pwlin.R

#Implements a piecewise linear function
#
#Args
#x          A vector of values to evaluate the function at
#xbp        A vector of strictly increasing values, the x-axis breakpoint values of the function
#ybp        A vector of the same length, the values of the function at the breakpoints
#
#Output
#A vector of values, NA for indices for which x was outside the range min(xbp) to 
#max(xbp)

pwlin<-function(x,xbp,ybp)
{
  #some small amount of error checking
  if (any(diff(xbp)<=0))
  {
    stop("Error in pwlin: xbp values must be strictly increasing")
  }
  
  #receptacle for results
  res<-NA*numeric(length(x))
  
  #now map each of the values of x that can be mapped
  inds<-which(x>=xbp[1] & x<=xbp[length(xbp)])
  if (length(inds)>0)
  {
    for (counter in 1:length(inds))
    {
      thisx<-x[inds[counter]]
      #wind1<-which(xbp<=thisx)
      #wind2<-which(xbp>=thisx)
      #if (length(wind1)==0 || length(wind2)==0)
      #{
      #  save(x,xbp,ybp,file="ErrorChecker.RData")
      #  stop("Error in pwlin, file saved for debugging")
      #}
      ilo<-max(which(xbp<=thisx))
      ihi<-min(which(xbp>=thisx))
      if (ilo==ihi)
      {
        res[inds[counter]]<-ybp[ilo]
      }else
      {
        res[inds[counter]]<-((thisx-xbp[ilo])/(xbp[ihi]-xbp[ilo]))*(ybp[ihi]-ybp[ilo])+ybp[ilo]
      }
    }
  }
  
  return(res)
}