Trimming a curve - how to find nearest number and replace

Question

Dom Smith 2017-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/332844-trimming-a-curve-how-to-find-nearest-number-and-replace

评论： Greg Dionne 2017-3-31

I have the x and y coordinates for a bathymetric curve eg...

I would like to trim this curve to a certain level line with at y=-0.15 and x from 0 to 45 I need to both replace the first coordinate with (0,-0.15) and make the final coordinate (45,-0.15) removing any x,y values greater than this.

I have this for a large number of data sets so of course a loop would be preferential but I cannot work out how to do these operations through code alone. The image below might clarify what I mean a little more, though it was made with different numbers.

Thanks in advance for any help

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Answer 1

Greg Dionne 2017-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/332844-trimming-a-curve-how-to-find-nearest-number-and-replace#answer_261095

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Try:

myLimit = -0.15;
iFirst = find(y<myLimit, 1, 'first');
iLast = find(y<myLimit, 1, 'last');
xdesired = x(iFirst:iLast);
ydesired = y(iFirst:iLast);
plot(x, y); 
hold on
plot(xdesired, ydesired);
legend('Original','Trimmed')

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Dom Smith 2017-3-31

With a bit of modification I've made this work thank you

Greg Dionne 2017-3-31

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OK. good.

For the concatenation I would probably do something like:

% use "," instead of ";" if xdesired and ydesired are row vectors
xdesired = [0; xdesired; 45];
ydesired = [mylimit; xdesired; mylimit];

Glad you got past this.

-G

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Answer 2

Image Analyst 2017-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/332844-trimming-a-curve-how-to-find-nearest-number-and-replace#answer_261113

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The data you supplied does not look like your plots. But to clamp data to some level, try this:

xy = [...
0   1.36
0.03  -0.92
0.13  -1.68
1.69  -3.72
6.63  -3.35
9.18  -3.36
20.03  -3.26
31.75  -2.93
40.15  -1.70
46.21   0.10
69.97   2.26];
x = xy(:, 1);
y = xy(:, 2);
subplot(2, 1, 1);
plot(x, y, 'b-');
grid on;
yClamped = y; % Initialize
clampLevel = 0.15;
yClamped(y>clampLevel) = clampLevel; % Do the clamping
subplot(2, 1, 2);
plot(x, yClamped, 'b-');
grid on;