How to select the maximum points of the absolute values of a sin function

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Mauricio Galván García Luna 2017-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/353050-how-to-select-the-maximum-points-of-the-absolute-values-of-a-sin-function

编辑： Adam 2017-8-17

I would like to get all the maximum values of x in another matrix any idea? Sorry I am new

   fs = 500;
t = 0:1/fs:10-1/fs;
x = abs(sin(2*pi*3*t));

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Adam 2017-8-17

编辑：Adam 2017-8-17

在 MATLAB Online 中打开

Due to discretisation and the imperfect reproduction of the sine wave you will only pick up certain peaks as being the maximum.

find( x == 1 )

will give the indices of these locations, but when I run it this gives me only every 3rd peak. If you want the actual peak locations of the continuous curve surely that is just trivial maths?

peakLocs = 1/12 : 1/6 : ( 10 - 1/fs );

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Answer 1

Torsten 2017-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/353050-how-to-select-the-maximum-points-of-the-absolute-values-of-a-sin-function#answer_278259

https://de.mathworks.com/help/signal/ref/findpeaks.html

Best wishes

Torsten.

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Mauricio Galván García Luna 2017-8-17

thank you so much!!

But once I get the maximum points is there a way to put them back in the time that they belong.

Torsten 2017-8-17

编辑：Torsten 2017-8-17

在 MATLAB Online 中打开

[xp,locs] = findpeaks(x);
for i=1:numel(xp)
  tp(i)=t(locs(i));
end

Best wishes

Torsten.

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Answer 2

Selva Karna 2017-8-17

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/353050-how-to-select-the-maximum-points-of-the-absolute-values-of-a-sin-function#answer_278256

在 MATLAB Online 中打开

fs = 500;
t = 0:1/fs:10-1/fs;
x = abs(sin(2*pi*3*t));
p=find(x==1)

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Stephen23 2017-8-17

编辑：Stephen23 2017-8-17

This answer is very fragile, and does not handle floating-point errors at all. This is not a general solution for this task (try changing fs to 499 and see what happens).

Read these to know why testing for equality of floating-point values is a bad way to write code:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

https://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matla?s_tid=answers_rc1-3_p3_MLT

and a hundred other discussion of this topic.

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How to select the maximum points of the absolute values of a sin function

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