[R] vector is not assigned correctly in for loop

[Rui Barradas]

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, 周果 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]]
>
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