How to access only the first element of the variables in a structure array as a vector?

Question

Darcy Cordell 2023-5-17

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1966429-how-to-access-only-the-first-element-of-the-variables-in-a-structure-array-as-a-vector

评论： Paul 2023-5-23

采纳的回答： Paul

在 MATLAB Online 中打开

Not sure I worded this question correctly, but I will try to explain.

I have a (1 x N) structure array (S) which contains the field "Location" like this:

%Pretend N = 3:
S(1).Location = [1 4];
S(2).Location = [4 15];
S(3).Location = [3 7];

I want to get a vector of the first entry of each "Location" variable, like this:

Location_1_Vector = [1 4 3]

Is there an efficient way to do this in one line without producing an intermediate variable?

The following do not work:

Location_1_Vector = [S(:).Location]; %Concatenates the Location variables together into [1 4 4 15 3 7]
Location_1_Vector = [S(:).Location(1)]; %Error: intermediate dot indexing produced comma separated ...

Of course, I can do it like this:

Location = reshape([S(:).Location],2,length(S));
Location_1_Vector = Location(1,:);

But I would prefer to avoid creating an intermediate "Location" variable with the additional clunkiness of "reshape".

Any help is appreciated.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Paul 2023-5-17

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1966429-how-to-access-only-the-first-element-of-the-variables-in-a-structure-array-as-a-vector#answer_1238069

在 MATLAB Online 中打开

One option

S(1).Location = [1 4];
S(2).Location = [4 15];
S(3).Location = [3 7];
L = arrayfun(@(S) S.Location(1),S)
L = 1×3
     1     4     3

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Paul 2023-5-23

在 MATLAB Online 中打开

Just realized the Question also asked for an efficient solution. arrayfun is one line and does not produce an intermediate variable, but not necessarily the most effiicient, at least in terms of run time.

S(1).Location = [1 4];
S(2).Location = [4 15];
S(3).Location = [3 7];
S = repmat(S,1,1000);
isequal(test1(S),test2(S),test3(S))
ans = logical
   1
timeit(@() test1(S))
ans = 0.0058
timeit(@() test2(S))
ans = 1.2132e-04
timeit(@() test3(S))
ans = 7.1075e-04
function L = test1(S)
L = arrayfun(@(S) S.Location(1),S);
end
function L = test2(S)
Location = reshape([S(:).Location],2,length(S));
L = Location(1,:);
end
function L = test3(S)
L = nan(1,numel(S));
for ii = 1:numel(S)
    L(ii) = S(ii).Location(1);
end
end