Random DD stuff

DDCalc.R

## Define function that will convert temperatures to Celcius and divide by 8 [same as * 1/8] (as each measurement represents 1/8 of a day)
#Note: need to add a part here where any value <0 = 0 and not a negative, as heating DD's are calculated only using the 'above the threshold'
extractDD=function(path) {
  # extract values and convert to kelvin
  vals=(values(raster(path,varname="TMP"))-273.15)/8
  # set any numbers  < 0 to zero
  if (sum(vals<0)>0) {
    vals[which(vals<0)]=0
  }
  # return error if any NAs
  if (sum(is.na(vals))>0)
    stop(paste0("this raster has na values: ", path))
  # return values
  return(vals)
}

so 1:
if any of the values in the table are <0, then:
2. use the 'which' function to convert vals <0 to 0?

very clever, i have not heard of the 'which' function, i need like a guidebook on base functions

The which is unnecessary, you could just use

...
vals[vals<0]=0
...

but it's a habit of mine.

which returns the indices that are true in a vector of logical (T/F) values. Eg. which(c(T,F,T,F)) yields c(1,3).

Cant believe you used unnecessary code... thats like 5 extra characters!