Strange result of find function

Question

Johan 2021-9-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1449179-strange-result-of-find-function

评论： Johan 2021-9-8

Hello,

I noticed a strange results of the find function today.

I am using the find function to extract the indices of a time serie that is contained inside a structure. The find function does find a result but it is however not the correct one and I do not understand why.

To be clearer here is the code I'm using:

t(1) = 10e-9;

t(2) = 25e-9;

t(3) = 35e-9;

t(4) = 40e-9;

Time_indices = find(abs([files(index).data.time]-t')< eps);

This gives the following result:

Time_indices =

100

651

1152

1603

However, my time series array size is 401x1 double so the indices are wrong apart from the first one. However, if I run the same find function but in a for loop, discretizing the t values I get the expected result:

for i=1:4

test(i) = find(abs([files(index).data.time]-t(i))< eps);

end

test

test =

100 250 350 400

I'm not sure if I'm using find wrong or if there is an issue here, if someone has any insight into this that would be helpful for my understanding.

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

Jan 2021-9-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1449179-strange-result-of-find-function#answer_783309

在 MATLAB Online 中打开

% Time_indices = find(abs([files(index).data.time]-t')< eps);
%                                                   ^

If t and [files(index).data.time] are row vectors, the transposition of t replies a matrix:

a = 1:4;
b = 1:4;
a - b'
ans = 4×4
     0     1     2     3
    -1     0     1     2
    -2    -1     0     1
    -3    -2    -1     0

Now the find() command gets the linear indices of the matrix elements.

Maybe using 2 outputs for the find() command solve your needs. Or try ismembertol .

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Johan 2021-9-8

Thanks I see what you mean,

I am creating a 401x4 matrix when doing the -t operation so the find function returns the correct indices but make the 401x4 matrix into a 1604x1 vector, thus the strange results.

Using two outputs does solve the problem.

请先登录，再进行评论。