import std.stdio;
import std.array;
import std.range;
import std.algorithm;

void main()
{
    auto arr = [64, 64, 64, 32, 31, 16, 32, 33, 64];

    auto newarr = arr[];
    bool state = true;
    while (arr.length)
    {
        newarr = state  ? array(until!("a < 32")(arr))
                               : array(until!("a >= 32")(arr));
        arr = arr[newarr.length .. $];
        state ^= 1;

        writeln(newarr);
    }
}

The idea is to find as many elements in a sequence that conform to some predicate, followed by as many elements that conform to another predicate. The two predicates are switched on each run.

The above code will print:
[64, 64, 64, 32]
[31, 16]
[32, 33, 64]

Is there a better way to do this, some std.range/algorithm function I don't know of?

Andrej Mitrovic:

> Is there a better way to do this, some std.range/algorithm function I don't know of?

Maybe this does what you want, but it's not very good:

import std.stdio, std.algorithm;

void main() {
    auto arr = [64, 64, 64, 32, 31, 16, 32, 33, 64];

    int last = 0;
    foreach (g; group!q{ (a < 32) == (b < 32) }(arr)) {
        writeln(arr[last .. last+g[1]]);
        last += g[1];
    }
}


With the change to group() Andrei talks about the code becomes a little better (untested code):

import std.stdio, std.algorithm;

void main() {
    auto arr = [64, 64, 64, 32, 31, 16, 32, 33, 64];

    foreach (g; group!q{ (a < 32) == (b < 32) }(arr))
        writeln(g[1]);
}


In Python groupby uses a key mapping function, like D schwartzSort():

>>> from itertools import groupby
>>> arr = [64, 64, 64, 32, 31, 16, 32, 33, 64]
>>> [list(g) for h,g in groupby(arr, key = lambda x: x < 32)]
[[64, 64, 64, 32], [31, 16], [32, 33, 64]]


If group uses a key mapping function as schwartzSort() the code improves (untested):

import std.stdio, std.algorithm;

void main() {
    auto arr = [64, 64, 64, 32, 31, 16, 32, 33, 64];

    foreach (g; group!q{ a < 32 }(arr))
        writeln(g[1]);
}


Bye,
bearophile

Those could be nice solutions, thanks.

My first code needlessly resizes the original array though. I could simply track the lower index instead:

void main()
{
    dchar[] arr = [64, 64, 64, 32, 31, 16, 32, 33, 64];
    dchar[] newarr;
    size_t index;

    bool state = true;
    while (index < arr.length)
    {
        newarr = state ? array(until!("a < 32")(arr[index..$]))
                              : array(until!("a >= 32")(arr[index..$]));
        index += newarr.length;
        state ^= 1;

        writeln(cast(int[])newarr);
    }
}

You know what sucks? I can't assign a range with different predicates
to the same variable. E.g. this won't compile:
       auto newarr = state ? (until!("a < 32")(arr[index..$]))
                                    : (until!("a >= 32")(arr[index..$]));

Error: incompatible types for
((until(arr[index..__dollar],cast(OpenRight)1)) ?
(until(arr[index..__dollar],cast(OpenRight)1))):
'Until!(pred,dchar[],void)' and 'Until!(pred,dchar[],void)'

I mean they are basically the same range type, with only a different predicate. Why every template instantiation has to be its own unique type, I'll never understand.