in R, customize names of columns created by dcast.data.table - r

Question

I am new to reshape2 and data.table and trying to learn the syntax.
I have a data.table that I want to cast from multiple rows per grouping variable(s) to one row per grouping variable(s). For simplicity, let's make it a table of customers, some of whom share addresses.
library(data.table)
# Input table:
cust <- data.table(name=c("Betty","Joe","Frank","Wendy","Sally"),
address=c(rep("123 Sunny Rd",2),
rep("456 Cloudy Ln",2),
"789 Windy Dr"))
I want the output to have the following format:
# Desired output looks like this:
(out <- data.table(address=c("123 Sunny Rd","456 Cloudy Ln","789 Windy Dr"),
cust_1=c("Betty","Frank","Sally"),
cust_2=c("Joe","Wendy",NA)) )
# address cust_1 cust_2
# 1: 123 Sunny Rd Betty Joe
# 2: 456 Cloudy Ln Frank Wendy
# 3: 789 Windy Dr Sally NA
I would like columns for cust_1...cust_n where n is the max customers per address. I don't really care about the order--whether Joe is cust_1 and Betty is cust_2 or vice versa.

Answer 1

Just pushed a commit to data.table v1.9.5. dcast now
allows casting on multiple value.var columns and multiple fun.aggregate functions
understands undefined variables/expressions in formula
With this, we can do:
dcast(cust, address ~ paste0("cust", cust[, seq_len(.N),
by=address]$V1), value.var="name")
# address cust1 cust2
# 1: 123 Sunny Rd Betty Joe
# 2: 456 Cloudy Ln Frank Wendy
# 3: 789 Windy Dr Sally NA

Answer 2

# My attempt:
setkey(cust,address)
x <- cust[,list(name, addr_cust_num=rank(name,ties.method="random")), by=address])
x[,addr_cust_num:=paste0("cust_",addr_cust_num)]
y <- dcast.data.table(x, address ~ addr_cust_num, value.var="name")
y
Note that I had to paste0 the "cust_" prefix. Before I added that step, I was using setnames(y, names(y), sub("(\\d+)","cust_\\1",names(y)) ) which seemed a clunkier (but probably faster) solution.
Wondering if there is a better way to do the prefixing.
Alternatively, you could just add the column directly to cust by reference:
# no need to set key
cust[, cust := paste("cust", seq_len(.N), sep="_"), by=address]
dcast.data.table(cust, address ~ cust, value.var="name")
# address cust_1 cust_2
# 1: 123 Sunny Rd Betty Joe
# 2: 456 Cloudy Ln Frank Wendy
# 3: 789 Windy Dr Sally NA