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performance tuning - Optimizing a simple calculus using a list of matrices - Repeated dot product between a vector and a list of matrices



I want to make the following operation :



$$ \begin{bmatrix} \dot q_1\\\dot q_2 \end{bmatrix} + \begin{bmatrix} \begin{bmatrix} q_1 & q_2 \end{bmatrix} \mathbf A_1 \begin{bmatrix} q_1\\ q_2 \end{bmatrix} \\ \begin{bmatrix} q_1 & q_2 \end{bmatrix} \mathbf A_2 \begin{bmatrix} q_1\\ q_2 \end{bmatrix} \end{bmatrix} = \begin{bmatrix} \dot q_1\\\dot q_2 \end{bmatrix} + \begin{bmatrix} \begin{bmatrix} q_1 & q_2 \end{bmatrix} \begin{bmatrix} 0&1\\ 1&1 \end{bmatrix} \begin{bmatrix} q_1\\ q_2 \end{bmatrix} \\ \begin{bmatrix} q_1 & q_2 \end{bmatrix} \begin{bmatrix} -1&0\\ 0&0 \end{bmatrix} \begin{bmatrix} q_1\\ q_2 \end{bmatrix} \end{bmatrix} = \begin{bmatrix} \dot q_1\\\dot q_2 \end{bmatrix} + \begin{bmatrix} {q_1}^2 + q_2(q_1+q_2)\\ -{q_1}^2 \end{bmatrix}$$


I also want to put the two $\mathbf A_1$ and $\mathbf A_2$ matrices inside a list of matrices A in mathematica.


So I started creating my list of matrices A and did the following :


A={{{0, 1}, {1, 1}},{{-1, 0}, {0, 0}}};
res={{q1, q2}.A[[1]].{q1, q2},{q1, q2}.A[[2]].{q1, q2}};
final={q1p,q2p}+res;

Now, my question : is there a way to optimize the calculation of res? The goal is to avoid calling A[[1]] and A[[2]] independently each time. The perspective is to extend the A list to a list of dimension $n \gt 2$ (something like this) :


$$ \begin{bmatrix} \dot q_1\\\dot q_2 \\ \vdots \\ \dot q_n \end{bmatrix} + \begin{bmatrix} \begin{bmatrix} q_1 & q_2 & \cdots & q_n\end{bmatrix} \mathbf A_1 \begin{bmatrix} q_1\\ q_2 \\ \vdots \\ \dot q_n \end{bmatrix} \\ \begin{bmatrix} q_1 & q_2 & \cdots & q_n \end{bmatrix} \mathbf A_2 \begin{bmatrix} q_1\\ q_2 \\ \vdots \\ \dot q_n \end{bmatrix} \\ \vdots \\ \begin{bmatrix} q_1 & q_2 & \cdots & q_n \end{bmatrix} \mathbf A_n \begin{bmatrix} q_1\\ q_2 \\ \vdots \\ q_n \end{bmatrix} \end{bmatrix}$$



Answer




Ok, so, thanks to Spawn1701D's comment here's the simplest way to proceed :


A={{{0, 1}, {1, 1}},{{-1, 0}, {0, 0}}};
res={q1,q2}.#.{q1,q2}&/@A;
final={q1p,q2p}+res;

To state it simple, Spawn1701D prescribes the usage of pure functions to make it extremely terse :



  • # function creates a slot between the two versions of the $\mathbf q$ vector

  • /@ is a shorthand of the Map function



Basically what this code does is : map the Dot operations to every element on the A list (which in this particular case is a list of matrices).


UPDATE


Another way to proceed (the simplest one in fact, thanks Michael E2) is to simply use the Dot product twice between the list A and vector q


q={q1,q2}
A={{{0, 1}, {1, 1}},{{-1, 0}, {0, 0}}};
res=A.q.q

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