Apply Functions

`tapply`

The documentation definition for tapply is a bit more specific than the others, where the arguments are now (X, INDEX, FUN), with X being an object where the split function applies, INDEX is a factor by which X is grouped, and FUN is function as before.

To simplify this definition, we can say tapply applies FUN to X when X is grouped by INDEX.

Examples

Use the sapply function to run this function on each month to get total amount of money spent on taxi cab rides each day. Use the tapply function to add up the amounts spent per day.

myfares <- function(mymonth) {
    myDF <- fread(paste0("/anvil/projects/tdm/data/taxi/yellow/yellow_tripdata_2018-", mymonth, ".csv"), select=c(2,17))
    mytable <- tapply(myDF$total_amount, as.Date(myDF$tpep_pickup_datetime), sum)
    return(mytable)
}

Click to see solution

myfares <- function(mymonth) {
    myDF <- fread(paste0("/anvil/projects/tdm/data/taxi/yellow/yellow_tripdata_2018-", mymonth, ".csv"), select=c(2,17))
    mytable <- tapply(myDF$total_amount, as.Date(myDF$tpep_pickup_datetime), sum)
    return(mytable)
}

library(data.table)
myresults <- sapply( sprintf("%02d", 1:12), myfares )

names(myresults) <- NULL
v <- do.call(c, myresults)
mytotals <- tapply(v, names(v), sum)

`sapply`

sapply will function identically to lapply unless the output can be simplified, in which case sapply executes that simplification. The following occurs when we run sapply in place of lapply on our squares vector.

Examples

Use the sapply function to run this function on each month to get total amount of money spent on taxi cab rides each day.

myfares <- function(mymonth) {
    myDF <- fread(paste0("/anvil/projects/tdm/data/taxi/yellow/yellow_tripdata_2018-", mymonth, ".csv"), select=c(2,17))
    mytable <- tapply(myDF$total_amount, as.Date(myDF$tpep_pickup_datetime), sum)
    return(mytable)
}

Click to see solution

myfares <- function(mymonth) {
    myDF <- fread(paste0("/anvil/projects/tdm/data/taxi/yellow/yellow_tripdata_2018-", mymonth, ".csv"), select=c(2,17))
    mytable <- tapply(myDF$total_amount, as.Date(myDF$tpep_pickup_datetime), sum)
    return(mytable)
}

library(data.table)
myresults <- sapply( sprintf("%02d", 1:12), myfares )

myresults