assigning a a structured 2x2 matrix using a loop

I have written code like this:

detresp = struct ('a','b','c','d');

and later in the code comes a while loop where i have to use an array of detresp structures.

centroid is a precomputed M*2 matrix. I need to assign centroid to 'a' field of each cell of the detresp array.

the line: detresp(framecount).a=centroid;

throws an error saying that the left hand side is not a valid target for assignment. Any help is highly appreciated.

Thank you.

Answer 1

[ChristianW]

M = 5;
centroid = rand(M,2);

detresp = struct('a',num2cell(centroid,2),'c','d');

or

detresp = struct('a',cell(M,1),'c','d');
for k = 1:M
    detresp(k).a = centroid(k,:);
end

Answer 2

[Image Analyst]

It's a bit tricky and unexpected. You need to preallocate all the members that you will ever need. See this example:

numberOfFrames = 5; % Whatever...
% Preallocate all the a, b, c, and d that we will ever need.
a = zeros(numberOfFrames, 2);
b = zeros(numberOfFrames, 1);
c = zeros(numberOfFrames, 1);
d = zeros(numberOfFrames, 1);

detresp = struct('a', 'b', 'c', 'd')
for k = 1 : numberOfFrames
  detresp(k).a = rand(2,1)
  detresp(k).b = rand(1,1)
  detresp(k).c = rand(1,1)
  detresp(k).d = rand(1,1)
end