Equal numbers shown as false when compared in R

Question

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In R, what is the difference between these two?
floating point issue in R?

This is part of a code I created. I spend days looking for the problem when finally realized that a comparison that should be TRUE was being calculated by R as FALSE. I'm using R 2.14.2 64 bits on windows. This is the code to reproduce the problem.

concList= c(1.15, 1.15, 1.15 ,1.15 ,1.15 ,1.15 )
concList=concList-0.4
a=sum(concList)
b=length(concList)*0.75
str(a)
str(b)
print(a==b)

The last print will result in FALSE even thou they are shown as exactly the same number. I tough this could be some problem on the floating point numerical representation of R, so I added the code below which solves the problem.

a=round(a,1)
b=round(b,1)
print(a==b)

My question is, is there any more elegant solution? Is this a bug that should be reported?

Thanks for your time.

Would `all.equal(a, b)` work for your purposes. This is different from `identical()` or what you have posted so far. — A5C1D2H2I1M1N2O1R2T1, Aug 08 '12 at 17:18
Very similar to http://stackoverflow.com/questions/6874867/floating-point-issue-in-r — csgillespie, Aug 08 '12 at 17:30
@csgillespie I saw that one too. I was certain there was another one with a fairly comprehensive answer; took me a few minutes to find it though. — joran, Aug 08 '12 at 17:31
See also http://stackoverflow.com/questions/9508518/why-are-these-numbers-not-equal — Brian Diggs, Aug 08 '12 at 17:43

score 6 · Accepted Answer · edited May 23 '17 at 11:56

6

Because they aren't exactly the same number. They differ by a very small amount due to the computer's representation of numbers, also known as floating point errors:

> a - b
[1] -8.881784e-16

Jon Skeet has an excellent blog post on this issue, which pops up on Stack Overflow with some regularity.

As @mrdwab suggests in the comments, you should use all.equal(a, b) to test for near equality.

edited May 23 '17 at 11:56

Community

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answered Aug 08 '12 at 17:16

David Robinson

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David has the point, see `?identical` for further assistance and examples. – rlegendi Aug 08 '12 at 17:19
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The all.equal is the command I needed, thanks for the helpful response. – Tiago Zortea Aug 08 '12 at 17:24

Equal numbers shown as false when compared in R

1 Answers1