R/cutoff.R

In jknowles/eeptools: Convenience Functions for Education Data

##' A function to calculate thresholds of cumulative sums in a vector.
##' @description This function tells us how far we have to
##' go before reaching a cutoff in a variable
##' by sorting the vector, then finding how far 
##' to go. Note that the cutoff is expressed in 
##' percentage terms (fixed cumulative sum)
##' @param x a numeric vector, missing values are allowed
##' @param cutoff a user defined numeric value to stop the cutoff specified as 
##' a proportion 0 to 1
##' @param na.rm logical, should missing values be excluded?
##' @details Calculates the distance through a numeric vector before a certain 
##' proportion of the sum is reached by sorting the vector and calculating the 
##' cumulative proportion of each element
##' @return An integer for the minimum number of elements necessary to reach cutoff
##' @author Jared E. Knowles
##' @export
##' @examples
##' # for vector
##' a <- rnorm(100, mean=6, sd=1)
##' cutoff(a, .7) #return minimum number of elements to account 70 percent of total
##' 
cutoff <- function(x, cutoff, na.rm = TRUE){
  if(cutoff <=0 | cutoff > 1 | is.na(cutoff)){
    stop("Cutoff value must be a numeric between 0 and 1 and nonmissing")
  }
  if(na.rm){
    x <- x[order(-x, na.last = TRUE)]
    xb <- cumsum(x)
    xc <- xb/sum(x, na.rm=TRUE)
    length(xc[xc < cutoff])
    
  } else{
    x <- x[order(-x)]
    xb <- cumsum(x)
    xc <- xb/sum(x, na.rm = FALSE)
    xc <- na.omit(xc)
    if(length(xc) == 0){
      NA
    } else {
      length(xc[xc < cutoff])
    }
  }
}