On Wed, Jul 8, 2009 at 4:22 AM, Petr PIKAL <petr.pi...@precheza.cz> wrote:
> Hi
>
> r-help-boun...@r-project.org napsal dne 07.07.2009 19:06:17:
>
> > Hi,
> >
> > I am confused about how to select elements from a list.
> >
> > I'm trying to select all rows of a table 'crossRsorted' such that the
> > mean of a related vector is > 0. The related vector is accessible as
> > a list element l[[i]] where i is the row index.
> >
> > I thought this would work:
> >
> > > crossRsorted[mean(q[[ crossRsorted[,1] ]], na.rm = TRUE) > 0, ]
> > Error in q[[crossRsorted[, 1]]] : no such index at level 2
>
> Strange, I got completely different error. Couldn't be that only ***you***
> have crossRsorted?
Ok, fair enough. I'm still thinking of a language in which the meaning of
operators is apparent from their syntactical structure - probably need to
read more of "The R Inferno".
Here's an example that reproduces the problem, I think (though the error
message is slightly different):
> q<-list()
> q[[105]] <- as.numeric(c(0,0,1))
> q[[104]] <- as.numeric(c(1,1,1))
> q[[10]] <- as.integer(c(3,3,1))
> crossRsorted <- data.frame(i = c(105, 104,10))
> q[[ crossRsorted[,1] ]]
Error in q[[crossRsorted[, 1]]] : recursive indexing failed at level 2
Even though the list 'q' has component 105, 104, and 10, the expression q[[
crossRsorted[,1] ]] causes an error.
Why?
And why does this work:
> q[[c(105)]]
[1] 0 0 1
but not this:
> q[[c(105,104)]]
Error in q[[c(105, 104)]] : subscript out of bounds
> q[[c(105,104,10)]]
Error in q[[c(105, 104, 10)]] : recursive indexing failed at level 2
even though q[[105]], q[[104], and q[[10]] are perfectly legitimate items?
Coming back to my question, how to I express "select all i in a vector for
which q[[i]] meets some predicate, where q is a list?"
Thank you for the tip about 'str' - that's the typeof function I've been
craving. (I thought 'attributes' or 'summary' was all there was...)
The output for str in the original problem:
In my original problem, the output is:
> str(crossRsorted)
'data.frame': 15750 obs. of 5 variables:
$ i : num 105 104 9 8 10 9 98 97 10 8 ...
$ j : num 104 105 8 9 9 10 97 98 8 10 ...
$ r : num -0.973 -0.973 0.764 0.764 0.744 ...
$ n : num 135 135 138 138 138 138 136 136 138 138 ...
$ pvalue: num 2.90e-86 2.90e-86 0.00 0.00 0.00 ...
and
> str(q)
List of 165
$ : NULL
$ : NULL
$ : NULL
$ : NULL
$ :'data.frame': 138 obs. of 1 variable:
..$ howdidyouhear: chr [1:138] "0 3" "3" "3" "3" ...
$ :'data.frame': 138 obs. of 1 variable:
..$ approximatelywhendidyoustart: int [1:138] 0 0 5 1 5 5 1 2 6 0 ...
[ main body deleted ]
$ :'data.frame': 138 obs. of 1 variable:
..$ revisiontestpage: num [1:138] 0 0 0 0 0 0 0 0 0 0 ...
basically - a heterogeneous sparse list of NULL and data.frames of types
character, num, and int.
However - by construction - the q[[i]] for i in crossRsorted[,1] are all
non-NULL, as in my small reproducible example above.
with data frame and list
>
> df1[sapply(list1,mean)>0,]
>
> selects rows of df1 which correspond to list elements with mean >0
>
I can't run 'sapply' over my list because sapply will also iterate over the
NULLs. I want to access only those components in list1 that occur in
df1[1,].
- Godmar
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