[R] Elementary sapply question
Sundar Dorai-Raj
sundar.dorai-raj at PDF.COM
Mon Jun 21 21:10:00 CEST 2004
Brian Desany wrote:
>
> Looking in ?mapply, I executed the examples:
>
>
>>mapply(rep, 1:4, 4:1)
>
> [[1]]
> [1] 1 1 1 1
>
> [[2]]
> [1] 2 2 2
>
> [[3]]
> [1] 3 3
>
> [[4]]
> [1] 4
>
>
>>mapply(rep, times=1:4, x=4:1)
>
> [[1]]
> [1] 4
>
> [[2]]
> [1] 3 3
>
> [[3]]
> [1] 2 2 2
>
> [[4]]
> [1] 1 1 1 1
>
>
> I can guess that because these are 2 examples, it is no surprise that the
> results are different. Why is this? If ?mapply is giving me a clue, I'm not
> seeing it.
>
> Thanks,
> -Brian.
>
The clue you seek is in ?rep which has "x" then "times" as its
arguments. The first example is equivalent to mapply(rep, x = 1:4, times
= 4:1) whereas the second example you were more explicit about the
arguments order.
--sundar
>
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Liaw, Andy
> Sent: Monday, June 21, 2004 11:50 AM
> To: 'Ajay Shah'; r-help
> Subject: RE: [R] Elementary sapply question
>
> At least two ways:
>
> 1. Use extra argument in the function being sapply()'ed; e.g.,
>
>
>>f <- function(x, y) x*x + y*y
>>x <- 3:5
>>sapply(x, f, 3)
>
> [1] 18 25 34
>
> [See the "..." argument in ?sapply.]
>
> 2. More generally, if both x and y are vectors (of the same length), then
> you can use mapply(); e.g.,
>
>
>>x <- 1:3
>>y <- 3:5
>>pyth <- function(x, y) x*x + y*y
>>mapply(pyth, x, y)
>
> [1] 10 20 34
>
> HTH,
> Andy
>
>
>>From: Ajay Shah
>>
>>I am discovering sapply! :-) Could you please help me with a very
>>elementary question?
>>
>>Here is what I know. The following two programs generate the
>>same answer.
>>
>>--------------------------------+-----------------------------
>>-----------
>> Loops version | sapply version
>>--------------------------------+-----------------------------
>>-----------
>> |
>>f <- function(x) { | f <- function(x) {
>> return(x*x) | return(x*x)
>>} | }
>>values = c(2,4,8) | values = c(2,4,8)
>>answers=numeric(3) | answers = sapply(values, f)
>>for (i in 1:3) { |
>> answers[i] = f(values[i]) |
>>} |
>>
>>and this is cool!
>>
>>My problem is this. Suppose I have:
>> pythagorean <- function(x, y) {
>> return(x*x + y*y)
>> }
>>
>>then how do I utilise sapply to replace
>> fixed.x = 3
>> y.values = c(3,4,5)
>> answers=numeric(3)
>> for (i in 1:3) {
>> answers[i] = pythagorean(fixed.x, y.values[i])
>> }
>>
>>?
>>
>>I have read the sapply docs, and don't know how to tell him that the
>>list values that he'll iterate over "fit in" as y.values[i].
>>
>>--
>>Ajay Shah Consultant
>>ajayshah at mayin.org Department of Economic Affairs
>>http://www.mayin.org/ajayshah Ministry of Finance, New Delhi
>>
>>______________________________________________
>>R-help at stat.math.ethz.ch mailing list
>>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
>>PLEASE do read the posting guide!
>>http://www.R-project.org/posting-guide.html
>>
>>
>
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
> http://www.R-project.org/posting-guide.html
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
More information about the R-help
mailing list