[R] lapply and runif issue?

[Ista Zahn]

Hi Bert,

On Tue, Nov 14, 2017 at 8:11 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
> Could someone please explain the following? I did check bug reports, but
> did not recognize the issue there. I am reluctant to call it a bug, as it
> is much more likely my misunderstanding. Ergo my request for clarification:
>
> ## As expected:
>
>> lapply(1:3, rnorm, n = 3)
> [[1]]
> [1] 2.481575 1.998182 1.312786
>
> [[2]]
> [1] 2.858383 1.827863 1.699015
>
> [[3]]
> [1] 1.821910 2.530091 3.995677
>

Exactly what expectation do you imagine the above is consistent with? Does

> lapply(100*(1:3), rnorm, n = 3)
[[1]]
[1] 100.35425  99.29429  98.69429

[[2]]
[1] 198.2963 201.1031 201.1077

[[3]]
[1] 299.7012 298.3700 298.0684

change your assessment?

>
> ## Unexpected by me:
>
>> lapply(1:3, runif, n = 3)
> [[1]]
> [1] 1 1 1
>
> [[2]]
> [1] NaN NaN NaN
>
> [[3]]
> [1] NaN NaN NaN
>
> Warning messages:
> 1: In FUN(X[[i]], ...) : NAs produced
> 2: In FUN(X[[i]], ...) : NAs produced

The first argument to runif is named 'n'. Thus,

lapply(1:3, runif)

means roughly

list(runif(n = 1), runif(n = 2), runif(n = 3))

But you specify than lapply(1:3, runif, n = 3). Since the first
argument ('n') is already specified, the X values from lapply get
"pushed" to the second argument. That is,

lapply(1:3, runif, n = 3)

means roughly

list(runif(n = 3, min = 1), runif(n = 3, min = 2), runif(n = 3, min = 3))

Note that this is exactly the same thing that happens with

lapply(1:3, rnorm, n = 3), though it becomes more obvious with

lapply(100*(1:3), rnorm, n = 3)

That is,

lapply(1:3, rnorm, n = 3)

means roughly

list(rnorm(n = 3, mean = 1), rnorm(n = 3, mean = 2), rnorm(n = 3, mean = 3))

>
>
> ## But note, as expected:
>
>> lapply(1:3, function(x)runif(3))
> [[1]]
> [1] 0.2950459 0.8490556 0.4303680
>
> [[2]]
> [1] 0.5961144 0.5330914 0.2363679
>
> [[3]]
> [1] 0.8079495 0.1431838 0.3671915

Sure, because you never use x in the body of your anonymous function.

As a final note, what you seem to expect can be achieved with

replicate(3, rnorm(n = 3), simplify = FALSE)

and

replicate(3, runif(n = 3), simplify = FALSE)


Best,
Ista

>
>
>
> Many thanks for any clarification.
>
> -- Bert
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.