分组后对data.table进行排序

Question

I have a data like this

z = data.table(name =sample(letters[1:6],10,replace=T),
Var1=rnorm(10,1,2),
Var2 =runif(10,1,10))

I want sum the var1by name and sort the sum like the fake code

z[order(sum(Var1)),sum(Var1),by =name]

But the code above doesn't work,any insights?

Answer 1

This can be done in a two step process - ie 1) create a sum column grouped by 'name' then order based on the new column ('V1') and assign that column to NULL (if the intention is to order the whole dataset)

z[, V1 := sum(Var1) , by = name][order(V1)][, V1 := NULL][]

If we are only ordering the summarized output

z[, sum(Var1) , by = name][order(V1)]

Or as @Frank suggested set functions will be more efficient

z[, V1 := sum(Var1) , by = name]
setorder(z, V1)[, V1 := NULL]