r - calculate daily averages for 3d array -

is possible perform daily averaging on 3d array in r?

for example:

i have 3d array of data points on lat/lon grid 2 days.

lat <- 50:51 lon <- 2:3 time <- as.posixct(c('2009-01-01 12:00','2009-01-01 15:00','2009-01-01 17:00','2009-01-02 12:00',                      '2009-01-02 16:00')) j <- array(c(1:6, 11:16, 21:26), c(2,2,5)) dim(j) [1] 2 2 5 

where first dimension refers latitude, second refers longitude, , third refers time (i.e. data @ each lat/lon through time).

how calculate daily averages of these values , return daily averaged 3d array?

the return array should have dimensions of

dim(j) [1] 2 2 2

where time dimensions correspond to:

new_time <- as.posixct(c('2009-01-01','2009-01-02)) 

is possible?

without taking account different days, can perform 3d averaging with:

apply(j, c(1,2), mean) 

but i'm unsure on how perform averaging on selected days.

any appreciated.

ok make sure, in example, there 5 days, right?

if want select days, e.g, 1st, 3rd , 5th, can specify them follows:

> apply(j[,,c(1,3,5)], c(1,2), mean)      [,1]     [,2] [1,]   13 6.333333 [2,]   14 7.333333 

edit

ok, here work around:

library(purrr) library(lubridate) 

if interested in unique days:

# days tx <- day(time) > tx [1] 1 1 1 2 2 

get unique days , count them:

# unique days txu <- unique(tx)  # number of unique days d <- length(unique(tx)) 

create dataframe hold data

# create df days df <- data.frame(day=1:d) 

find indecies of entries corresponding each days , add them in new column

# add column entries corresponding each days  df <- df %>%          mutate(days_entries=map(day, function(x)  which(tx %in% txu[x]) ))  > df   day days_entries 1   1      1, 2, 3 2   2         4, 5 

add means of values each day in new column

df <- df %>%          mutate(day_mean=map(days_entries,                              {function(x) apply(j[,,x], c(1,2), mean)}                             )                ) 

put means in 1 array:

> k <- array( c( sapply(df$day_mean, function(x) x)) , dim = c( 2 ,2 ,d ) )     > dim(k)     [1] 2 2 2     > k     , , 1               [,1]      [,2]     [1,] 6.333333  9.666667     [2,] 7.333333 10.666667      , , 2           [,1] [,2]     [1,]   23   12     [2,]   24   13