Adding and Sorting

Part of the Logic & Flow section of Coddy's R journey. Lesson 45 of 64.

Assigning to a new column name adds the column. Arithmetic between columns works row by row, like any vector arithmetic:

orders <- data.frame(item = c("tea", "jam", "oil"), qty = c(2, 5, 1), price = c(4, 2.5, 9))
orders$total <- orders$qty * orders$price
print(orders)

Output:

  item qty price total
1  tea   2   4.0   8.0
2  jam   5   2.5  12.5
3  oil   1   9.0   9.0

df[order(df$col), ] sorts the rows by a column. The row numbers on the left stay attached to the original rows. Use a minus sign or decreasing = TRUE for high to low, and a second key for ties:

orders <- data.frame(item = c("tea", "jam", "oil"), qty = c(2, 5, 1), price = c(4, 2.5, 9))
orders$total <- orders$qty * orders$price
print(orders[order(-orders$total, orders$item), c("item", "total")])

Output:

  item total
2  jam  12.5
3  oil   9.0
1  tea   8.0

A new column can be text built from other columns. sprintf() works on whole columns at once:

orders <- data.frame(item = c("tea", "jam"), qty = c(2, 5), price = c(4, 2.5))
orders$label <- sprintf("%d x %s", orders$qty, orders$item)
print(orders$label)

Output:

[1] "2 x tea" "5 x jam"

rbind() adds a row from a one-row data frame with the same columns, and head(df, n) keeps the first n rows:

orders <- data.frame(item = c("tea", "jam"), qty = c(2, 5))
orders <- rbind(orders, data.frame(item = "oil", qty = 1))
print(nrow(orders))
print(head(orders[order(-orders$qty), ], 2))

Output:

[1] 3
  item qty
2  jam   5
1  tea   2

Challenge

Easy

Complete order_totals(df). df has the columns item, qty and price. Add a total column (quantity times price), sort the rows by total from high to low (items with equal totals alphabetically), and return one line per row, jam: 12.50, with the total to two decimals.

The supplied code reads the table from the input with read.csv(text = input): the first line holds the column names and every following line is one row, with values separated by commas. The code prints each returned line.

Try it yourself

order_totals <- function(df) {
  # Write your code here
  character(0)
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
df <- read.csv(text = input)
cat(order_totals(df), sep = "\n")