SO R benchmarking

# benchmarking for
# http://stackoverflow.com/questions/20538638/how-to-pick-a-value-in-data-table

codoremifa <- function() {
  for ( i in unique(y[!is.na(y$len),'len']))  {
    y[y$len == i & !is.na(y$len),'Output'] <- y[y$len == i & !is.na(y$len),paste0('Day_',i)]
  }
  y
}

sven <- function() {
  cbind(y, output = y[2:4][cbind(seq.int(nrow(y)), y$len)])
}

shadow <- function() {
  require(data.table)
  dt <- data.table(y)
  dt[, output:=ifelse(len==1, Day_1, ifelse(len==2, Day_2, Day_3))]
}

tonytonov <- function() {
  mat <- rbind(y$Day_1, y$Day_2, y$Day_3)
  y$output <- diag(mat[y$len, ])
  y
}

user <- function() {
  cbind(y, output=apply(y, 1, function(r) r[r["len"]+2]))
}

N <- 1000
df <- data.frame(len = sample(c(NA, 1:3), size=N, replace=T),
                 Day_1 = 5 + runif(N),
                 Day_2 = rnorm(N),
                 Day_3 = rbinom(N, 1, 1/2))

require(rbenchmark)
benchmark(default(), tonytonov(), sven(), shadow(), codoremifa(), replications=100, order="relative")