[R] Extract elements from objects in a list

[Bill.Venables at csiro.au]

> x <- lapply(1:100, function(x) summary(runif(100)))

> head(x, 4)
[[1]]
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
0.02922 0.38330 0.58120 0.58230 0.83430 0.99870 

[[2]]
    Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
0.004903 0.281400 0.478900 0.497100 0.729900 0.990700 

[[3]]
    Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
0.002561 0.283100 0.567400 0.516500 0.736800 0.986700 

[[4]]
    Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
0.004827 0.264000 0.499500 0.503200 0.732300 0.998500 

> sapply(x, "[", 3)[1:4]
Median Median Median Median 
0.5812 0.4789 0.5674 0.4995 
>  

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Jeremy Miles
Sent: Wednesday, 29 June 2011 9:23 AM
To: r-help
Subject: [R] Extract elements from objects in a list

Hi All,

I want to extract elements of elements in a list.

Here's an example of what I mean:

If I create a list:

x <- as.list(100)
for(loop in c(1:100)) {
	x[[loop]] <- summary(runif(100))
	}


> head(x)
[[1]]
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
0.02271 0.25260 0.58130 0.52120 0.77270 0.99670

[[2]]
    Min.  1st Qu.   Median     Mean  3rd Qu.     Max.
0.006796 0.259700 0.528100 0.515500 0.781900 0.993100

[[3]]
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
0.00927 0.22800 0.40780 0.46410 0.69460 0.98780

I want to extract (say) the medians as a vector.  This would be:
x[[1]][[3]]
x[[2]][[3]]
x[[3]][[3]]

I thought there would be a way of doing this with something like
apply(), but I cannot work it out.  Is there a way of doing this
without a loop?  Thanks,

Jeremy

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