Reference to non-existent field 'i'.

 %% To Create Symbolic symmentric P matrix of Order n*n

n=2;
%n=3;
p_n=(n*(n+1))/2;
P = sym('a', [n n],'real');
P_list = sym('p', [1 p_n],'real');
%logic to generate the Matrix
index=1;
for i=1:n
    for j=1:n
        if(i<=j)
            P(i,j)=P_list(index);
            index=index+1;
        else
            P(i,j)=P(j,i);
        end
    end
end
P

%% To Create symbolic A in CCF form

%Logic to generate A matrix(In controllable canonical form)
A_list = sym('a', [1 n],'real');
A_list = -A_list;
A=zeros(n-1,n);
for i=1:(n-1)
    for j=1:n
        if j==i+1
            A(i,j)=1;
        end
    end
end
A=[A ; A_list];
A

%% Solve the symbolic equation using traditional method

% Solving Lyapunov equation symbollicaly 
% Uses above generated A and P Matrix
Q_N=-eye(n);
Condition=A'*P + P*A;
Eq=Condition==Q_N;
eqns=[Eq(:)];
vars = P_list;
S = solve(eqns,vars,'Real',true);

%% Display results  

%Extracting values from structure S
p1=S.p1
p2=S.p2
p3=S.p3

 % p4=S.p4
 % p5=S.p5
 % p6=S.p6
 

I am trying to make the above extraction of fields [p1 p2 p3 ....] from structure S to work for any value(integer) of n instead of hardcoding everytime as I doing currently.

I tried with for loop as below but getting the error Reference to non-existent field 'i'.

Is there any way to achieve the task of extraction of fields from Struct (Symbolic Data Type) without hardcoding.

%%  Generic way to Display result

%**************THIS SESSION IS CREATING ERROR************************


%n=2 has P_list=[p1,p2,p3]
%n=3 has P_list=[p1,p2,p3,p4,p5,p6]
%n=4 has P_list=[p1,p2,p3,p4,p5,p6,p7,p8,p9,p10]

for i=P_list
    i=S.i
end

[KSSV]

Clearly there S do not have i as a field.

[KESAVKUMAR A]

Thanks for response.

Yes you are right i is not a field in S but when it is assigned to P_list and itterated as below

i got ans as

i=p1

i=p2

i=p3

So tried using S.i inside for loop and got error Reference to non-existent field 'i'.I am confused what does this mean.

for i=P_list
    i
end

Answer 1

[Walter Roberson]

S.(char(P_list(i)))

[KESAVKUMAR A]

Thanks,that works well but I would like to know why the below fails.

What is problem with assigning result to char(P_list(i))?

for i=1:length(P_list)
  char(P_list(i))=S.(char(P_list(i)))
end

Argument to dynamic structure reference must evaluate to a valid field name.

Error in P_traditional (line 58)

char(P_list(i))=S.(char(P_list(i)))

% Using fprintf, it works Fine

for i=1:length(P_list)
  fprintf('%s = %s',char(P_list(i)), S.(char(P_list(i))))
  fprintf('\n')
end

OUTPUT:

p1 = (a1 + a1^2 + a2^2)/(2*a1*a2)

p2 = 1/(2*a1)

p3 = (a1 + 1)/(2*a1*a2)

[Walter Roberson]

After the first iteration you have replaced the function named char with an array named char

In MATLAB it is never possible to compute the name of the output variable in a direct assignment statement. Basically... Don't Do That, it doesn't end well in the long run.

[KESAVKUMAR A]

Thanks.That helps!!!!

Answer 2

[KSSV]

Read about getfield.

for i=1:length(P_list)
    val = getfield(S,P_list{i})
end

[KESAVKUMAR A]

Thanks.

I tried the code provided but stuck with this below error.

for i=1:length(P_list)
    val = getfield(S,P_list{i})
end

Brace indexing is not supported for variables of this type.

Error in sym/subsref (line 898)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in P_traditional (line 56)

val = getfield(S,P_list{i})

% I changed the brace as well from {} ---> () as below
for i=1:length(P_list)
    val = getfield(S,P_list(i))
end

Now I am getting error as

Error using getfield (line 71)

Inputs must be strings, character vectors, or cell array of indices.

% I also tried this to check if getfield can help in anyway

Val=P_list(1);
% Now Val will be assigned as p1 (whose class is 'sym not 'char')

getfield(S,Val)

Error using getfield (line 71)

Inputs must be strings, character vectors, or cell array of indices.

getfield is not able to handle symbolic datatype so I am running out of help with this option also

[KSSV]

S.a = rand ; 
S.b = rand ; 
S.c = rand ; 
P_list = fieldnames(S) ; 
for i = 1:length(P_list)
  val = getfield(S,P_list{i})
end

[KESAVKUMAR A]

Thanks.This answer is also acceptable.