Select the first row by group

Question

From a dataframe like this

test <- data.frame('id'= rep(1:5,2), 'string'= LETTERS[1:10])
test <- test[order(test$id), ]
rownames(test) <- 1:10

> test
    id string
 1   1      A
 2   1      F
 3   2      B
 4   2      G
 5   3      C
 6   3      H
 7   4      D
 8   4      I
 9   5      E
 10  5      J

I want to create a new one with the first row of each id / string pair. If sqldf accepted R code within it, the query could look like this:

res <- sqldf("select id, min(rownames(test)), string 
              from test 
              group by id, string")

> res
    id string
 1   1      A
 3   2      B
 5   3      C
 7   4      D
 9   5      E

Is there a solution short of creating a new column like

test$row <- rownames(test)

and running the same sqldf query with min(row)?

Answer 1

You can use duplicated to do this very quickly.

test[!duplicated(test$id),]

Benchmarks, for the speed freaks:

ju <- function() test[!duplicated(test$id),]
gs1 <- function() do.call(rbind, lapply(split(test, test$id), head, 1))
gs2 <- function() do.call(rbind, lapply(split(test, test$id), `[`, 1, ))
jply <- function() ddply(test,.(id),function(x) head(x,1))
jdt <- function() {
  testd <- as.data.table(test)
  setkey(testd,id)
  # Initial solution (slow)
  # testd[,lapply(.SD,function(x) head(x,1)),by = key(testd)]
  # Faster options :
  testd[!duplicated(id)]               # (1)
  # testd[, .SD[1L], by=key(testd)]    # (2)
  # testd[J(unique(id)),mult="first"]  # (3)
  # testd[ testd[,.I[1L],by=id] ]      # (4) needs v1.8.3. Allows 2nd, 3rd etc
}

library(plyr)
library(data.table)
library(rbenchmark)

# sample data
set.seed(21)
test <- data.frame(id=sample(1e3, 1e5, TRUE), string=sample(LETTERS, 1e5, TRUE))
test <- test[order(test$id), ]

benchmark(ju(), gs1(), gs2(), jply(), jdt(),
    replications=5, order="relative")[,1:6]
#     test replications elapsed relative user.self sys.self
# 1   ju()            5    0.03    1.000      0.03     0.00
# 5  jdt()            5    0.03    1.000      0.03     0.00
# 3  gs2()            5    3.49  116.333      2.87     0.58
# 2  gs1()            5    3.58  119.333      3.00     0.58
# 4 jply()            5    3.69  123.000      3.11     0.51

Let's try that again, but with just the contenders from the first heat and with more data and more replications.

set.seed(21)
test <- data.frame(id=sample(1e4, 1e6, TRUE), string=sample(LETTERS, 1e6, TRUE))
test <- test[order(test$id), ]
benchmark(ju(), jdt(), order="relative")[,1:6]
#    test replications elapsed relative user.self sys.self
# 1  ju()          100    5.48    1.000      4.44     1.00
# 2 jdt()          100    6.92    1.263      5.70     1.15