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周果

2012-Oct-06 06:14 UTC

[R] vector is not assigned correctly in for loop

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
}

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?
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]]

Berend Hasselman

2012-Oct-06 10:25 UTC

head link

[R] vector is not assigned correctly in for loop

On 06-10-2012, at 08:14, ?? <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
> }
> 
> 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.
And the 7th is also incorrect.
> Is this a bug or I am missing something?
It is not a bug in R.
Yes you are indeed missing something. Read R FAQ 7.31.
Answer is: floating point inaccuracy.

Insert

print(formatC(idx/interval,format="f",digits=17))   
print(as.integer(idx/interval))

immediately after the opening { of the for loop.
If you insist on copying breaks to bins in the way you are doing you could use
round(idx/interval,3) for example.

Berend

R. Michael Weylandt

2012-Oct-06 10:29 UTC

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[R] vector is not assigned correctly in for loop

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, ?? <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.

Rui Barradas

2012-Oct-06 10:33 UTC

head link

[R] vector is not assigned correctly in for loop

Hello,

This seems to be a case for FAQ 7.31 Why doesn't R think these numbers 
are equal?

See this example:

3/5 - 1/5 - 2/5  # not zero
3/5 - (1/5 + 2/5)  # not zero, different from above

In your case, try

for(idx in breaks){
     print(idx / interval, digits = 16)  # see problem indices
     bins[idx / interval] = idx
}
b2 <- breaks

identical(bins, b2)  # FALSE

What happens is that instead of 7, the value of idx/interval is 
6.9999999 with integer part 6. So bins[6] is assigned twice, first 1.2 
then this valuew is overwritten by 1.4 and bins[7] is never written to. 
The same goes with indices 9 and 10.

Avoid this type of indexing. And if possible use the vectorized 
instruction b2 <- breaks.

Hope this helps,

Rui Barradas

Em 06-10-2012 07:14, ?? escreveu:> 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
> }
>
> 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?
> 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.

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