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Thread can do this

Thread[{a, {b, c, d}}]
{{a, b}, {a, c}, {a, d}}

But when above a, b, c and d are themselves Lists, for instance, used as a representation for 2D/3D points, it will not work as a naive generalization

Thread[{{a, b}, {{x, y}, {z, w}, {u, v}}}]

which just raises an error.

Here what is intended for is

{{{a, b}, {x, y}}, {{a, b}, {z, w}}, {{a, b}, {u, v}}}

So can it still be realized by Thread? If yes, how? If no, then do there exist any concise and efficient workarounds?

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1 Answer 1

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A level specification can help (we have to specify the head to thread over, too):

Thread[{{a, b}, {{x, y}, {z, w}, {u, v}}}, List, {2}]

{{{a, b}, {x, y}}, {{a, b}, {z, w}}, {{a, b}, {u, v}}}

Note that this unpacks arrays, so I would not suggest that for big datasets. I that case, I would propose a ConstantArray/Transpose combo:

n = 1000000 ;
a = RandomReal[{-1, 1}, 2];
b = RandomReal[{-1, 1}, {n, 2}];

Thread[{a, b}, List, {2}] // Developer`PackedArrayQ // AbsoluteTiming

Transpose[{ConstantArray[a, n],b}] // Developer`PackedArrayQ // AbsoluteTiming

{0.347755, False}

{0.046035, True}

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    $\begingroup$ It seems it. So any clarification for the curly brackets around 2 here? The F1 documentation of Thread does not mention level specification. $\endgroup$ Commented Sep 17, 2018 at 4:19
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    $\begingroup$ For level specifications, f[...,n] with integer n usually means "until level n" while f[...,{n}] means "only at level n". $\endgroup$ Commented Sep 17, 2018 at 4:21
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    $\begingroup$ Sorry, do you mean f[..., {n}] in the latter? $\endgroup$ Commented Sep 17, 2018 at 4:23
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    $\begingroup$ Yes. But I fixed the comment already. Thanks for the accept btw. $\endgroup$ Commented Sep 17, 2018 at 4:51
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    $\begingroup$ Your information on packed array is valuable. $\endgroup$ Commented Sep 17, 2018 at 5:54

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