Question

Can someone explain why in the below example the column name for why appears to remain x even though it is clearly named why and can be called as such but not for the column zz?

df<-data.frame(x=1:5,y=1:5)
df$z<-"a"
df$zz<-df$x*df$y
df$why<-df[1]*df[2]

df

df["why"]

Answer 1

Because you’re actually storing a dataframe into why – not a vector.

> str(df)
'data.frame':   5 obs. of  4 variables:
 $ x  : int  1 2 3 4 5
 $ y  : int  1 2 3 4 5
 $ z  : chr  "a" "a" "a" "a" ...
 $ why:'data.frame':    5 obs. of  1 variable:
  ..$ x: int  1 4 9 16 25
> str(df[1]*df[2])
'data.frame':   5 obs. of  1 variable:
 $ x: int  1 4 9 16 25
> str(df[,1] * df[,2])
 int [1:5] 1 4 9 16 25  
> df$why2 <- df[,1]*df[,2]
> df
  x y z  x why2
1 1 1 a  1    1
2 2 2 a  4    4
3 3 3 a  9    9
4 4 4 a 16   16
5 5 5 a 25   25

df[1] returns the first element of df as a sublist. A dataframe is a special type of list which is why you can use this type of indexing to grab columns. However only using the single bracket tells it to return a sublist containing the element of interest (instead of just the element of interest).