[R] problem with lists...

[Chuck Cleland]

A.R. Ebrahimi wrote:
> Hi guys,
> I am new to R, so sorry if my problem seems trivial.
> 
> Sometimes I encounter some lists, which I cannot index their components 
> with [ . ]
> For instance the prcomp() function returns a 'prcomp' object whose 
> components are some 'lists'. the second component is a list that 
> comtains the following:
> 
>  > mylist <- churn[2]
> 
>  > class(mylist)
> [1] "list"
> 
>  > mylist
> 
>                             PC1                    PC2                
> PC3                    PC4                    PC5                    PC6
> Account_Length    -8.930570e-03    2.383719e-03    -2.414908e-02    
> -1.004704e-01    9.941468e-01    2.867606e-02
> Area_Code            1.543767e-02    -1.710566e-02    -6.258129e-03    
> 9.945271e-01    1.008478e-01    -8.890155e-03
> Intl_Plan                -2.627796e-04    -1.809011e-04    
> -9.542965e-05    3.230055e-04    2.148520e-04    -1.622996e-05
> VMail_Plan            4.879640e-06    -1.339026e-04    1.639344e-04    
> -1.367628e-05    2.897648e-05    5.548126e-04
> VMail_Message    -4.174810e-04    -2.969518e-03    4.791422e-03    
> -5.643246e-04    -1.752559e-03    1.433728e-02
> Day_Mins            -9.845791e-01    2.049519e-02    -4.038624e-02    
> 1.619195e-02    -8.308484e-03    4.217712e-04
> Day_Calls            -2.810874e-03    1.428161e-02    2.837638e-03    
> -7.647877e-03    2.532260e-02    -9.431350e-01
> Day_Charge        -1.673781e-01    3.483165e-03    -6.864827e-03    
> 2.752626e-03    -1.412836e-03    7.042886e-05
> Eve_Mins            -4.029002e-02    -8.265860e-01    5.544912e-01    
> -1.166711e-02    1.426697e-02    -1.087738e-02
> 
> It look more like a matrix!
> but I cannot access individual items in this list.
> 
>  > mylist[1]
> 'returns the whole think'
> 
>  > mylist[1,1]
> Error in mylist[1, 1] : incorrect number of dimensions
> 
>  > mylist[2]
> $<NA>
> NULL
> 
> 
> Would you please help me on this

  Below are some possibilities.  What exactly are you trying to extract
for the prcomp results?

> pc <- prcomp(USArrests, scale = TRUE)

> str(pc)
List of 5
 $ sdev    : num [1:4] 1.575 0.995 0.597 0.416
 $ rotation: num [1:4, 1:4] -0.536 -0.583 -0.278 -0.543  0.418 ...
  ..- attr(*, "dimnames")=List of 2
  .. ..$ : chr [1:4] "Murder" "Assault" "UrbanPop" "Rape"
  .. ..$ : chr [1:4] "PC1" "PC2" "PC3" "PC4"
 $ center  : Named num [1:4]   7.79 170.76  65.54  21.23
  ..- attr(*, "names")= chr [1:4] "Murder" "Assault" "UrbanPop" "Rape"
 $ scale   : Named num [1:4]  4.36 83.34 14.47  9.37
  ..- attr(*, "names")= chr [1:4] "Murder" "Assault" "UrbanPop" "Rape"
 $ x       : num [1:50, 1:4] -0.976 -1.931 -1.745  0.140 -2.499 ...
  ..- attr(*, "dimnames")=List of 2
  .. ..$ : chr [1:50] "Alabama" "Alaska" "Arizona" "Arkansas" ...
  .. ..$ : chr [1:4] "PC1" "PC2" "PC3" "PC4"
 - attr(*, "class")= chr "prcomp"

> pc[[2]]
                PC1        PC2        PC3         PC4
Murder   -0.5358995  0.4181809 -0.3412327  0.64922780
Assault  -0.5831836  0.1879856 -0.2681484 -0.74340748
UrbanPop -0.2781909 -0.8728062 -0.3780158  0.13387773
Rape     -0.5434321 -0.1673186  0.8177779  0.08902432

> pc[[2]][,1:2]
                PC1        PC2
Murder   -0.5358995  0.4181809
Assault  -0.5831836  0.1879856
UrbanPop -0.2781909 -0.8728062
Rape     -0.5434321 -0.1673186

> pc$rotation[,1:2]
                PC1        PC2
Murder   -0.5358995  0.4181809
Assault  -0.5831836  0.1879856
UrbanPop -0.2781909 -0.8728062
Rape     -0.5434321 -0.1673186

> Regards
> 
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> and provide commented, minimal, self-contained, reproducible code.

-- 
Chuck Cleland, Ph.D.
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