[R] strange logical results

[Chuck Cleland]

Erin Hodgess wrote:
> Dear R People:  
> 
> I am getting some odd results when using logical operators:
> 
>> x <- seq(from=-1,to=1,by=0.1)
> 
>>> x
>  [1] -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1  0.0  0.1  0.2  0.3  0.4
> [16]  0.5  0.6  0.7  0.8  0.9  1.0
>> x == -1
>  [1]  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.9
>  [1] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.8
>  [1] FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.7
>  [1] FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.6
>  [1] FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.5
>  [1] FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> x == -0.4
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> 
> Should this show as true also, please?

  They are not exactly equal.  You could use all.equal() to test for
nearly equal.

> x[7] == -0.4
[1] FALSE

> all.equal(x[7], -0.4)
[1] TRUE

> all.equal(x[7], -0.4, tol = 0)
[1] "Mean relative  difference: 2.775558e-16"

> I saw this in both Windows and LINUX Versions 2.4.0
> 
> Thanks in advance,
> Sincerely,
> Erin Hodgess
> Associate Professor
> Department of Computer and Mathematical Sciences
> University of Houston - Downtown
> mailto: hodgess at gator.uhd.edu
> 
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-- 
Chuck Cleland, Ph.D.
NDRI, Inc.
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