One of the coolest things you can do in R is write custom functions to solve your own unique problems. I’m not sure I’m brave enough to try my hand at more complex functions with loops and conditionals and such but, for now, I thought I’d share something simple.

Suppose you have a list of transactions and you are trying to get the total amount from all of them. Of course, you can just call the sum() function on the individual transactions. But what if you get a discount on all the transactions? Then, what if there are taxes? Obviously, you can just do this with simple math. But, humor me. Here’s a function for it…

### create subtotal function
sub_tot <- function(costs,discount = 0,tax = 0) {
raw_sub <- sum(costs) - (sum(costs) * discount) + (sum(costs) * tax)
round(raw_sub, digits = 2)
}

The sub_tot function that I’ve created takes three arguments. The first is a vector of data that you want to total. The second is the percentage discount you get on all the transactions–expressed as a decimal. The third argument is the tax rate that is applied after the discounts are taken into consideration.

If you look at the function, all I do is subtract the discounted amount from the total, add the amount of taxes, and then round to two decimal places. Pretty simple, right? Now, let’s try it out on something…

### install and load readr package
install.packages("readr")
library(readr)

### import data set of sales transactions
sales <- read_csv("http://samplecsvs.s3.amazonaws.com/SalesJan2009.csv")

### print sum of all purchases before any discounts or taxes
sum(sales$Price)

### print sum of all purchases after discounts and taxes
sub_tot(sales$Price,0.15,0.08)

### taking taxes into account, how much did we save with the discount?
sum(sales$Price) - sub_tot(sales$Price,0.15,0.08)

Let’s pull a random set of real estate transactions from the web and store it into a data frame called “sales.” If we view the data set, we’ll see that there is a column called “Price.”

Now, let’s suppose you are a real estate developer who is buying up all these properties. You get a 15% discount on all that you buy, and there is an 8% tax rate. How much do you spend in all?

When we use the sub_tot function, here’s what we get…

So, before the discounts and taxes, you spend ~$1.6 million. After the discounts and taxes, you spent ~$1.5 million–which mean that you save a total of ~$100k.

But, suppose you didn’t get the discount. How much would you spend without the discount but taking into consideration the taxes? And, how much after-tax money do you save by getting the 15% discount vs not getting it? Let’s write some more code…

### what if you don't get a discount? how much do you spend?
sub_tot(sales$Price,,0.08)

### how much do you save with the discount vs not having the discount?
sub_tot(sales$Price,,0.08) - sub_tot(sales$Price,0.15,0.08)

Since “0” is set as the default argument for discount and tax, we simply skip the discount argument when there is no discount. So, when we run the above code, we find that the subtotal without a discount is ~$1.76 million and the after-tax amount you save by getting the discount is ~$250k.

Now, want to send some of that money my way???