Say I have a data.frame:

df <- data.frame(A=c(10,20,30),B=c(11,22,33), C=c(111,222,333))

  A  B  C

1 10 11 111

2 20 22 222

3 30 33 333

If I select two (or more) columns I get a data.frame:

x <- df[,1:2]

   A  B

 1 10 11

 2 20 22

 3 30 33

This is what I want. However, if I select only one column I get a numeric vector:

x <- df[,1]

[1] 1 2 3

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Use drop=FALSE

> x <- df[,1, drop=FALSE]

> x

   A

1 10

2 20

3 30

From the documentation (see ?"[") you can find:

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

    

    

    

    

===================

Omit the ,:

x <- df[1]

   A

1 10

2 20

3 30

From the help page of ?"[":

    Indexing by [ is similar to atomic vectors and selects a list of the specified element(s).

A data frame is a list. The columns are its elements.

 

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REF:

https://stackoverflow.com/questions/21025609/how-do-i-extract-a-single-column-from-a-data-frame-as-a-data-frame