Error converting expression in double array

Inspite of defining a column of matrix as symbolic using 'syms', MATLAB gives the same error, that it cannot convert eypression to double.

A part of the code goes somewhat like this (for a start i can be taken equal to P1):

epsilon_s_2(i-P1+2,1:151) = sym (zeros (1,151));
  Fs(1:151,1) = sym (zeros (151,1));




for j=1:50000 % Limiting the inner iterations to 50000
    if L<=1.5
        syms x positive 
        
 
 epsilon_p_a2(i-P1+2,1)= epsilon_p_a2(i-P1+1,1); % Force finding in prestressing cable
 Fp (((L-L_ung)/0.1)+1,1)= Ecfk_p*Ap*epsilon_p_a2(i-P1+2,1);
 


 
     epsilon_s_2(i-P1+2,(round (L/0.1))+1) = ((ds/x) -1) * epsilon_c_2(i-P1+2,(round (L/0.1))+1);

[Walter Roberson]

We do not know the datatypes of your various arrays. For example, assigning sym zeros into epsilon_s_2 does not force epsilon_s_2 to be symbolic unless it did not happen to exist before.

I suspect your epsilon_s_2 was not initialized to symbolic.

[Prathamesh Khorgade]

Hii Mr. Roberson,

all the other arrays given in the program are 'double' arrays.

I am afraid that I can not completely follow your answer. It would be truly kind of you if you could explain, what is meant by this statement of yours, 'assigning sym zeros into epsilon_s_2 does not force epsilon_s_2 to be symbolic unless it did not happen to exist before.'

Moreover, what can be done in your opinion to tackle the problem, of the part of matrix not getting initialized as symbolic?

Thanks in advance.

[Walter Roberson]

AAA = [0 0 0];      %initialize as double
AAA(2) = sym(3);    %put a symbolic number in there
AAA                 %show the content
class(AAA)          %did AAA become symbolic?
syms x
AAA(2) = x;         %we assigned AAA(2) to symbolic value. Can we now store symbolic variables into there?

BBB = sym([0 0 0]);  %initialize as symbolic
BBB(2) = sym(3);     %put a symbolic number in there
BBB                  %show the content
class(BBB)           %is BBB (still) symbolic ?
syms x
BBB(2) = x;          %we assigned BBB(2) to symbolic value. Can we now store symbolic variables into there?

When you have a double() array, then when you assign sym() of a numeric value into the array, the symbolic number is converted into double precision and the double precision is stored; the array does not suddenly become symbolic. And because it is not changed to symbolic when you assign a symbolic numeric value into part of it, then it is still not symbolic later when you try to assign an expression involving x into it, where x is a symbolic variable that has not been asssigned a numeric value.

[Prathamesh Khorgade]

Hello Mr. Roberson,

thanks a lot for the detailed explanation. I have not tried it yet, however, I have understood the logic and recognised the mistake, I did. I first define the part of array as double and then the other part as symbolic, which is more or less equivalent to overwriting.

I am pretty sure, that with this background it works. I once again sincerely acknowledge your inputs.