using logical indexing to find events

Question

William Rose 2021-12-16

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https://ww2.mathworks.cn/matlabcentral/answers/1612090-using-logical-indexing-to-find-events

评论： Mathieu NOE 2021-12-17

I have three vectors of the same length: t, x, y. I also have tevent, whose elements are a subset of the elements in t. Can I use logical indexing, without a for loop, to find all the elements of x and y whose times are the times in tevent?

Example:

t=0:0.01:10; x=sin(2*pi*t)+t/10; y=sin(2*pi*t+pi/4);

tevent=[5.3,3.2,8.7,2,0.9];

%xevent=x(t==tevent)); %I wish this would work, but it causes an error.

%Next line is not practical when tevent has many elements,

%and is impossible, if length(tevent) is not known in advance.

xevent=x(t==tevent(1)|t==tevent(2)|t==tevent(3)|t==tevent(4)|t==tevent(5));

yevent=y(t==tevent(1)|t==tevent(2)|t==tevent(3)|t==tevent(4)|t==tevent(5));

%for loop works, but seems ugly

xe2=zeros(size(tevent)); ye2=xe2;

for i=1:length(tevent)

xe2(i)=x(t==tevent(i));

ye2(i)=y(t==tevent(i));

end

plot(x,y,'-r',xevent,yevent,'bx',xe2,ye2,'g+');

There must be a better way than what I did above.

Another thing: certain event times do not get found by the search above. I guess this is a rounding thing. For example, the line below should find two values in the vector t, but it fails to find the element t=5.4.

fprintf('Find and print two values: %.1f, %.1f.\n',t(t==5.3),t(t==5.4)); 
Find and print two values: 5.3, .

A workaround is

fprintf('Find and print two values: %.1f, %.1f.\n',t(abs(t-5.3)<1e-6),t(abs(t-5.4)<1e-6)); 
Find and print two values: 5.3, 5.4.

Thanks in advance.

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Answer 1

Mathieu NOE 2021-12-16

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hello

there is a very simple answer using interp1

t=0:0.01:10; x=sin(2*pi*t)+t/10; y=sin(2*pi*t+pi/4);
tevent=[5.3,3.2,8.7,2,0.9];
xevent=interp1(t,x,tevent);
yevent=interp1(t,y,tevent);
plot(x,y,'-r',xevent,yevent,'bd');

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Answer 2

Steven Lord 2021-12-16

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https://ww2.mathworks.cn/matlabcentral/answers/1612090-using-logical-indexing-to-find-events#answer_856635

在 MATLAB Online 中打开

Use ismembertol (to avoid floating-point issues) or ismember (if your time data contains exactly represtentable times.)

t=0:0.01:10;
x=sin(2*pi*t)+t/10;
y=sin(2*pi*t+pi/4);
tevent=[5.3,3.2,8.7,2,0.9];
eventLocations = ismembertol(t, tevent);
tsort = sort(tevent);
times = [t(eventLocations); tsort]
times = 2×5
    0.9000    2.0000    3.2000    5.3000    8.7000
    0.9000    2.0000    3.2000    5.3000    8.7000
checkX = [x(eventLocations); sin(2*pi*tsort)+(tsort/10)]
checkX = 2×5
   -0.4978    0.2000    1.2711    1.4811   -0.0811
   -0.4978    0.2000    1.2711    1.4811   -0.0811
checkY = [y(eventLocations); sin(2*pi*tsort+pi/4)]
checkY = 2×5
    0.1564    0.7071    0.8910    0.4540   -0.8910
    0.1564    0.7071    0.8910    0.4540   -0.8910