How to call element of matrix of symbolic variables

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I have two symbolic variables
syms th1(t) Lc1
and generated a matrix in terms of th(t) and Lc1
tc1(t) =
[ cos(th1(t)), -sin(th1(t)), 0, Lc1*cos(th1(t))]
[ sin(th1(t)), cos(th1(t)), 0, Lc1*sin(th1(t))]
[ 0, 0, 1, 0]
[ 0, 0, 0, 1]
now when I try to call the first three elements of the last column
pc1 = tc1(1:3,4);
I got this error:
Error using symfun/subsref (line 169)
Symbolic function expected 1 inputs and received 2.
Error in Fk (line 10)
pc1 = tc1(1:3,4);

Accepted Answer

Walter Roberson
Walter Roberson on 17 Jul 2018
It is not possible to index the entries of a symbolic function that computes a matrix.
The closest you can get is to call the function passing in variables for the parameters, get an array output, index into the array, and use the resulting expression or turn it into a function if needed.

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More Answers (1)

Nathan Hardenberg
Nathan Hardenberg on 19 May 2022
Since the code answer is a bit hidden in the comments I'm posting the solution as full code here:
syms th1(t) Lc1
tc1(t) = [
cos(th1(t)), -sin(th1(t)), 0, Lc1*cos(th1(t));
sin(th1(t)), cos(th1(t)), 0, Lc1*sin(th1(t));
0, 0, 1, 0;
0, 0, 0, 1;]
pc1 = tc1(t); % call the function with t first
pc1 = pc1(1:3,4) % now you can index
Stu on 19 Oct 2022
Wow, thanks for that. It's beautiful.
Now I can pass a vector (which itself contains symbolic variables) into another symbolic function which requires that vector input.

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