[R] How to replace a column in a data frame with another one with a different size

[Michael Weylandt]

On Jul 8, 2012, at 9:31 AM, Stathis Kamperis <ekamperi at gmail.com> wrote:

> Hello everyone,
> 
> I have a dataframe with 1 column and I'd like to replace that column
> with a moving average.
> Example:
> 
>> library('zoo')
>> mydat <- seq_len(10)
>> mydat
> [1]  1  2  3  4  5  6  7  8  9 10
>> df <- data.frame("V1" = mydat)
>> df
>   V1
> 1   1
> 2   2
> 3   3
> 4   4
> 5   5
> 6   6
> 7   7
> 8   8
> 9   9
> 10 10
>> df[df$V1 <- rollapply(df$V1, 3, mean)]
> Error in `$<-.data.frame`(`*tmp*`, "V1", value = c(2, 3, 4, 5, 6, 7, 8,  :
>  replacement has 8 rows, data has 10
>> 
> 

I'm not sure you need the outer df[...] -- I think you just want

df$V1 <- rollapply(df$V1,3,mean)

However, this will still give you the error message you're seeing because rollapply() only returns 8 values here (you don't get the "endpoints" by default). To get the right number of rows, you want

rollapply(df$V1, 3, mean, fill = NA) # Change NA if desired

which will put NA's on each end and give you a length 10 result, as needed. 

Best, 
Michael

> I could use a temporary variable to store the results of rollapply()
> and then reconstruct the data frame, but I was wondering if there is a
> one-liner that can achieve the same thing.
> 
> Best regards,
> Stathis
> 
> P.S. If you don't mind, cc me at your reply because I'm not subscribed
> to the list (but I will check the archive anyway).
> 
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