[R] I can't get seq to behave how I think it should

Thu Jan 17 17:52:11 CET 2019

> On 17 Jan 2019, at 15:56 , POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST) via R-help <r-help using r-project.org> wrote:
> 
> Well I get the issue with finite precision. As in SQRT(2) * SQRT(2) is not 2.

As Jeff indicates, you also need to get that just like 3rds and 7ths cannot be represented exactly in base 10, 5ths and 10ths cannot be represented exactly in base 2 (only powers of 1/2 and their multiples can).

Specifically, 1.4 decimal is 1.0110011001100.... binary

-pd

> 
> What surprised me was that seq(1.4, 2.1, by=0.001) starts at 1.3999999999999999 and not 1.4!
> 
> 
> -----Original Message-----
> From: PIKAL Petr [mailto:petr.pikal using precheza.cz]
> Sent: 17 January 2019 14:30
> To: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST); Ben Tupper
> Cc: r-help using r-project.org
> Subject: RE: [R] I can't get seq to behave how I think it should
> 
> Hi
> 
> It is not seq problem, it is floating point numbers representation in finit precision problem. Ben pointed to it and you could learn about it from FAQ 7.31.
> 
> Cheers
> Petr
> 
>> -----Original Message-----
>> From: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION
>> TRUST) <calum.polwart using nhs.net>
>> Sent: Thursday, January 17, 2019 2:56 PM
>> To: PIKAL Petr <petr.pikal using precheza.cz>; Ben Tupper
>> <btupper using bigelow.org>
>> Cc: r-help using r-project.org
>> Subject: RE: [R] I can't get seq to behave how I think it should
>> 
>> Thanks guys.
>> 
>> I've used Petr's method and its working for me.
>> 
>> If the data had been from a calculation I'd have rounded it... just
>> didn't expect seq to break it!
>> 
>> C
>> 
>> -----Original Message-----
>> From: PIKAL Petr [mailto:petr.pikal using precheza.cz]
>> Sent: 17 January 2019 13:53
>> To: Ben Tupper; POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS
>> FOUNDATION TRUST)
>> Cc: r-help using r-project.org
>> Subject: RE: [R] I can't get seq to behave how I think it should
>> 
>> Hi
>> 
>> Or you could use rounding.
>> which(round(lut, 3)==1.8)
>> [1] 401
>> 
>> Cheers
>> Petr
>> 
>>> -----Original Message-----
>>> From: R-help <r-help-bounces using r-project.org> On Behalf Of Ben Tupper
>>> Sent: Thursday, January 17, 2019 2:43 PM
>>> To: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS
>> FOUNDATION TRUST)
>>> <calum.polwart using nhs.net>
>>> Cc: r-help using r-project.org
>>> Subject: Re: [R] I can't get seq to behave how I think it should
>>> 
>>> Hi,
>>> 
>>> This looks like a floating point reality bump - see
>>> 
>>> https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-thin
>>> k- these-numbers-are-equal_003f
>>> <https://cran.r-project.org/doc/FAQ/R-
>>> FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f>
>>> 
>>> You can use other methods to finding your row - I would opt for
>>> findInterval()
>>> 
>>>> lut = seq(1.4, 2.1, by=0.001)
>>>> findInterval(1.8, lut)
>>> [1] 401
>>> 
>>> findInterval() uses a rapid search to find the index in the look up
>>> table (lut) that is just less than  or equal to the search value (in
>>> your example
>> 1.8).
>>> 
>>> Cheers,
>>> Ben
>>> 
>>>> On Jan 17, 2019, at 8:33 AM, POLWART, Calum (COUNTY DURHAM AND
>>> DARLINGTON NHS FOUNDATION TRUST) via R-help <r-help using r-project.org>
>>> wrote:
>>>> 
>>>> I am using seq with the expression seq(1.4, 2.1, by=0.001) to
>>>> create a sequence of references from 1.4 to 2.1 in 0.001
>>>> increments.  They appear to be created correctly.  They have a
>>>> related pair of data which for the purposes of this we will call
>>>> val.  I'm interested in the content on the row with seq = 1.8. But
>>>> I can't seem to get it returned.  I can get other values but not
>>>> 1.8!  yet looking at row
>>>> 401 there is nothing to indicate an issue
>>>> 
>>>>> a = 1.4
>>>>> b = 2.1
>>>>> seq = seq(a, b, by=0.001)
>>>>> val = ceiling(seq * 50)
>>>>> s=data.frame(seq, val)
>>>>> s$val[seq==1.799]
>>>> [1] 90
>>>>> s$val[s$seq==1.8]
>>>> numeric(0)
>>>>> s$val[seq==1.8]
>>>> numeric(0)
>>>>> s$val[s$seq==1.800]
>>>> numeric(0)
>>>>> s$val[s$seq==1.801]
>>>> [1] 91
>>>>> head(s[s$seq>1.798,])
>>>>     seq val
>>>> 400 1.799  90
>>>> 401 1.800  90
>>>> 402 1.801  91
>>>> 403 1.802  91
>>>> 404 1.803  91
>>>> 405 1.804  91
>>>> 
>>>> 
>>>> Can anyone explain what's going on here and how I would correctly
>>>> find the
>>> content of row 401 by using an expression to equal the seq column?
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>> 
>> *******************************************************************
>>> ***
>>>> **********************************************
>>>> 
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>>>> ...{{dropped:25}}
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-- 
Peter Dalgaard, Professor,
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