Group Summaries

Part of the Logic & Flow section of Coddy's R journey. Lesson 40 of 64.

tapply(values, groups, f) groups the values and applies f to every group in one call. It gives the same result as sapply(split(values, groups), f):

amounts <- c(10, 20, 30, 40)
category <- c("food", "rent", "food", "fun")
print(tapply(amounts, category, sum))
print(tapply(amounts, category, max))

Output:

food  fun rent 
  40   40   20 
food  fun rent 
  30   40   20

The result is named by group in sorted order, so [[ ]] reads one group and names() lists them:

amounts <- c(10, 20, 30, 40)
category <- c("food", "rent", "food", "fun")
avg <- tapply(amounts, category, mean)
print(avg[["food"]])
print(names(avg))

Output:

[1] 20
[1] "food" "fun"  "rent"

The summary function can be your own. Here it counts the orders above 25 in each group:

amounts <- c(10, 20, 30, 40)
category <- c("food", "rent", "food", "fun")
print(tapply(amounts, category, function(v) sum(v > 25)))

Output:

food  fun rent 
   1    1    0

The result is a named vector, so sort() and which.max() find the biggest group. which.max() gives the first of equal values:

amounts <- c(10, 20, 30, 40, 5)
category <- c("food", "rent", "food", "fun", "rent")
totals <- tapply(amounts, category, sum)
print(names(totals)[which.max(totals)])
print(names(sort(totals, decreasing = TRUE)))

Output:

[1] "food"
[1] "food" "fun"  "rent"

Challenge

Easy

Complete city_report(cities, temps). Return one line per city in alphabetical order, oslo: avg 4.5, max 9, with the mean temperature rounded to one decimal and the highest temperature. Then add Warmest: followed by the city with the highest mean (the alphabetically first one when means tie).

The supplied code reads the cities and the temperatures as two comma-separated lines of equal length and prints each returned line.

Try it yourself

city_report <- function(cities, temps) {
  # Write your code here
  character(0)
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
cat(city_report(strsplit(input[1], ",")[[1]], as.numeric(strsplit(input[2], ",")[[1]])), sep = "\n")