> x <- c(1,2,3) # a vector of numbers, such as snowfallsum
> (cx <- cumsum(x)) # a vector of cumulative sums.
1 3 6
> i <- 1 # The starting point.
> j <- 2 # The ending point.
> cx[j] - cx[i-1] # sum of x[i] + ... + x[j]
ERROR!
> cx <- c(0, cx) # Oops, we need this step.
> cx[j+1] - cx[i]
So using c(0,cumsum(x)) you take O(#x) time and O(#x) space and get a data
structure that will answer any (i,j) -> x[i]+...+x[j] query in constant time.
Let's now suppose you fix delta = 3 (days) and some threshold:
indices <- (delta + 1):length(cx)
which(cx[indices] - cx[indices - delta] > threshold)
On Fri, 30 Jul 2021 at 19:24, Stefano Sofia
<stefano.so...@regione.marche.it> wrote:
>
> Dear R users,
> I have a data frame with daily snow cumulates (these quantities are known as
> "hn" and are expressed in cm), from the 1st of December to the 30th of April,
> for more than twenty years.
>
> I would need to find days when the sum of a given short interval (I might
> choose two consecutive days, three consecutive days or something like that)
> is higher than a threshold (it might be 80 cm, or 100 cm).
>
> I am trying with rle, but I really struggle to find an efficient algorithm.
> Could somebody help me with some hints?
>
> Thank you for your attention and your help
> Stefano
>
>
> init_day <- as.POSIXct("2018-02-01", format="%Y-%m-%d", tz="Etc/GMT-1")
> fin_day <- as.POSIXct("2018-02-20", format="%Y-%m-%d", tz="Etc/GMT-1")
> mydf <- data.frame(data_POSIX=seq(init_day, fin_day, by="1 day"))
> mydf$hn <- c(30, 0, 10, 50, NA, 40, 70, 0, 0, 0 , NA, 10, 50, 30, 30, 10, 0,
> 0, 90, 0)
>
> - if I choose a threshold of 100 cm in two days, I should get the 6th of
> February;
> - if I choose a threshold of 80 cm in two days I should get the 6th and the
> 13th of February, but not the 19th of February because this is a single day;
> - f I choose a threshold of 100 cm in four days, I should get the 12th of
> February.
>
> (oo)
> --oOO--( )--OOo--------------------------------------
> Stefano Sofia PhD
> Civil Protection - Marche Region - Italy
> Meteo Section
> Snow Section
> Via del Colle Ameno 5
> 60126 Torrette di Ancona, Ancona (AN)
> Uff: +39 071 806 7743
> E-mail: stefano.so...@regione.marche.it
> ---Oo---------oO----------------------------------------
>
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