R help - Jan 2008 - how to extract data by specific months from a monthly ts object

Hi,

I am trying to extract data from a ts object by month, e.g., extract Jan,
Feb, and Aug data from a monthly ts object. I tried the following but it
didn't work:
> xa=1:50
> ta=ts(xa,start=c(1990,1),frequency=12)
> ta[cycle(ta)==c(1,2)] # this method works but it's not what I want
 [1]  1  2 13 14 25 26 37 38 49 50
> ta[cycle(ta)==c(1,2,8)] # this method doesn't work, unfortunately
 [1]  1  2 13 14 25 26 37 38 49 50
Warning message:
In `==.default`(cycle(ta), c(1, 2, 8)) :
  longer object length is not a multiple of shorter object length

somehow ta[cycle(ta)==c(1,2)] works. But what I really need is for
ta[cycle(ta)==c(1,2,8)] to work, and it doesn't. Does anyone know how could
to do this kinds of data extraction?

Thanks!

-- 
Tom

	[[alternative HTML version deleted]]

tom soyer wrote:
> Hi,
> 
> I am trying to extract data from a ts object by month, e.g., extract Jan,
> Feb, and Aug data from a monthly ts object. I tried the following but it
> didn't work:
> 
>> xa=1:50
>> ta=ts(xa,start=c(1990,1),frequency=12)
>> ta[cycle(ta)==c(1,2)] # this method works but it's not what I want
>  [1]  1  2 13 14 25 26 37 38 49 50
>> ta[cycle(ta)==c(1,2,8)] # this method doesn't work, unfortunately
>  [1]  1  2 13 14 25 26 37 38 49 50
> Warning message:
> In `==.default`(cycle(ta), c(1, 2, 8)) :
>   longer object length is not a multiple of shorter object length
> 
> somehow ta[cycle(ta)==c(1,2)] works. But what I really need is for
> ta[cycle(ta)==c(1,2,8)] to work, and it doesn't. Does anyone know how
could
> to do this kinds of data extraction?
> 
> Thanks!
Tom,

You need to use %in% when you want to test whether one or more values
are in a second set of values.

In this case:
> ta[cycle(ta) %in% c(1, 2, 8)]
 [1]  1  2  8 13 14 20 25 26 32 37 38 44 49 50



BTW, the use of:
> ta[cycle(ta) == c(1, 2)]
 [1]  1  2 13 14 25 26 37 38 49 50

results in a correct response, but largely by chance in this case
because of the way in which the data is stored. You should be using:
> ta[cycle(ta) %in% c(1, 2)]
 [1]  1  2 13 14 25 26 37 38 49 50


Note however, that if we use:
> ta[cycle(ta) == c(1, 3)]
[1]  1 13 25 37 49

we do not get the correct answer, which should be:
> ta[cycle(ta) %in% c(1, 3)]
[1]  1  3 13 15 25 27 37 39 49


See ?"%in%" for more information.

HTH,

Marc Schwartz

Try %in%

ta[ cycle(ta) %in% c(1, 2, 8) ]

cycle(ta) == c(1, 2)

works only in this case since c(1, 2) is recycled to

 cycle(ta) == rep(c(1, 2), length = length(ta))

It would not have worked had your data had started
on an even numbered month.

This also works:

cyc <- cycle(ta)
ta[ cyc == 1 | cyc == 2 | cyc == 8 ]


On Jan 20, 2008 8:09 PM, tom soyer <tom.soyer at gmail.com>
wrote:
> Hi,
>
> I am trying to extract data from a ts object by month, e.g., extract Jan,
> Feb, and Aug data from a monthly ts object. I tried the following but it
> didn't work:
>
> > xa=1:50
> > ta=ts(xa,start=c(1990,1),frequency=12)
> > ta[cycle(ta)==c(1,2)] # this method works but it's not what I want
>  [1]  1  2 13 14 25 26 37 38 49 50
> > ta[cycle(ta)==c(1,2,8)] # this method doesn't work, unfortunately
>  [1]  1  2 13 14 25 26 37 38 49 50
> Warning message:
> In `==.default`(cycle(ta), c(1, 2, 8)) :
>  longer object length is not a multiple of shorter object length
>
> somehow ta[cycle(ta)==c(1,2)] works. But what I really need is for
> ta[cycle(ta)==c(1,2,8)] to work, and it doesn't. Does anyone know how
could
> to do this kinds of data extraction?
>
> Thanks!
>
> --
> Tom
>
>        [[alternative HTML version deleted]]
>
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