[datatable-help] calculating proportions on 192 column data table grouped by one column

[carlsutton]

I can do the calculation on a couple columns, but how do I automate this for
192 column data table?

#  calculating proportion for column variables
library(data.table)
a <- rep(1:5, 2)
b <- sample(20:50, size = 10, replace = TRUE)
c <- sample(80:130, size = 10, replace = TRUE)
dt1 <- data.table(a,b,c)
dt1
d <- sum(dt1$b)
e <- sum(dt1$c)
proportion_b <- dt1$b/d
proportion_c <- dt1$c/e
proportion_b
proportion_c
dt_manipulated <- data.table(dt1,proportion_b, proportion_c)
dt_manipulated
#  now an attempt to add column proportions to the data table
dt2 <- dt1[, `:=` (proportion = b/sum(b), proportion = c/sum(c))]
identical(dt_manipulated,dt2)
#  Nice that this works, but I sure do NOT want to do this for 191 columns
of data!
#  My instinct is to use lapply, but how do I define the column to divide by
column sum???





-----
Carl Sutton
--
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