How can I use syms like theta_(i-1)?

I like to express theta_(i-1) in matlab like theta_i, how can I solve this?

If i use 'theta_i_minus_1' results theta_(i,1) ...

Answer 1

[Walter Roberson]

Unfortunately, the current notation does not support subscripts that are general expressions. The current notion supports only (groups of) A to Z and a to z and 0 to 9 as subscripts (does not support underscore in a subscript), with multiple subscripts automatically being spliced together with commas.

I was going to say that "You can have multiple annotations on the main variable but you cannot have annotations on the subscripts. You cannot, for example, construct (theta x double dot)" but that turns out to be at least partly wrong

syms theta_ddot_x theta_x_ddot theta__ddot_x theta_ddot__x theta__ddot__x theta_x_ddot theta_x__ddot theta__x_ddot theta__x__ddot
[theta_ddot_x theta_x_ddot theta__ddot_x theta_ddot__x theta__ddot__x theta_x_ddot theta_x__ddot theta__x_ddot theta__x__ddot]

ans = 

[서호 이]

Thank you!

[Walter Roberson]

parts = ["theta", "x", "ddot"];
j = ["_", "__"];
orders = [1 2 3; 1 3 2; 2 1 3; 2 3 1; 3 1 2; 3 2 1];
for oidx = 1 : size(orders,1)
    tp = parts(orders(oidx,:));
    s = [tp(1) + j(1) + tp(2) + j(1) + tp(3), ...
         tp(1) + j(1) + tp(2) + j(2) + tp(3), ...
         tp(1) + j(2) + tp(2) + j(1) + tp(3), ...
         tp(1) + j(2) + tp(2) + j(2) + tp(3)];
    disp(s); disp(sym(s));
end

    "theta_x_ddot"    "theta_x__ddot"    "theta__x_ddot"    "theta__x__ddot"

    "theta_ddot_x"    "theta_ddot__x"    "theta__ddot_x"    "theta__ddot__x"

    "x_theta_ddot"    "x_theta__ddot"    "x__theta_ddot"    "x__theta__ddot"

    "x_ddot_theta"    "x_ddot__theta"    "x__ddot_theta"    "x__ddot__theta"

    "ddot_theta_x"    "ddot_theta__x"    "ddot__theta_x"    "ddot__theta__x"

    "ddot_x_theta"    "ddot_x__theta"    "ddot__x_theta"    "ddot__x__theta"

[서호 이]

Thank you for your effort! I learned more about this :)