Looping Over Names

Part of the Logic & Flow section of Coddy's R journey. Lesson 10 of 64.

A for loop over names(x) visits every key, and x[[key]] gives the value for that key:

scores <- c(ada = 91, bo = 78, cy = 85)
for (name in names(scores)) {
  cat(name, "scored", scores[[name]], "\n")
}

Output:

ada scored 91 
bo scored 78 
cy scored 85

Entries keep the order they were added in. Loop over sort(names(x)) for alphabetical order:

scores <- c(cy = 85, ada = 91, bo = 78)
for (name in sort(names(scores))) {
  cat(name, scores[[name]], "\n")
}

Output:

ada 91 
bo 78 
cy 85

Looping over the input and updating a lookup table counts how often each value appears. The first time a word is seen it gets 1, after that it goes up by 1:

words <- c("red", "blue", "red", "green", "red")
counts <- c()
for (w in words) {
  if (w %in% names(counts)) {
    counts[w] <- counts[w] + 1
  } else {
    counts[w] <- 1
  }
}
print(counts[["red"]])
print(names(counts))

Output:

[1] 3
[1] "red"   "blue"  "green"

To build output lines, collect them in a vector inside the loop and return or print them at the end:

scores <- c(ada = 91, bo = 78)
lines <- c()
for (name in names(scores)) {
  lines <- c(lines, paste0(name, ": ", scores[[name]]))
}
cat(lines, sep = "\n")

Output:

ada: 91
bo: 78

Challenge

Easy

Complete word_counts(words). Return one line per distinct word, in alphabetical order, formatted as word: count.

The supplied code reads the words as one line separated by spaces and prints each returned line.

Try it yourself

word_counts <- function(words) {
  # Write your code here
  character(0)
}

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