## digitalmars.D.bugs - [Issue 10042] New: std.range.inits and tails

d-bugmail puremagic.com writes:
```http://d.puremagic.com/issues/show_bug.cgi?id=10042

Summary: std.range.inits and tails
Product: D
Version: D2
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P2
Component: Phobos
AssignedTo: nobody puremagic.com
ReportedBy: bearophile_hugs eml.cc

--- Comment #0 from bearophile_hugs eml.cc 2013-05-07 17:38:31 PDT ---
I suggest to add to Phobos the inits() and tails() random-access ranges similar

This is a bare-bones forward-range implementation, with an usage example, a
inefficient function that finds the maximum subsequence:

import std.stdio, std.algorithm, std.range, std.typecons;

mixin template InitsTails(T) {
T[] data;
size_t pos;
property bool empty() { return pos > data.length; }
void popFront() { pos++; }
}

struct Inits(T) {
mixin InitsTails!T;
property T[] front() { return data[0 .. pos]; }
}

auto inits(T)(T[] seq) { return seq.Inits!T; }

struct Tails(T) {
mixin InitsTails!T;
property T[] front() { return data[pos .. \$]; }
}

auto tails(T)(T[] seq) { return seq.Tails!T; }

auto maxSubseq(T)(T[] seq) {
return seq
.tails
.map!inits
.join
.map!q{ tuple(reduce!q{a + b}(0, a), a) }
.reduce!max[1];
}

void main() {
[-1, -2, 3, 5, 6, -2, -1, 4, -4, 2, -1].maxSubseq.writeln;
[-1, -2, -3, -5, -6, -2, -1, -4, -4, -2, -1].maxSubseq.writeln;
}

Using the key function for the max() function as proposed in Issue 4705 the
function becomes quite short:

auto maxSubseq(T)(T[] seq) {
return seq.tails.map!inits.join.reduce!(max!q{ a.sum });
}

Using maxs:

auto maxSubseq(T)(T[] seq) {
return seq.tails.map!inits.join.maxs!q{ a.sum };
}

--
Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
```
May 07 2013
d-bugmail puremagic.com writes:
```http://d.puremagic.com/issues/show_bug.cgi?id=10042

--- Comment #1 from bearophile_hugs eml.cc 2013-10-16 15:58:15 PDT ---
Another usage example:

sequence!q{n}.inits.map!sum.take(20).writeln;

Should generate the the triangle number sequence (http://oeis.org/A000217 ),