"find" function doesn't work

2011 10 4
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Hi~ I got a weird problem here. See the attached picture. "find" works for some values and doesn't for some others. It could a stupid question but can anybody help? Thanks a lot!
So the wavelength vector "lem" has some value but "find" can't find it.
The following is what happens in the command window. It can find the number value but can't find the variable x which equals that value.
>> x
x =
507.0350
>> jj = find(lem==x)
jj =
Empty matrix: 1-by-0
>> jj = find(lem==507.0350)
jj =
3408

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Walter Roberson
Walter Roberson 2011-10-4

0 个投票

find() works fine. Your array does not contain the value in x. If you were to try
lem(3408) - x
you would find that the result is non-zero.
See this FAQ

6 个评论
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Yuji Zhang
Yuji Zhang 2011-10-4
oh Thanks a lot Walter!
yes you're right.
I got this:
>> lem(3408)-x
ans =
5.6843e-014
Let me carefully check the values.
Yuji Zhang
Yuji Zhang 2011-10-4
Hi Walter, Hi everybody, could you help more?
I actually don't know how to handle this small difference in value.
>> format('long')
>> x
x =
5.070350000000000e+002
>> lem(3408)
ans =
5.070350000000000e+002
>> lem(3408)-x
ans =
5.684341886080802e-014
Yuji Zhang
Yuji Zhang 2011-10-4
they are double precision variables. Thanks~
>> whos x
Name Size Bytes Class Attributes
x 1x1 8 double
>> whos lem
Name Size Bytes Class Attributes
lem 1x80001 640008 double
Yuji Zhang
Yuji Zhang 2011-10-4
Hi~ Thanks for the help, Walter.
I read something about this flow point error here http://www.mathworks.com/help/techdoc/matlab_prog/f2-12135.html?#bqxyrhp.
I currently use this to deal:
>> jj = find(floor(10000000000*(lem-x))==0)
jj =
3408
it works. Thanks! Let me know if there are better ways to solve this problem. Thanks!
Yuji Zhang
Yuji Zhang 2011-10-4
actually
jj = find(abs(10000000000*(lem-x))<1)
is a better way. because the error could be >0 or <0.
if it was -1e-14, floor(-1e-14)=-1
Walter Roberson
Walter Roberson 2011-10-4
Please examine the documentation for eps()

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