Find a value in vector

Question

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I have a vector with a 2 million components, how do i find the index of the component which got the value 29.6 in it?

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Answer 1

KSSV 2017-8-31

2 个投票

idx = find(abs(v-29.6)<=10^-5) ;

where v is your vector with a 2 million components.

Jan 2017-8-31

Or: <= 1e-10 or what ever limit is matching.

The idea is that comparisons for equality are weak for floating point values. Perhaps the 29.6 is the result of a calculation and is actually a 29.5999999999999. See https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

Note that 10^-5 is an expensive power operation , while 1e-5 is a cheap constant. Matlab does not optimize such constant expressions yet.

José-Luis 2017-8-31

Also, this is probably somewhere you want to use eps().

Depending on the order of magnitude of OP's data, @KSSV's solution might yield junk.