CODEWITHSUNDEEP
rlapplyradix
Vyvanse
Vyvanse

Reputation: 1

What does "[" do in R?

So I have a list of length 9 where each element is a dataframe. I wanted to extract specific columns from each dataframe in the most efficient way possible, so I used the below function.

Down <- lapply(tables, "[", 2)

This successfully extracted the information I wanted, but why? What is "[" and how does it satisfy the semantics of lapply?

Upvotes: 0

Views: 59

Answers (1)

SteveM
SteveM

Reputation: 2301

To augment Ritchie Sacramento's very nice explanation, you can also access the dataframe columns in a list using conventional notation:

cars1 <- mtcars
cars2 <- cars1
cars3 <- cars2
tables <- list(cars1, cars2, cars3)
lapply(tables, function(x) x$cyl)
[[1]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

[[2]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

[[3]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4
lapply(tables, function(x) x[, 2])
[[1]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

[[2]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

[[3]]
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

Upvotes: 1

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