[R] Take average of previous weeks

[Gabor Grothendieck]

There is no  `value` column in the `dput` output shown in the
question so using `tmin` instead note that the `width=` argument
of `rollapply` can be a list containing a vector of offsets (-1 is prior
value, -2 is value before that, etc.) and that we can use `rollapplyr`
with an `r` on the end to get right alignment.  See `?rollapply`

  library(dplyr)
  library(zoo)

  roll <- function(x, k) rollapplyr(x, list(-seq(1:k)), mean, fill = NA)
  df %>%
      group_by(citycode) %>%
      mutate(mean2 = roll(tmin, 2), mean3 = roll(tmin, 3), mean4 =
roll(tmin, 4)) %>%
      ungroup

(The code above has been indented 2 spaces so you can
identify inadvertent line wrapping by the email system.)


On Sun, Mar 25, 2018 at 10:48 AM, Miluji Sb <milujisb at gmail.com> wrote:
> Dear all,
>
> I have weekly data by city (variable citycode). I would like to take the
> average of the previous two, three, four weeks (without the current week)
> of the variable called value.
>
> This is what I have tried to compute the average of the two previous weeks;
>
> df = df %>%
>   mutate(value.lag1 = lag(value, n = 1)) %>%
>   mutate(value .2.previous = rollapply(data = value.lag1,
>                                      width = 2,
>                                      FUN = mean,
>                                      align = "right",
>                                      fill = NA,
>                                      na.rm = T))
>
> I crated the lag of the variable first and then attempted to compute the
> average but this does not seem to to what I want. What I am doing wrong?
> Any help will be appreciated. The data is below. Thank you.
>
> Sincerely,
>
> Milu
>
> dput(droplevels(head(df, 10)))
> structure(list(year = c(1970L, 1970L, 1970L, 1970L, 1970L, 1970L,
> 1970L, 1970L, 1970L, 1970L), citycode = c(1L, 1L, 1L, 1L, 1L,
> 1L, 1L, 1L, 1L, 1L), month = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L,
> 2L, 3L), week = c(1L, 2L, 3L, 4L, 5L, 5L, 6L, 7L, 8L, 9L), date =
> structure(c(1L,
> 2L, 3L, 4L, 5L, 5L, 6L, 7L, 8L, 9L), .Label = c("1970-01-10",
> "1970-01-17", "1970-01-24", "1970-01-31", "1970-02-07", "1970-02-14",
> "1970-02-21", "1970-02-28", "1970-03-07"), class = "factor"),
>     value = c(-15.035, -20.478, -22.245, -23.576, -8.84099999999995,
>     -18.497, -13.892, -18.974, -15.919, -13.576)), .Names = c("year",
> "citycode", "month", "week", "date", "tmin"), row.names = c(NA,
> 10L), class = "data.frame")
>
>         [[alternative HTML version deleted]]
>
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