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digitalmars.D.learn - lookahead on ranges

reply "Sebastian Graf" <SebastianGraf t-online.de> writes:
Hi,

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);

on a range? I think I could possibly do this with zip, but I am 
curious if there is something more to the point to do it in 
phobos. In particular something that just buffers n eles and does 
not copy the input range to popFront() all of them.
Apr 29 2013
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
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
Apr 29 2013
parent "Sebastian Graf" <SebastianGraf t-online.de> writes:
On Monday, 29 April 2013 at 16:39:21 UTC, bearophile wrote:

 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


Thanks. Sounds good.
Apr 29 2013