compounding precipitation based on whether falls within a day

Thanks for the reprex. Wouldn't have bothered without it.

The following is I believe **almost** what you want. It seems a bit clumsy
to me, so others may provide you something neater. But anyway...

## Convert POSIXct vector to dates
## There are 22 different days, not 21
date <- as.Date(prec_idx)

## Sum results by date at each i,j of the last 2 array dimensions
z <- lapply(unique(date),function(d)
   apply(prec[date==d,,],2:3,sum)
   )

## This gives a list with 22 3x4 matrices of sums.
## Convert to 3x4x22 array with

prec_daily <- array(unlist(z),dim=c(3,4,22))

## This is the **almost** part. You can use the aperm() function to reshape
the array if you like. I leave those pleasures to you.

HTH.

Cheers,
Bert



Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Wed, Sep 13, 2017 at 9:52 AM, Morway, Eric <emorway at usgs.gov> wrote:

Using the small reproducible example below, I'm wondering how best to
complete the following task:

In the small reproducible example below, the 3D array prec has indexes

that

correspond to time, x, y (i.e., prec[time, x, y]).  In this case, the

time

index is hours since some predefined start time.  I'd like to add up all
the time indexes in 'prec' based on whether or not the corresponding

hours

fall within a given day.  So, at the end of the small example below,

there

are two variables that I'm left with, prec_idx (an hourly sequence from
beg_time to end_time) whose length is equal to the first index (the time
index) of the 3D array of precipitation called prec.  That is, I'd like

to

get a 3D array called prec*_daily* that has dimension prec*_daily*[21, 3,
4],
where 21 is the number of days and the value in say prec*_daily*[1,x,y]

is

equal to prec[1,x,y] + prec[2,x,y] + ... + prec[24,x,y]


ndays <- 21
base_time <- as.character('2001-12-31T23:00:00Z')
hrs_since_base <- 1

# adding an extra second to the end b/c I'm paranoid about the midnight
time stamp not being explicit
beg_time <- as.POSIXct(base_time, format = "%Y-%m-%dT%H:%M:%SZ") +
(hrs_since_base * 60 * 60) + 1

max_hr_since <- 24 * ndays
end_time <- as.POSIXct(base_time, format = "%Y-%m-%dT%H:%M:%SZ") +
(max_hr_since * 60 * 60) + 1

prec_idx <- seq(beg_time, end_time, by='hour')

prec <- array(abs(rnorm((24*ndays) * 3 * 4)) , dim=c(24*ndays, 3, 4))

length(prec_idx)
# 504
dim(prec)
# 504   3   4

# How do I aggregate prec to get daily sums of precipitation based on the
prec_idx array?

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