# How to use the fields in the structure?

4 views (last 30 days)
Praveen Kumar Pakkirisamy on 28 Feb 2019
My structure has 83 Fields, and each field has 1059 *3 Double. Firstly how do I access these fields from the structure and then how do I multiply a matrix(rotation matrix) on these fields?. I know that I have to use for loop to access all the values and I have found this code But I do not know how It does function.
for i = fieldnames(markerStruct)'
newMarkerStruct.(i{1}) = markerStruct.(i{1});
end
I understand that values from markerStruct gets copied to newmarkerStruct. But I don't understand what i{1} means and How the value is getting assigned.

Adam Danz on 28 Feb 2019
Edited: Adam Danz on 28 Feb 2019
I've responded to your quesitons below. First I make some fake data to work with.
% Fake data
S.f1 = rand(1059, 3);
S.f2 = rand(1059, 3);
S.f3 = rand(1059, 3);
"Firstly how do I access these fields from the structure?"
% Field f1 is accessed like this
S.f1
% or dynamically
fn = fieldnames(S); %list of field names
S.(fn{1})
S.f4 = rand(1059, 3);
"How do I multiply a matrix(rotation matrix) on these fields?"
% rotation matrix
ry = [ cos(pi) 0 sin(pi);
0 1 0 ;
-sin(pi) 0 cos(pi)] ;
% Multiply the matrix by the first field .
f1Rot = S.f1 * ry;
"I know that I have to use for loop to access all the values "
% Actually, not true. You can use structfun() to apply a function to all fields.
% In this example, it's expected that all fields have 3 columns of data.
S2 = structfun(@(x) x * ry, S, 'UniformOutput', false);
% But if you must use a loop
fn = fieldnames(S); %list of field names
S2 = struct; % create new structure (no fields yet)
for i = 1:length(fn)
S2.(fn{i}) = S.(fn{i}) * ry; %store results in a new struct with same field names
end
"I have found this code But I do not know how It does function"
% This for-loop does nothing. It's just looping through each field and re-assigning the identical value.
% It's like A = A.
for i = fieldnames(markerStruct)'
newMarkerStruct.(i{1}) = markerStruct.(i{1});
end
fieldnames() lists all field names in a cell array of strings.
Adam Danz on 28 Feb 2019
Edited: Adam Danz on 28 Feb 2019
Yeah, to be honest, this is my first time seeing it.
If you're going to use a loop (rather than structfun, which I showed in my examples), then this is the better approach:
fn = fieldnames(S); %list of field names
for i = 1:length(fn)
S.(fn{i}) * ry
end
Praveen Kumar Pakkirisamy on 28 Feb 2019
Thank you so much for the answer. Appreciate it.