Loop through data-struct and its sub-structs; Trouble with 'indexing' - error

Hi,

I have a data struct called "DATA" with N fields (let's say 5 in to make it easier on the mind). Each of these fields has two more fields, which have a big matrix.

I am trying to create a loop to go through these fields and select one certain column (each have names) of the matrix.

So let's say I go with DATA.N.second-field.columnOfChoice; That means I went with field #N of my struct 'DATA' and wanted to chose the second 'sub'-field of N, which is a matrix. Out of this matrix I am trying to get the values of a certain row, which has a certain name, which I am using instead of 'columnOfChoice'.

I am having trouble to include the variable N into the loop. My thought was to go something like this:

fn = fieldnames(DATA);  % Gives me a cell of N fields, each containing the name of the name of field #N

for N = 1:length(fn)
    x = DATA.fn(N).second-field.columnOfChoice1;
    y = DATA.fn(N).second-field.columnOfChoice2;
    % Should select the preferred column of the N-th DATA-field to plot
    % them later on.
    plot(x,y);
    hold on;
end

I get the error

"Unrecognized field name "N".",

which is kinda weird as I have defined N in the loop earlier.

Ich think the issue might lie in the fact that fn gives me a cell array, meaning the entries are for instance {'One'}, {'Two'}, etc.

What would be a possible way to run a loop through all of the DATA fields and select the columns of choice. Wether it is to plot them or not is not that important, as I am trying to use them for multiple reasons.

Answer 1

[Jan]

You want to use "dynamic field names". Then the field name must be included in parentheses:

x = DATA.(fn{N}).second-field.columnOfChoice1;
%        ^  ^ ^^

In addition you need curly braces to address an element of the cell string fn.

".columnOfChoice1" is not a valid indexing in Matlab. Maybe you mean:

x = DATA.(fn{N}).second-field(:, columnOfChoice1);

[Sokratis Panagiotidis]

Okay, firstly THANK YOU!

Now to your addition:

Let's say 'second-field' is actually called 'b' and the column of Choice is 'height', so by plotting I would have made

y = DATA.(fn{N}).b.height;

and for x I'd make the lenght of the column, which let's say it's

x = DATA.(fn{N}).b.length;

So to plot them I always used

plot(x,y, '.'); % Needed them as dots instead of a line.

so I am not sure if I understand your comment on that.