using split() and lapply instead of tapply

[Martyn Colins <martyncollins10@yahoo.com>]

 

Does anyone know how to use the commands split() and lapply() to do the following: suppose I have the following data.frame

 

w <- data.frame(y1 =1:10, y2=as.factor(rep(LETTERS[1:5],2)), 
y3=as.factor(rep(c("level1", "level2"),5)), y4=c(rep(c("sub1", "sub2", 
"sub3"), 3), "sub3"))

this is the output that i'm after

> tapply(w$y1, list(w$y2, w$y3), sum)
  level1 level2
A      1      6
B      7      2
C      3      8
D      9      4
E      5     10

 

I would like to do this using the split and lapply commands as these are more efficient than tapply.

I can split w on one variable

w2 <- split(w, w$y2)

but how do I split on both variables to produce the equivalent output?

 

Thanks in advance for any help.

 

Regards,

Martyn

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