what's wrong with 'smooth' and 'find' commands in this code?

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Taehwan
Taehwan 2012-1-31
x = [0 0 0 1 1 1 0 1 0 0 1 1 1 0 0 0 0];
y = smooth(x,5);
find(y(:) == 0.6)
ans = Empty matrix: 0-by-1
Running above script gives empty matrix though definitely 0.6 elements exit. What have I done wrong?

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Grzegorz Knor
Grzegorz Knor 2012-1-31
Look at result of this line:
y- round(y*10)/10
In vector y there is no exact 0.6 value.
Try this code:
x = [0 0 0 1 1 1 0 1 0 0 1 1 1 0 0 0 0];
y = smooth(x,5);
find(abs(y(:)-0.6)<eps)
  2 个评论
Taehwan
Taehwan 2012-1-31
Thank you for the comment and the results. One thing I still can't catch is that why 'smooth' doesn't give exact 0.6 (= 3/5) answer... while others are exact 0.2, 0.4, 0.8, etc.
Grzegorz Knor
Grzegorz Knor 2012-1-31
It is 0.6, because difference between result and exact value is 1.11022302462516e-16 (on my PC) and it is smaller than floating-point relative accuracy 2.22044604925031e-16 (on my PC).
Compare with:
format long
x = single([0 0 0 1 1 1 0 1 0 0 1 1 1 0 0 0 0])
y = smooth(x,5)

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更多回答（1 个）

Jan
Jan 2012-1-31
This is one of the mayn effects of the limited precision of floating point values. See: Answers: Tag=faq6.1.

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