On the basis of the vector Fields create a numeric vector ID containing the ID numbers of every field (i.e. the value following the underscore).

 

> head(Fields, n = 10)
 [1] "MiddleID_54"  "HighID_115"   "LowID_109"    "HighID_346"   "MiddleID_298" "HighID_139"   "MiddleID_221"
 [8] "MiddleID_447" "HighID_27"    "LowID_331"   
 ...