How to call element of matrix of symbolic variables

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Hazim Nasir
Hazim Nasir am 16 Jul. 2018
Kommentiert: Walter Roberson am 2 Feb. 2023
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);
thanks

Akzeptierte Antwort

Walter Roberson
Walter Roberson am 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.
  10 Kommentare
Walter Roberson
Walter Roberson am 20 Okt. 2022
Using () indexing to access the components of a symbolic function that defines an array, is something that is not going to be permitted. The () indexing method for symbolic function is already overloaded to evaluate the function, and there is no way to say "when I write these () brackets I mean indexing instead of evaluation".
There is already formula to retrieve the array expression in a way that can be later indexed.
Walter Roberson
Walter Roberson am 20 Okt. 2022
@ermiyas lakew I think you might be looking for argnames

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Weitere Antworten (2)

Nathan Hardenberg
Nathan Hardenberg am 19 Mai 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
  2 Kommentare
Osvaldo
Osvaldo am 30 Aug. 2022
Very usefull! Tks
Stu
Stu am 19 Okt. 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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João Vitor Fraga
João Vitor Fraga am 1 Feb. 2023
Bearbeitet: João Vitor Fraga am 1 Feb. 2023
Here's a simple solution that doesn't turn your symfun into a sym: simply left-multiply and right-multiply your symfun by appropriate matrices whose entries are either zero or one, depending on which elements you wish to extract. Your symfun will remain a symfun. In your case:
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];
Lmatrix = [1 0 0 0
0 1 0 0
0 0 1 0];
Rmatrix = [0
0
0
1];
pc1 = Lmatrix*tc1*Rmatrix
pc1(t) = 
class(pc1)
ans = 'symfun'
This can easily be condensed into a simple function you could call whenever needed.

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