How to get the first row of a dataframe with names when there is only one column? [duplicate]

Question

I'm facing an unexpected behavior with base R.

I want to retrieve the first row of a dataframe with its colnames as a vector, and this common method works perfectly for most cases.

df = data.frame(A=c(12,13), B=c(24,25))
unlist(df[1,]) #class=numeric
#    A  B
#   12 24

But when the dataframe is only composed of one column, the result is coerced to an unnamed vector:

df = data.frame(A=c(12,13))
unlist(df[1,]) #class=numeric too
# 12

How to keep the name is the second case?

Answer 1

When extracting a dataframe using [ by default drop argument is TRUE.

From ?Extract

drop - If TRUE the result is coerced to the lowest possible dimension.

Also you can check the class for both the dataframe after extracting the row.

df1 = data.frame(A=c(12,13), B=c(24,25))
df2 = data.frame(A=c(12,13))

class(df1[1, ])
#[1] "data.frame"
class(df2[1, ])
#[1] "numeric"

As we can see df2 is coerced to a vector. Using drop = FALSE will keep it as dataframe and not drop the dimensions.

df2[1,, drop = FALSE]
#   A
#1 12

class(df[1,, drop = FALSE])
#[1] "data.frame"