I am running some calculations with vectors (lists) and in order to perform them, they need to be flattened. Is there a better or neater way to do this?

ReactionPower = Flatten[RF1].Flatten[RF1vel] + Flatten[RF2].Flatten[RF2vel]; 

All the variables are of the form:

RF1 = {{x},{y}}

x and y are not integers, but long expressions with trigonometric functions.

  • $\begingroup$ What do RF1, RF1vel, RF2, and RF2vel look like before Flatten[]-ing? $\endgroup$ Commented Apr 15, 2017 at 0:56
  • $\begingroup$ @J.M. All are of the form {{ }, { }} $\endgroup$ Commented Apr 15, 2017 at 0:59
  • $\begingroup$ Ah, so ReactionPower is a scalar? $\endgroup$ Commented Apr 15, 2017 at 1:01
  • $\begingroup$ @J.M. Correct and the variables on the RHS are x and y components $\endgroup$ Commented Apr 15, 2017 at 1:04
  • 1
    $\begingroup$ Maybe a route that is "better ... than using multiple Flatten[]s..." can be had if you modify the process generating those lists to be Flatten[]-ed. $\endgroup$ Commented Apr 15, 2017 at 1:06

2 Answers 2



rf1 = {{x1}, {y1}};
rf2 = {{x2}, {y2}};
rf1vel = {{vx1}, {vy1}};
rf2vel = {{vx2}, {vy2}};

Here is a way using Total:

reactionPower = Total[rf1*rf1vel + rf2*rf2vel, 2]
(* vx1 x1 + vx2 x2 + vy1 y1 + vy2 y2 *)

reactionPower === Flatten[rf1].Flatten[rf1vel] + Flatten[rf2].Flatten[rf2vel]
(* True *)

This will certainly be less efficient, but is slightly shorter:

ReactionPower = Tr[RF1.Transpose[RF1vel] + RF2.Transpose[RF2vel]];

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