[R] Which() missing a number

[Duncan Murdoch]

On 18/03/2014 7:14 AM, Thomas wrote:
> Does anyone know why this is happening? Which() is picking up the
> indices of the numbers 13 and 15 in neilist, but it's missing out the
> 13 at index 6.
>
> Thank you,
>
> Thomas Chesney
>
>   > neilist
>    [1] 13 15 28 29 30 13 14 15 28 30 43 44 45 14 15 29 44 45
>
>   > pfriends
> [1] 13 15
>
>   > which(neilist==pfriends)
> [1] 1 2 8

Two problems with your code.  The first is recycling.  Your expression 
neilist == pfriends compares neilist[1] with pfriends[1], then 
neilist[2] with pfriends[2], then neilist[3] with pfriends[1], etc.

The second problem is that pfriends is not an integer vector, so it may 
actually contain 13.00000000001 and 14.9999999999
(or similar numbers that round to 13 and 15 when printed), and those 
won't test equal to 13 and 15.

Duncan Murodch
>
>   > neilist[6]
> [1] 13
>
>   > str(neilist)
>    int [1:18] 13 15 28 29 30 13 14 15 28 30 ...
>
>   > str(pfriends)
>    num [1:2] 13 15
>
>   > sessionInfo()
> R version 2.15.3 (2013-03-01)
> Platform: i386-apple-darwin9.8.0/i386 (32-bit)
>
> locale:
> [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
>
> attached base packages:
> [1] splines   grid      stats     graphics  grDevices utils
> datasets  methods
> [9] base
>
> other attached packages:
>    [1] spdep_0.5-56    coda_0.16-1     deldir_0.0-21   maptools_0.8-23
> foreign_0.8-52
>    [6] nlme_3.1-108    MASS_7.3-23     Matrix_1.0-11   lattice_0.20-13
> boot_1.3-7
> [11] sp_1.0-8        igraph_0.6.5-1
>
> loaded via a namespace (and not attached):
> [1] LearnBayes_2.12 tools_2.15.3
>    
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