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Anyone know an R function that will return the number of items in a list?

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3 Answers

up vote 36 down vote accepted


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I spent ages trying to figure this out but it is simple! You can use length(·). length(mylist) will tell you the number of objects mylist contains.

... and just realised someone had already answered this- sorry!

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It's still an answer, and still a valid one at that, so +1. –  PaulHurleyuk Jun 14 '10 at 18:04
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Advice for R newcomers like me : beware, the following is a list of a single object :

> mylist <- list (1:10)
> length (mylist)
[1] 1

In such a case you are not looking for the length of the list, but of its first element :

> length (mylist[[1]])
[1] 10

This is a "true" list :

> mylist <- list(1:10, rnorm(25), letters[1:3])
> length (mylist)
[1] 3

Also, it seems that R considers a data.frame as a list :

> df <- data.frame (matrix(0, ncol = 30, nrow = 2))
> typeof (df)
[1] "list"

In such a case you may be interested in ncol() and nrow() rather than length() :

> ncol (df)
[1] 30
> nrow (df)
[1] 2

Though length() will also work (but it's a trick when your data.frame has only one column) :

> length (df)
[1] 30
> length (df[[1]])
[1] 2
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