Adding and Removing Keys

Part of the Logic & Flow section of Coddy's R journey. Lesson 9 of 64.

Assigning to a name updates its value when the key exists and adds a new entry when it does not:

stock <- c(apples = 12, pears = 3)
stock["pears"] <- 5
stock["plums"] <- 8
print(stock)

Output:

apples  pears  plums 
    12      5      8

To change a value based on its current value, read and write the same key:

stock <- c(apples = 12, pears = 3)
stock["apples"] <- stock["apples"] - 2
print(stock[["apples"]])

Output:

[1] 10

A lookup table can start empty with c() and grow one key at a time:

ages <- c()
ages["Ada"] <- 36
ages["Linus"] <- 21
print(names(ages))

Output:

[1] "Ada"   "Linus"

Remove an entry from a named vector by keeping every other name:

stock <- c(apples = 12, pears = 3, plums = 8)
stock <- stock[names(stock) != "pears"]
print(names(stock))

Output:

[1] "apples" "plums"

For a list, assigning NULL removes the entry:

user <- list(name = "Ada", temp = TRUE)
user[["temp"]] <- NULL
print(names(user))

Output:

[1] "name"

Challenge

Easy

Complete apply_delivery(stock, item, qty). Add qty to the stock of item, or add item with the quantity qty when it is not stocked yet. Return the updated stock.

The supplied code reads the stock as name=quantity pairs, then the item and the quantity. It prints each entry of the returned stock as name: quantity, in the stock's order.

Try it yourself

apply_delivery <- function(stock, item, qty) {
  # Write your code here
  stock
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
stock <- c()
for (pair in strsplit(input[1], ",")[[1]]) {
  kv <- strsplit(pair, "=")[[1]]
  stock[kv[1]] <- as.numeric(kv[2])
}
stock <- apply_delivery(stock, input[2], as.numeric(input[3]))
for (name in names(stock)) {
  cat(name, ": ", stock[[name]], "\n", sep = "")
}