[R] vector is not assigned correctly in for loop

[R. Michael Weylandt]

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.
>
>
>
> On 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]]
>>
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