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Why find cannot handle this very simple task?

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Mr M.
Mr M. on 19 Apr 2021
Answered: Jan on 27 Apr 2021
X = -0.1:.001:.25;
find(X == .077)
I get the following:
ans = 1×0 empty double row vector
However X(178) = 0.077. How to get back index 178?

Answers (2)

Stephan
Stephan on 19 Apr 2021
You are dealing with doubles, they are not precisly 0.077 - use round:
X = -0.1:.001:.25;
find(round(X,3) == .077)
  3 Comments
Steven Lord
Steven Lord on 27 Apr 2021
I wouldn't use round here. Decide how close is "close enough" and use that as the tolerance.
X = -0.1:0.001:0.25;
closeEnough = 1e-8;
X(find(abs(X-0.077) < closeEnough))
ans = 0.0770
Or work with integer values and convert to non-integer values as needed:
X2 = -100:250;
X2(X2 == 77)/1000
ans = 0.0770
Or use ismembertol which tries to choose a good "close enough" tolerance.
X(ismembertol(X, 0.077))
ans = 0.0770

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Jan
Jan on 27 Apr 2021
Welcome to the world of calculations with numbers represented with limited precision.

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