Indexing structure array. Every first entry of an vector.

Hello, I have stored a lot of vectors in the fields of an structure array and i want to get every first element. s is an 1xN struct. field1 contains the vector

a = s(1:end).field1(1)

or

a = s.field1(1)

give the same error "Expected one output from a curly brace or dot indexing expression, but there were N results."

[Stephen23]

"Expected one output from a curly brace or dot indexing expression, but there were N results."

Because you are generating a comma-separated list:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

https://www.mathworks.com/matlabcentral/answers/344486-generate-comma-separated-list-in-single-line-of-code

Answer 1

[Bruno Luong]

If speed is matters, use for-loop

s = struct('field1',num2cell(rand(1,100000)));

tic
a = arrayfun(@(S) S.field1(1), s);
toc % Elapsed time is 0.598622 seconds.

tic
a = zeros(size(s));
for k=1:length(a)
    a(k) = s(k).field1(1);
end
toc % Elapsed time is 0.051761 seconds.

[Alexander Höll]

Thanks for the fast response :), seems like you can't always get around the for loop.

[Walter Roberson]

You can use hidden for loops such as arrayfun or cellfun, but there is no built-in operation for this situation.

Answer 2

[Walter Roberson]

arrayfun(@(S) S.field1(1), s)