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df <- data.frame( x = 1:3,
y = c("a", "b", "c"),
stringsAsFactors = FALSE)
typeof(df)

## [1] "list"

df[[1]] 

## [1] 1 2 3

df[1,]

##   x y
## 1 1 a

df[1]

##   x
## 1 1
## 2 2
## 3 3

df[2]

##   y
## 1 a
## 2 b
## 3 c

df[[2]]

## [1] "a" "b" "c"

ba <- data.frame(x=c(1,2,NA),y=1:3)
ba[1]

##    x
## 1  1
## 2  2
## 3 NA