从向量列表的元素中提取向量

Question

I have some json data [{a:10, b:123,c:4.5},{a:2,b:5,c:33}] and so on that I read into R via json_data <- fromJSON(paste(json_file, collapse="")) (json_file is the input url). So far so fine. Now I would like to create vectors from this input which fromJSON has converted into a List of vectors where the vectors have components a,b,c.

Is there a better way than looping over the input list and doing this manually by concatenating the individual vector components on the new target vector(s)?

Answer 1

If you have a list like this:

l <- list(c(a=10, b=123, c=4.5),c(a=2,b=5,c=33))

You could just do something like the following:

df <- data.frame(do.call(rbind, l))
#    a   b    c
# 1 10 123  4.5
# 2  2   5 33.0
as.list(df)
# $a
# [1] 10  2
# $b
# [1] 123   5
# $c
# [1]  4.5 33.0

(The do.call(rbind, X) construct is handy, allowing you to rbind together the elements of a list of arbitrary length. You can then slice and dice the resulting matrix as you see fit --- I just converted it to a data.frame and then to a list to show a couple of possibilities.)