Need help pairing data

Question

I'm looking for what I'm sure is a quick answer. I'm working with a data set that looks like this:

    Week      Game.ID           VTm VPts HTm HPts Differential HomeWin 
    1  NFL_20050908_OAK@NE      OAK   20 NE  30           10   TRUE 
    1 NFL_20050911_ARI@NYG      ARI   19 NYG 42           23   TRUE 
    1 NFL_20050911_CHI@WAS      CHI    7 WAS 9            2    TRUE 
    1 NFL_20050911_CIN@CLE      CIN   27 CLE 13          -14   FALSE 
    1  NFL_20050911_DAL@SD      DAL   28 SD  24           -4   FALSE 
    1 NFL_20050911_DEN@MIA      DEN   10 MIA 34           24   TRUE 

NFL data. I want to come up with a way to pair each HTm with its Differential, and store these values in another table. I know it's easy to do, but all the methods I am coming up with involve doing each team individually via a for loop that searches for [i,5]=="NE", [i,5]=="NYG". I'm wondering if there's a way to systematically do this for all 32 teams. I would then use the same method to pair VTM of the same team code ("NYG" or "NE") with VPTs and a VDifferential.

Thanks for the help.

Answer 1

Im not sure if i understood your question correctly(you need a function like select in a database?) but:

cbind(matr[,x], matr[,y])

selects column x and y and creates a new matrix

Answer 2

It sounds like you'd like to perform operations on your data frame based on a grouping variable. For that, there are many functions, among which is tapply() . For example, if your data is in a data.frame object named nflDF , you could get the maximum Differential for each home team HTm by

tapply(nflDF$Differential, nflDF$HTm, FUN = max)

Which would return (with your sample data)

CLE MIA  NE NYG  SD WAS 
-14  24  10  23  -4   2

Alternatively, you could use by :

by(nflDF, nflDF$HTm, FUN = function(x) max(x$Differential))

HTm: CLE
[1] -14
------------------------------------------------------------ 
HTm: MIA
[1] 24
------------------------------------------------------------ 
HTm: NE
[1] 10
------------------------------------------------------------ 
HTm: NYG
[1] 23
------------------------------------------------------------ 
HTm: SD
[1] -4
------------------------------------------------------------ 
HTm: WAS
[1] 2

To perform more complicated operations, change the values supplied to the FUN arguments in the appropriate function.