Logical indexing in maltab

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Bharat
Bharat 2015-6-17
评论： James Tursa 2015-6-17
Hello,
I need help regarding a code.:
Sal = para(i,7); y= X(X<Sal); this doesn't change the output
The vaulue of Sal is = 0.005. If I use this, then it returns the right value.
y = X(X<0.005); this returns the correct output.
How do I resolve this issue.
Thanks.
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Stephen23
Stephen23 2015-6-17
编辑：Stephen23 2015-6-17
Learn about binary floating point numbers, and how they cannot accurately represent all decimal fractions:
http://de.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
You might also like to try using this, to find out what the value really is:
http://de.mathworks.com/matlabcentral/fileexchange/22239-num2strexact--exact-version-of-num2str-

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回答（2 个）

James Tursa
James Tursa 2015-6-17
If I understand your question, you are saying that y= X(X<Sal) returns a different result than y = X(X<0.005), even though Sal = 0.005.
I would hazard a guess that Sal is not really 0.005, but is a number close to 0.005 that displays the same as 0.005. What do you get with the following:
Sal == 0.005
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Bharat
Bharat 2015-6-17
I just checked. y=X(X<0.005010368459505) also doens't work. however y=X(X<0.005) works, which suggests that it related to number of significant digits. How do I change the number of significant digits?
Thanks
James Tursa
James Tursa 2015-6-17
I think you now need to give us a more complete picture of what you are doing and what "doesn't work" means. If y=X(X<0.005) "works", then why aren't you doing that explicitly? Is the real problem that you need to round Sal to the nearest 3-decimal place number for some reason? Or what?

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Walter Roberson
Walter Roberson 2015-6-17

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