Delete every second element in array
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Hello everyone,
for my research project I'm trying to let a robot move a sine curve (only in a z axis up and down). The problem is, that if two points are very close to each other (i.e. 0.2mm) the robot gets really slow, stays still for like half a second and then moves on, so the total time isn't the same. Now in my code I'm trying to delete every second point (and then double the time in between these points in RoboDK), but I can't figure out how to do this in an if loop. Like really important is, that the extreme values must be kept, so the robot can move to the endpositions and I'll get a sine curve.
Here's my code so far:
clear;
A = 50;
t = 0.05;
x = 0:0.05:10*pi;
y = -A*sin(x) + 389.387; %389.387 is the start position of the robot, he only moves in z-axis
T = (1:length(x))*t;
figure(1)
plot(T,y,'.')
xlabel('time')
ylabel('distance')
hold on
%%
newpoints = zeros(1,(length(y)));
newpoints(1,1) = y(1,1);
for i = 3:length(y)
if (y(i) - y(i-1)) < 0.2 %Check, if distance under 0.2mm
if (sign(y(i)-y(i-1))) ~= (sign((y(i-1)-y(i-2)))) %Check if it's an extreme value, because of sign change
newpoints(i-1) = y(i-1);
elseif i == 1:2:length(y) %This is where the trouble begins, I dont't know how to tell Matlab to delete every second point
newpoints(i-1) = [];
end
end
end
figure(1)
plot(T,newpoints,'r.')
xlabel('time')
ylabel('distance')
I'm using Matlab R2020 for academic use. I'd be really glad if you guys could help me out!
0 Comments
Accepted Answer
Adam Danz
on 3 May 2021
Edited: Adam Danz
on 3 May 2021
To delete every second element in an array starting with element #2,
x(2:2:end) = [];
or starting with element #1,
x(1:2:end) = [];
But to ensure you never delete the endpoints,
x(2:2:end-1) = [];
To preserve indexing, you can insert NaN values instead of removing elements,
x(2:2:end) = nan;
x(1:2:end) = nan;
x(2:2:end-1) = nan;
7 Comments
Adam Danz
on 4 May 2021
Edited: Adam Danz
on 4 May 2021
> I need a loop to clarify if the distance between two points is nearer than 0.2mm.
Scott's advice is spot-on.
Alternatively, instead of
z = sqrt(diff(x).^2 + diff(y).^2);
you could use,
z = hypot(diff(x), diff(y));
which is probably the same thing in Scott's approach (we don't have access to the hypot code) but the documentation claims that hypot avoids underflow and overflow. I compared both approaches and they are equally speedy and the difference in results is less than 4.4409e-16 which is just roundoff error.
More Answers (1)
Scott MacKenzie
on 3 May 2021
Edited: Scott MacKenzie
on 3 May 2021
As per your question (Delete every second element in array) ...
x = 1:10
y = x(rem(x,2)==1)
Output:
x =
1 2 3 4 5 6 7 8 9 10
y =
1 3 5 7 9
2 Comments
Philipp Brogli
on 12 Jan 2024
The problem with this aproach is that it just removes any value that is divideable by 2. If you have a different vector x, for example [1,3,5,7,9], then it wouldn't work anymore.
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