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

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