N <- 100

df <- data.frame(
  Z = sample(1:3, N, replace = TRUE),
  Y_1 = rnorm(N),
  Y_2 = rnorm(N),
  Y_3 = rnorm(N)
)

# an annoying solution
df <- within(df,{
  Y <- rep(NA, nrow(df))
  Y[Z == 1] <- Y_1[Z == 1]
  Y[Z == 2] <- Y_2[Z == 2]
  Y[Z == 3] <- Y_3[Z == 3]
})

head(df)

Z         Y_1        Y_2         Y_3           Y
1 3  0.89124772  1.4377700  0.05226285  0.05226285
2 1  0.89186873 -0.6984839 -0.86141525  0.89186873
3 1 -0.01315678  1.5193461  0.18290065 -0.01315678
4 3 -0.57857274 -1.4445197  2.03764943  2.03764943
5 3 -0.19793692 -0.1818225  1.10270877  1.10270877
6 2  1.48291431  2.7264541  0.70129357  2.72645413

df$Y <- apply(df, 1, function(x) x[x[1]+1] )
head(df)
#  Z        Y_1        Y_2        Y_3          Y
#1 1 -0.8598997 -0.3180947  1.9374462 -0.8598997
#2 2 -0.2392902  0.2266245  0.2364991  0.2266245
#3 1 -0.8733609 -1.3892361  0.3351359 -0.8733609
#4 3 -0.6533548 -1.1042993 -0.2906852 -0.2906852
#5 1 -1.7424126 -0.2101860  0.1198945 -1.7424126
#6 2 -1.9746651 -0.4308746 -0.7849773 -0.4308746

get_result <- function(dfrow){
  x <- unlist(dfrow[,1:4])
  Y <- x[x[1] + 1]
}

library(purrr)
newdf <- by_row(df, get_result)

df$Y <- df[-1][cbind(1:nrow(df), df$Z)]
df
#  Z         Y_1        Y_2         Y_3           Y
#1 3  0.89124772  1.4377700  0.05226285  0.05226285
#2 1  0.89186873 -0.6984839 -0.86141525  0.89186873
#3 1 -0.01315678  1.5193461  0.18290065 -0.01315678
#4 3 -0.57857274 -1.4445197  2.03764943  2.03764943
#5 3 -0.19793692 -0.1818225  1.10270877  1.10270877
#6 2  1.48291431  2.7264541  0.70129357  2.72645410

df <- structure(list(Z = c(3L, 1L, 1L, 3L, 3L, 2L), Y_1 = c(0.89124772,
0.89186873, -0.01315678, -0.57857274, -0.19793692, 1.48291431
), Y_2 = c(1.43777, -0.6984839, 1.5193461, -1.4445197, -0.1818225,
2.7264541), Y_3 = c(0.05226285, -0.86141525, 0.18290065, 2.03764943,
1.10270877, 0.70129357)), .Names = c("Z", "Y_1", "Y_2", "Y_3"
), row.names = c("1", "2", "3", "4", "5", "6"), class = "data.frame")

N <- 100000

df <- data.frame(
  Z = sample(1:3, N, replace = TRUE),
  Y_1 = rnorm(N),
  Y_2 = rnorm(N),
  Y_3 = rnorm(N)
)


ind_split <-
system.time({
  df$Y <- sapply(split(df, 1:nrow(df)), function(x) x[, paste0("Y_", x$Z)])
  head(df)
})

revealer <-
  function(list_element){
    col_name <- paste0("Y_", list_element[1, "Z"])
    list_element$Y <- list_element[,col_name]
    return(list_element)
  }

group_split <-
system.time({
  split_list <- split(df, df$Z)
  df <- do.call(what = rbind, lapply(split_list, revealer))
  head(df)
})


by_hand <-
system.time({
  # an annoying solution
  df <- within(df,{
    Y <- rep(NA, nrow(df))
    Y[Z == 1] <- Y_1[Z == 1]
    Y[Z == 2] <- Y_2[Z == 2]
    Y[Z == 3] <- Y_3[Z == 3]
  })
  head(df)
})


ind_split
group_split
by_hand

> ind_split
   user  system elapsed
  1.023   0.083   1.136
> group_split
   user  system elapsed
  0.011   0.002   0.013
> by_hand
   user  system elapsed
  0.001   0.000   0.001

df$Y <- df[cbind(1:nrow(df), match(paste0('Y_', df$Z), names(df)))]