R: How can I use apply on rows of a data.frame and get out $column_name?
I'm trying to access $a using the following example:
df<-data.frame(a=c("x","x","y","y"),b=c(1,2,3,4))
> df
a b
1 x 1
2 x 2
3 y 3
4 y 4
test_fun <- function (data.frame_in) {
print (data.frame_in[1])
}
I can now access $a if I use an index for the first column:
apply(df, 1, test_fun)
a
"x"
a
"x"
a
"y"
a
"y"
[1] "x" "x" "y" "y"
But I cannot access column $a with the $ notation: error: "$ operator is invalid for atomic vectors"
test_fun_2 <- function (data.frame_in) {
print (data.frame_in$a)
}
>apply(df, 1, test_fun_2)
Error in开发者_Go百科 data.frame_in$a : $ operator is invalid for atomic vectors
Is this not possible?
You could use adply
from the plyr package instead:
library(plyr)
adply(df, 1, function (data.frame_in) print(data.frame_in$a))
because data.frame.in
is not a data.frame:
apply(df, 1, function(v){print(class(v))})
but you can access the named elements with:
test_fun_2 <- function (data.frame_in) {
+ print (data.frame_in['a'])}
Because apply
changes the data type in your function:
> apply(df, 1, class)
[1] "character" "character" "character" "character"
> apply(df, 1, colnames)
NULL
Since there are no column names, you can't reference the values with the $
operator.
From the apply
documentation:
If X is not an array but has a dimension attribute, apply attempts to coerce it to an array via as.matrix if it is two-dimensional (e.g., data frames) or via as.array.
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