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If the following code is executed MATLAB makes a mistake. Can someone verify this?

floor([0.1:0.1:2]/0.01)

So what is the 129 doing here??

ans = 10 20 30 40 50 60 70 80 90 100 110 120 129 140 150 160 170 180 190 200
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3 Answers 3

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It is a floating point rounding error because of the colon-generated vector.
Like Rasman said, if you do:

floor((0.1:0.1:2 + eps) / 0.01)

There will be no rounding errors.

However, based on how the colon operator works, I suggest that you do the same calculation like this:

floor([(1:20)/10] / 0.01)

[Edit: following Rasman's comment, I will add that the latter approach works for negative values as well, while adding eps sometimes fails]

The bottom line is that it is better using the colon-operator with integer numbers to minimize rounding errors.

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It is probably doing a floating point calculation resulting in an inexact value of 129.99999999999999... something instead of 130. and then you floor it to 129.

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it's a rounding approximation brought on by the array construction. The solution would be to add eps:

floor([0.1:0.1:2]/0.01+ eps([0.1:0.1:2]/0.01))
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  • Thank you, is this a workaround for this specific situation? Or do you recommend this kind of code always in such cases?
    – mmumboss
    Commented Jun 7, 2012 at 11:36
  • Most of the time I've seen a rounding problem it is because it is off by a single least significant bit (perhaps 2, although I can't recall seeing one. You can be super-safe by adding 2*eps, but for this type of problem it works: any((floor([0.1:0.1:200]/0.01 + eps([0.1:0.1:200]/0.01))) - (10:10:20000))
    – Rasman
    Commented Jun 7, 2012 at 11:42
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    I take that back, it works while you are in positive territory
    – Rasman
    Commented Jun 7, 2012 at 11:45
  • @EitanT I completely disagree, you're trying to add a value-specific eps, not some generic number. Use my example and compare: any((floor([0:0.1:2]/0.01 +eps)) - (0:10:200)) vs any((floor([0:0.1:2]/0.01 +eps([0:0.1:2]/0.01))) - (0:10:200))
    – Rasman
    Commented Jun 7, 2012 at 11:49
  • @Rasman +1 for beating me to the answer with eps, and +1 for the comment on the negative values ;) About the second comment, look at what I wrote in my answer, in the first approach. Add the eps to the colon-generated vector, before the division.
    – Eitan T
    Commented Jun 7, 2012 at 11:50

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