using logical indexing to find events

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.

Answer 1

[Mathieu NOE]

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');

Answer 2

[Steven Lord]

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

[William Rose]

@Steven Lord, thank you for telling me about ismember() and ismembertol().

I would accept your answer too, if it were possible to accept more than one.

Answer 3

[William Rose]

@Mathieu NOE, Thank you very much!

[Mathieu NOE]

My pleasure !