vector is not assigned correctly in for loop

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

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