How do I write a user defined function in r?

Question

Suppose I have 2 columns in a table

Quantity int Price decimal

And I want to calculate the value of a 3rd column called Total.

In SQL Server\'s transact-sql

Answer 1

In R this kind of operation is vectorized, meaning that you can create the column total just by multiplying the other columns:

mytable$total <- mytable$quantity * mytable$price

or a little cleaner:

mytable$total <- with(mytable, quantity * price)

The "tidyverse" way uses dplyr::mutate:

library(dplyr)
mytable <- mytable %>%
    mutate(total = quantity * price)

If you want a function:

calcTotal <- function(x, y) {
  x * y
}

And then you can use e.g.

mytable <- mytable %>%
  mutate(total = calcTotal(quantity, price))

However, be aware that not all functions work "row-wise" in this way, in which case you might use the mapping functions from the purrr package. For simple operations, it's probably not worth writing the function.