how to conver a string to a array or matrix

hi. i have a string. its something like a='[0 1;J 2]' . i want to convert this string to a array. but as u see there is a varible in the string (J) . how can i convert this string to array so that J changes to a varible too.

[Turlough Hughes]

How did you get the string? Also what is stored in J?

[reza hamzeh]

i got it by get(handles.text,'string') . i want to store 0:0.1:10 in J

[Turlough Hughes]

Does the following help? I generate a 3d matrix here and he different values of J.

% set J to temp value, NaN, and then find index
J=NaN; 
data1=eval(a);
[row,col]=find(isnan(data1));

J = 0:0.1:10;
data = repmat(data1,[1 1 length(J)]);
data(row,col,:) = J;

(Assumes you've assigned the variable to J and that J appears only once). I'm guessing the string is coming from some sort of user input? This approach is definitely not ideal. Can you explain more about what you are trying to do overall?

[reza hamzeh]

it gets the info from user like this figure. user insert a hamiltonian matrix. it should have only 1 or 2 varibles till the program can plot the hamiltonian versus the varibles (after doing some calculations on the matrix). the user have to defind the varible and its range in the textbox varibles (the varible that it put it in hamiltonian matrix) .

Answer 1

[Stephen23]

You can easily avoid evil eval by using str2func:

>> a = '[0 1;J 2]';
>> v = 'J'; % comma-separated variables, e.g. 'J,K,L'
>> f = str2func(sprintf('@(%s)%s;',v,a));
>> f(4)
ans =
   0   1
   4   2

This approach is also likely to be more efficient if you will call this function repeatedly.

Answer 2

[Bhaskar R]

J must be initialized scaler values,

J = 2;    % assumed(must a single value 1x1 size)
a='[0 1;J 2]';
result = eval(a);

Answer 3

[Turlough Hughes]

You can make a 3d matrix with the third dimensions being the same size as your range. You could then generate the results as follows:

range=1:10;
J=0; 
temp=eval(a);
H=NaN(size(temp,1),size(temp,2),length(range)); % Preallocate space for data
ii=0;
for c=range
    ii=ii+1;    % Incrementing variable provides index for where to store in H on each iteration
    J=c;
    H(:,:,ii)=eval(a);
end

Here I have just assigned values to range but you should write some code to update the variable range according to user input, whether it be 1:10 or 0:0.1:10.