lookahead on ranges

[bearophile]

Sebastian Graf:

> is there any way to to something like
>
>     auto arr = [1,2,3,4,5];
>     auto delta = arr.lookahead!"b-a"(1); // or probably pass 1 
> as template arg
>     assert(equal(delta[], [1,1,1,1][]);
>
> or like
>
>     // lookahead returns range of tuples (template arg) or 
> arrays (runtime arg)
>     foreach (a, b; arr.lookahead!1)
>         writeln(b-a);

I think there isn't something like that in Phobos (I can't be 
fully sure because std.algorithm and std.range contain lot of 
powerful stuff, and it's not easy to know every possible 
combination of them).

So I think you should use zip.

Time ago I have suggested to add a second argument to chunks:
http://d.puremagic.com/issues/show_bug.cgi?id=6621

With that you can do something like:

auto arr = [1,2,3,4,5];
auto delta = arr.chunks(2, 1).map!(p => p[1] - p[0]);


Note that today this:

assert(equal(delta[], [1,1,1,1][]));

is written like this:

assert(equal(delta, [1,1,1,1]));

Or even:

assert(delta.equal([1,1,1,1]));

The online documentation is old.

Bye,
bearophile