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Function std.range.roundRobin

roundRobin(r1, r2, r3) yields r1.front, then r2.front, then r3.front, after which it pops off one element from each and continues again from r1. For example, if two ranges are involved, it alternately yields elements off the two ranges. roundRobin stops after it has consumed all ranges (skipping over the ones that finish early).

auto roundRobin(Rs...) (
  Rs rs
if (Rs.length > 1 && allSatisfy!(isInputRange, staticMap!(Unqual, Rs)));


import std.algorithm.comparison : equal;

int[] a = [ 1, 2, 3 ];
int[] b = [ 10, 20, 30, 40 ];
auto r = roundRobin(a, b);
assert(equal(r, [ 1, 10, 2, 20, 3, 30, 40 ]));


roundRobin can be used to create "interleave" functionality which inserts an element between each element in a range.

import std.algorithm.comparison : equal;

auto interleave(R, E)(R range, E element)
if ((isInputRange!R && hasLength!R) || isForwardRange!R)
    static if (hasLength!R)
        immutable len = range.length;
        immutable len =;

    return roundRobin(
        element.repeat(len - 1)

assert(interleave([1, 2, 3], 0).equal([1, 0, 2, 0, 3]));


Andrei Alexandrescu, David Simcha, Jonathan M Davis, and Jack Stouffer. Credit for some of the ideas in building this module goes to Leonardo Maffi.


Boost License 1.0.