Lesson 6 of 17 · Week 03 · Thu Oct 8

Control Flow

Making decisions: if / else

x <- 12
if (x > 10) {
  print("big")
} else if (x > 5) {
  print("medium")
} else {
  print("small")
}

The condition inside if ( ) must be a single TRUE or FALSE. When you have a whole vector of comparisons, squash it into one answer with any() (is at least one TRUE?) or all() (are they all TRUE?). Combine conditions with && (and) and || (or).

any(gm$year == 1987)                 # TRUE: at least one row is from 1987
if (any(gm$lifeExp > 80)) print("Someone lives past 80 on average")

if can also return a value, which is handy for labels:

label <- if (x %% 2 == 0) "even" else "odd"

Adapted from Software Carpentry's “Control Flow” episode (CC BY 4.0, The Carpentries). Text, examples and exercises have been rewritten for this course.

Repeating steps: for loops

for (yr in c(1967, 2007)) {
  print(paste("The year is", yr))
}

The loop variable (yr) takes each value of the vector in turn. To loop over positions instead, use seq_along(v), which is safer than 1:length(v) when v might be empty.

Saving results from a loop. You can grow a vector with out <- c(out, new_value), but that gets slow for long loops. It's better to make the output first and fill it by position:

squares <- numeric(5)            # five zeros, ready to be filled
for (i in 1:5) {
  squares[i] <- i ^ 2
}

With a nested loop you have to work out each result's position yourself, e.g. (i - 1) * n_inner + j.

A preview: picking values with a condition

Loops over groups need one trick that the next lesson covers fully: v[condition] keeps only the elements of v where the condition is TRUE.

gm$lifeExp[gm$continent == "Europe"]          # Europe's life expectancies
for (cont in levels(gm$continent)) {
  m <- mean(gm$lifeExp[gm$continent == cont])
  print(paste(cont, round(m, 1)))
}

To store one result per group, use a named vector: res <- character(0) before the loop and res[cont] <- "something" inside it. startsWith(as.character(gm$country), "B") tests whether names start with a letter.

while loops

balance <- 1000
years <- 0
while (balance < 2000) {
  balance <- balance * 1.07
  years <- years + 1
}
years

A while loop runs as long as its condition stays TRUE. Make sure something inside it eventually makes the condition FALSE, or it runs forever (press Esc in Positron's console to stop it).

Loops are fine, but in R many loops can be replaced by one vectorized operation. That's the subject of Week 04.

Exercises

Each exercise gives you up to 10 checked attempts. Use Run as many times as you like to experiment first, then click Check answer when you're ready. Your exercises use numbers/subsets unique to you, so answers won't match a classmate's exactly.