[R] vector is not assigned correctly in for loop

[Bert Gunter]

But the OP should not be doing this **at all.** He apparently has not
bothered to read the Intro to R tutorial as he appears not to know
about vectorized calculations.

-- Bert

On Sat, Oct 6, 2012 at 3:29 AM, R. Michael Weylandt
<michael.weylandt at gmail.com> wrote:
> Forgot to cc the list.
>
> RMW
>
> On Sat, Oct 6, 2012 at 11:29 AM, R. Michael Weylandt
> <michael.weylandt at gmail.com> wrote:
>> A case study of a good question! Would that all posters did such a good job.
>>
>>n Sat, Oct 6, 2012 at 7:14 AM, 周果 <guo.chow at gmail.com> wrote:
>>> Hi there,
>>> Here is a minimum working example:
>>> ----------------------------------------------------------------
>>> lower = 0
>>> upper = 1
>>> n_bins = 50
>>> interval = (upper - lower) / n_bins
>>> bins = vector(mode="numeric", length=n_bins)
>>> breaks = seq(from=lower  + interval, to=upper, by=interval)
>>>
>>> for(idx in breaks)
>>> {
>>> bins[idx / interval] = idx
>>> }
>>>
>>
>> Note that this could slightly move idiomatically be done as
>>
>> bins[breaks / interval] = breaks
>>
>>> print(bins)
>>> ----------------------------------------------------------------
>>> which outputs:
>>> ----------------------------------------------------------------
>>>  [1] 0.02 0.04 0.06 0.08 0.10 0.14 0.00 0.16 0.20 0.00 0.22 0.24 0.26 0.28
>>> [15] 0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56
>>> [29] 0.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.84
>>> [43] 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00
>>> ----------------------------------------------------------------
>>> It turns out that some elements are incorrect, such as the 6th
>>> element 0.14, which should be 0.12 in fact.
>>> Is this a bug or I am missing something?
>>
>> Take a look at
>>
>> as.integer(breaks / interval)
>>
>> You're hitting up on floating-point issues (see the link in R FAQ 7.31
>> for the definitive reference, but it's a large and complicated field
>> with many little manifestations like this)
>>
>> What's basically happening is that the 7 you see in breaks / interval,
>> is actually 6.999999999999 (or so) which gets printed as a 7 by
>> print() but truncated to a 6 for subsetting as mentioned in ?`[`. If
>> you were to turn on more digits for printing, you'd see it's not
>> really a 7.
>>
>> You'd probably rather have
>>
>> bins[round(breaks / interval)] = breaks
>>
>> Cheers and thanks again for spending so much time to make a good question,
>>
>> Michael
>>
>>> And here is the output of sessionInfo():
>>> ----------------------------------------------------------------
>>> R version 2.15.0 (2012-03-30)
>>> Platform: x86_64-pc-mingw32/x64 (64-bit)
>>>
>>> locale:
>>> [1] LC_COLLATE=Chinese (Simplified)_People's Republic of China.936
>>> [2] LC_CTYPE=Chinese (Simplified)_People's Republic of China.936
>>> [3] LC_MONETARY=Chinese (Simplified)_People's Republic of China.936
>>> [4] LC_NUMERIC=C
>>> [5] LC_TIME=Chinese (Simplified)_People's Republic of China.936
>>>
>>> attached base packages:
>>> [1] stats     graphics  grDevices utils     datasets  methods   base
>>>
>>> loaded via a namespace (and not attached):
>>> [1] cubature_1.1-1 tools_2.15.0
>>> ----------------------------------------------------------------
>>> Thanks in advance.
>>>
>>> Regards,
>>> Guo
>>>
>>>         [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.



-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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