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Is there a way to dot two vectors that are included in a list? Let's say I have two lists that contain a couple of vectors

list1 = {{1,2,3},{2,4,6},{1,5,7}}
list2 = {{4,1,9},{1,4,3},{2,8,3}}

and I would like to take the dot product between each of the vectors and give out the result in a new list. This function function[list1,list2] would then do something like

{{1,2,3}.{4,1,9},{2,4,6}.{1,4,3},{1,5,7}.{2,8,3}}
(*
{33, 36, 63}
*)
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  • 4
    $\begingroup$ MapThread[Dot, {list1, list2}, 1] $\endgroup$ – C. E. Aug 23 '19 at 18:01
  • $\begingroup$ Or Dot @@@ Transpose[{list1, list2}]. $\endgroup$ – march Aug 28 '19 at 16:24
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As mentioned in the comments, the obvious approach is to use MapThread. However, if you have large lists of vectors, a faster approach is to rely on listability:

a = RandomReal[1, {10^5, 100}];
b = RandomReal[1, {10^5, 100}];

r1 = Total[a b, {2}]; //RepeatedTiming
r2 = MapThread[Dot,{a, b}]; //RepeatedTiming

r1 == r2

{0.021, Null}

{0.187, Null}

True

So, about an order of magnitude faster. If you want an even faster option, you might consider the internal, undocumented function NDSolve`FEM`MapThreadDot:

r3 = NDSolve`FEM`MapThreadDot[a, b]; //AbsoluteTiming

r1 == r3

{0.012493, Null}

True

The internal function is about 2 times faster.

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