[R] Behavior of apply()

[R. Michael Weylandt]

It's because your first example produces answers of varying length so
there's no natural way to coerce it to a matrix. Your second has a
consistent length so the result can be made into a matrix.

Michael

On Thu, Dec 8, 2011 at 4:30 PM, Doran, Harold <HDoran at air.org> wrote:
> Suppose I have the following matrix
>
>> class(cov_50)
> [1] "matrix"
>> cov_50
>          [,1]     [,2]
> [1,] 0.3201992 2.308084
> [2,] 6.7312928 5.719641
>
> I then use the following function via apply and get the desired output, a list
>
> signif <- function(x) which(abs(x) > 1.96)
> apply(cov_50, 1, signif)
>
>> apply(cov_50, 1, signif)
> [[1]]
> [1] 2
>
> [[2]]
> [1] 1 2
>
> However, I can't see why the following occurs
>
>> class(cov_25)
> [1] "matrix"
>> cov_25
>          [,1]     [,2]
> [1,] 12.116106 14.81248
> [2,]  5.492176  4.73138
>
>>  apply(cov_25, 1, signif)
>     [,1] [,2]
> [1,]    1    1
> [2,]    2    2
>
> So, using the same function (signif) on a different object of the same class yields a matrix and *not* a list as desired.
>
> I can't see any reason for the different output.
>
>> sessionInfo()
> R version 2.12.2 (2011-02-25)
> Platform: x86_64-pc-mingw32/x64 (64-bit)
>
> locale:
> [1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252
> [3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C
> [5] LC_TIME=English_United States.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> other attached packages:
> [1] quantreg_4.57 SparseM_0.86
>
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>
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