[R] Filtering data

[John Fox]

At 08:39 PM 11/6/2001 -0700, Matt Pocernich wrote:

>I am having difficulty filtering data.  I am working with flow data
>collected at a stream gage.  For each record, I have a date and flow
>value.  I have filtered this data to only include days when flow values
>exceed a given threshold.
>
>Here is my problem.  Within this subset of data, I often have several
>consecutive days above the threshold.  From this group of days, I wish to
>select the record (both date and flow) containing the maximum flow.  If an
>exceedance is isolated ( the preceeding and succeeding day is below the
>threshold) I also wish to select that record.
>
>For example from the data set
>
>Day     Flow
>
>1       10
>4       13
>5       20
>6       15
>9       13
>
>I would like the 1st, 3rd and 5th record filered.
>
>Any ideas on how I would write such and algorithm would be appreciated.

Dear Matt,

Here's a function that does what you want with loops. Perhaps someone else 
will produce a more elegant solution:

     > select.rows <- function(data) {
     +     indices <- data[,1]
     +     values <- data[,2]
     +     n <- length(indices)
     +     if (n == 0) stop('no data')
     +     if (n == 1) return(data)
     +     selection <- rep(0, n)  # so as not to grow the selection vector
     +     current <- 1
     +     number <- 1
     +     for (i in 2:n){
     +         if (indices[i] == 1 + indices[i - 1]){
     +             if (values[i] > values[current]) current <- i
     +             }
     +         else {
     +             selection[number] <- current
     +             number <- number + 1
     +             current <- i
     +             }
     +         }
     +     selection[number] <- current
     +     data[selection,]
     +     }
     >
     > data <- matrix(c(1,4,5,6,9, 10,13,20,15,13), 5, 2)
     > colnames(data) <- c('Day', 'Flow')
     > select.rows(data)
         Day Flow
     [1,]   1   10
     [2,]   5   20
     [3,]   9   13


I hope that this isn't too inefficient.

John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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