R: Count number of objects in list

R: Count number of objects in list

length(x)

Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined.

lengths(x)

Get the length of each element of a list or atomic vector (is.atomic) as an integer or numeric vector.

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 its a trick when your data.frame has only one column) :

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

R: Count number of objects in list

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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