent reply Ary Borenszweig <ary esperanto.org.ar> writes:
On 7/25/14, 6:39 PM, Jonathan M Davis wrote:

 On Friday, 25 July 2014 at 21:33:23 UTC, Timothee Cour via
 Digitalmars-d-learn wrote:

 Is there a function for doing this?
 myrange.at(i)
 (with meaning of myrange.dropExactly(i).front)
 it's a common enough operation (analog to myrange[i]; the naming is from
 C++'s std::vector<T>::at)


 That would require a random access range, in which case you can just
 index directly. For a non-random access range, which you're doing would
 be the most direct way of doing it.

 - Jonathan M Davis


No, the OP said the meaning was `myrange.dropExactly(i).front`, which is 
not a random access.

Sometimes you *do* want the n-th element of a range even if the range is 
not a random access.
Jul 25 2014
next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Saturday, 26 July 2014 at 00:28:32 UTC, Ary Borenszweig wrote:

 On 7/25/14, 6:39 PM, Jonathan M Davis wrote:

 On Friday, 25 July 2014 at 21:33:23 UTC, Timothee Cour via
 Digitalmars-d-learn wrote:

 Is there a function for doing this?
 myrange.at(i)
 (with meaning of myrange.dropExactly(i).front)
 it's a common enough operation (analog to myrange[i]; the 
 naming is from
 C++'s std::vector<T>::at)


 That would require a random access range, in which case you 
 can just
 index directly. For a non-random access range, which you're 
 doing would
 be the most direct way of doing it.

 - Jonathan M Davis


 No, the OP said the meaning was `myrange.dropExactly(i).front`, 
 which is not a random access.

 Sometimes you *do* want the n-th element of a range even if the 
 range is not a random access.


That is an inherently expensive operation, so it would be a very 
bad idea IMHO to support it. The OP referenced vector, which has 
random access, and that's a completely different ballgame.

In general, when operating on ranges, you should be trying to 
iterate over them only once and to backtrack as little as 
possible if you have backtrack. It's true that's not always 
possible, but if at() were O(n), then it would make inefficient 
code less obvious.

I'd argue against at() working on non-random access ranges for 
the same reason that std.container doesn't support containers 
with a length property of O(n) - because it's a function that 
looks like it's O(1), and programmers will consistently think 
that it's O(1) and misuse it. C++ has had that problem with 
std::list' size function which is O(n). at() looks like it would 
be O(1) (and it always is in C++), so it would be inappropriate 
to have it in cases where it would need to be O(n), and since we 
already have [], why add at()? It exists on vector in addition to 
[] to give it range checking random-access. We already have that 
in D with [].

myrange.dropExactly(i).front makes it much more obvious what 
you're doing and that it's inefficient. It might be necessary in 
some cases, but we don't want to give the impression that it's 
cheap, which at() would do.

- Jonathan M Davis
Jul 25 2014
prev sibling parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Saturday, 26 July 2014 at 00:28:32 UTC, Ary Borenszweig wrote:

 No, the OP said the meaning was `myrange.dropExactly(i).front`, 
 which is not a random access.

 Sometimes you *do* want the n-th element of a range even if the 
 range is not a random access.


What he did also say is he wanted the equivalent of C++'s "at", 
which is the equivalent of "checked random-access" (or "checked 
dictionary access").

So the actual requirements aren't very clear. In terms of "C++ 
at" equivalent, I don't think we have anything equivalent to 
offer. That said, I've never seen anyone use "at" in C++ ever.
Jul 26 2014
next sibling parent Timothee Cour via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
Just for clarification, I wanted 'myrange.at(i)' to be the same as
`myrange.dropExactly(i).front`
(so I don't assume it's a random access range).


 myrange.dropExactly(i).front makes it much more obvious what you're




doing and that it's inefficient. It might be necessary in some cases, but
we don't want to give the impression that it's cheap, which at() would do.

I think it's already clear that it's potentially O(n) [n=i] cost as we're
not using myrange[i]. But fine, call it atWalk/walkAt/whatever. Point is
it's a common enough operation.





On Sat, Jul 26, 2014 at 11:15 AM, monarch_dodra via Digitalmars-d-learn <
digitalmars-d-learn puremagic.com> wrote:


 On Saturday, 26 July 2014 at 00:28:32 UTC, Ary Borenszweig wrote:


 No, the OP said the meaning was `myrange.dropExactly(i).front`, which is
 not a random access.

 Sometimes you *do* want the n-th element of a range even if the range is
 not a random access.


 What he did also say is he wanted the equivalent of C++'s "at", which is
 the equivalent of "checked random-access" (or "checked dictionary access").

 So the actual requirements aren't very clear. In terms of "C++ at"
 equivalent, I don't think we have anything equivalent to offer. That said,
 I've never seen anyone use "at" in C++ ever. I'd assume it's more of a
Jul 27 2014
prev sibling next sibling parent "H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:
On Sun, Jul 27, 2014 at 07:42:17PM -0700, Timothee Cour via Digitalmars-d-learn
wrote:

 Just for clarification, I wanted 'myrange.at(i)' to be the same as
 `myrange.dropExactly(i).front`
 (so I don't assume it's a random access range).
 

 myrange.dropExactly(i).front makes it much more obvious what you're




 doing and that it's inefficient. It might be necessary in some cases,
 but we don't want to give the impression that it's cheap, which at()
 would do.
 
 I think it's already clear that it's potentially O(n) [n=i] cost as
 we're not using myrange[i]. But fine, call it atWalk/walkAt/whatever.
 Point is it's a common enough operation.


[...]

You could just define your own function for it, right?

	// or call it whatever you want
	auto getNth(R)(R range, size_t index)
		if (isInputRange!R)
	{
		return range.dropExactly(index).front;
	}


T

-- 
Making non-nullable pointers is just plugging one hole in a cheese grater. --
Walter Bright
Jul 27 2014
prev sibling parent Timothee Cour via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On Sun, Jul 27, 2014 at 9:20 PM, H. S. Teoh via Digitalmars-d-learn <
digitalmars-d-learn puremagic.com> wrote:


 On Sun, Jul 27, 2014 at 07:42:17PM -0700, Timothee Cour via
 Digitalmars-d-learn wrote:

 Just for clarification, I wanted 'myrange.at(i)' to be the same as
 `myrange.dropExactly(i).front`
 (so I don't assume it's a random access range).


 myrange.dropExactly(i).front makes it much more obvious what you're




 doing and that it's inefficient. It might be necessary in some cases,
 but we don't want to give the impression that it's cheap, which at()
 would do.

 I think it's already clear that it's potentially O(n) [n=i] cost as
 we're not using myrange[i]. But fine, call it atWalk/walkAt/whatever.
 Point is it's a common enough operation.


 [...]

 You could just define your own function for it, right?

         // or call it whatever you want
         auto getNth(R)(R range, size_t index)
                 if (isInputRange!R)
         {
                 return range.dropExactly(index).front;
         }


 T

 --
 Making non-nullable pointers is just plugging one hole in a cheese grater.
 -- Walter Bright



Obviously I did that, but I thought it belonged in phobos. Anyway, closing
this.
Jul 29 2014