I can't get seq to behave how I think it should
On 17 Jan 2019, at 15:56 , POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST) via R-help <r-help at 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 at precheza.cz] Sent: 17 January 2019 14:30 To: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST); Ben Tupper Cc: r-help at 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 at nhs.net> Sent: Thursday, January 17, 2019 2:56 PM To: PIKAL Petr <petr.pikal at precheza.cz>; Ben Tupper <btupper at bigelow.org> Cc: r-help at 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 at precheza.cz] Sent: 17 January 2019 13:53 To: Ben Tupper; POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST) Cc: r-help at 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 at 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 at nhs.net> Cc: r-help at 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 at 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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