The R Book

Using the match Function in Dataframes

The worms dataframe (above) contains fields of five different vegetation types:

unique(worms$Vegetation)

[1]  Grassland  Arable  Meadow Scrub Orchard

and we want to know the appropriate herbicides to use in each of the 20 fields. The herbicides are in a separate dataframe that contains the recommended herbicides for a much larger set of plant community types:

herbicides<-read.table("c:\\temp\\herbicides.txt",header=T)
herbicides

Type  Herbicide
1    Woodland   Fusilade
2     Conifer   Weedwipe
3      Arable   Twinspan
4        Hill   Weedwipe
5     Bracken   Fusilade
6       Scrub   Weedwipe
7    rassland   Allclear
8       Chalk   Vanquish
9      Meadow   Propinol
10       Lawn   Vanquish
11    Orchard   Fusilade
12      Verge   Allclear

The task is to create a vector of length 20 (one for every field in worms) containing the name of the appropriate herbicide. The first value needs to be Allclear because Nash's Field is grassland, and the second needs to be Twinspan because Silwood Bottom is arable, and so on. The first vector in match is worms$Vegetation and the second vector in match is herbicides$Type. The result of this match is used as a vector of subscripts to extract the relevant herbicides from herbicides$Herbicide like this:

herbicides$Herbicide[match(worms$Vegetation,herbicides$Type)]

  [1]  Allclear  Twinspan  Allclear  Propinol  Weedwipe  Allclear
       Allclear  Twinspan
  [9]  Fusilade  Allclear  Weedwipe  Allclear  Allclear  Allclear
       Propinol  Propinol
 [17]  Weedwipe  Twinspan  Allclear  Weedwipe

You could add this information as ...