r - Set column name ddply -

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how set column name of summarized data in 

library(plyr) ddply(data,.(col1,col2),nrow)

like in

ddply(data,.(col1,col2),function(x) data.frame(number=nrow(x)))

perhaps looking summarize (or mutate or transform, depending on want do).

a small example:

set.seed(1) data <- data.frame(col1 = c(1, 2, 2, 3, 3, 4),                    col2 = c(1, 2, 2, 1, 2, 1),                    z = rnorm(6)) ddply(data,.(col1,col2), summarize,        number = length(z), newcol = mean(z)) #   col1 col2 number     newcol # 1    1    1      1 -0.6264538 # 2    2    2      2 -0.3259926 # 3    3    1      1  1.5952808 # 4    3    2      1  0.3295078 # 5    4    1      1 -0.8204684

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