[R] Error with lapply

[megh]

I have written following codes, with intention to get a list with values
1,2,9,16 :

fn <- function(i) return(i^2)
lapply(1:4, fn, i)

However I got following error :
Error in FUN(1:4[[1L]], ...) : unused argument(s) (1)

Can anyone please tell me what will be the correct code here?

Regards,

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[baptiste auguie]

Hi,

you are feeding lapply "i" as an optional argument, which is passed to
fn() and causes an error. Just use lapply(1:4, fn), or better yet,
sapply,

> fn <- function(i) return(i^2)
> sapply(1:4, fn)
[1] 1 4 9 16

Hope this helps,

baptiste

_____________________________

Baptiste Auguié

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[Marc Schwartz]

on 11/20/2008 10:31 AM megh wrote:
> I have written following codes, with intention to get a list with values
> 1,2,9,16 :
>
> fn <- function(i) return(i^2)
> lapply(1:4, fn, i)
>
> However I got following error :
> Error in FUN(1:4[[1L]], ...) : unused argument(s) (1)
>
> Can anyone please tell me what will be the correct code here?
>
> Regards,

Try this:

fn <- function(i) i^2

> lapply(1:4, fn)
[[1]]
[1] 1

[[2]]
[1] 4

[[3]]
[1] 9

[[4]]
[1] 16

The error message indicates that the argument 'i' that you have in your
initial attempt to use lappply() is unused, because the values 1:4 are
passed to the function's first argument by default already. Thus,
specifying 'i' again is an error, since there is not a second argument
in your function fn().

Note also that 'return()' is not needed, as per the Details in ?return:

"If the end of a function is reached without calling return, the value
of the last evaluated expression is returned."

Note also, that since lapply() effectively uses an internal loop, a
faster "vectorized" approach would be:

> as.list((1:4) ^ 2)
[[1]]
[1] 1

[[2]]
[1] 4

[[3]]
[1] 9

[[4]]
[1] 16

For example, using 100,000 instead of 4:

> system.time(x1 <- lapply(1:100000, fn))
user system elapsed
0.500 0.015 0.600

> system.time(x2 <- as.list((1:100000) ^ 2))
user system elapsed
0.018 0.004 0.039

> identical(x1, x2)
[1] TRUE

HTH,

Marc Schwartz

[Gabor Grothendieck]

lapply already passes the first arg to fn and by specifying the
i (which is undefined -- its only defined within fn) it would be
trying to to pass a second arg to fn yet fn takes only takes
one arg. Try these:

lapply(1:4, fn)
lapply(1:4, "^", 2)

[Bert Gunter]

To be clear, the problem is not the return statement in your function, but
the extra argument, i, in your lapply statement:

lapply(1:4,fn)

works just fine with your original function. You need to read ?lapply more
carefully: fn receives the values of the first argument (1:4) in turn
automatically; other arguments in lapply are passed in as **additional**
arguments to fn, of which there are none here.
Compare:

fn <- function(i,a)i^2+a
lapply(1:4,fn) ## error
lapply(1:4,fn,5)

Cheers,
Bert

Bert Gunter
Genentech

[megh]

I need one more clarification here :

Here I did :

fn <- function(i) return(list(i, i^2))
ss = sapply(1:4, fn)

Here the object "ss" should be a matrix object :
is.matrix(ss)

However I feel it lacks some the matrix object properties. For example the
syntax "min(ss[1,])" generates an error :
"Error in min(ss[1, ]) : invalid 'type' (list) of argument".

What should be the way out? Am I missing something ?

Regards,

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[Peter Dalgaard]

megh wrote:
> I need one more clarification here :
>
> Here I did :
>
> fn <- function(i) return(list(i, i^2))
> ss = sapply(1:4, fn)
>
> Here the object "ss" should be a matrix object :
> is.matrix(ss)
>
> However I feel it lacks some the matrix object properties. For example the
> syntax "min(ss[1,])" generates an error :
> "Error in min(ss[1, ]) : invalid 'type' (list) of argument".
>
> What should be the way out? Am I missing something ?

ss is a dim'ed list object, because fn returns a list. This appears to
be a feature, although rarely used. One point is that you can do

> fn <- function(i) return(list(as.character(i), i^2))
> ss <- sapply(1:4, fn)
> ss
[,1] [,2] [,3] [,4]
[1,] "1" "2" "3" "4"
[2,] 1 4 9 16
> ss[,1]
[[1]]
[1] "1"

[[2]]
[1] 1

I.e., the rows can be of different mode.

The easiest way out is just not to do that, i.e. return a vectore c(i,
i^2) instead.

--
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[Gabor Grothendieck]

Try str(ss) to see what it really looks like. You probably want:

fn <- function(i) c(i, i^2)