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hadley edited this page Feb 15, 2013 · 32 revisions

Functionals

Introduction

"To become significantly more reliable, code must become more transparent. In particular, nested conditions and loops must be viewed with great suspicion. Complicated control flows confuse programmers. Messy code often hides bugs." --- Bjarne Stroustrup

Higher-order functions encompass any functions that either take a function as an input or return a function as output. We've already seen closures, functions returned by another function. The complement to a closure is a functional, a function that takes a function as an input and returns a vector as output.

Here's a simple functional, it takes an input function and calls it with some random input:

randomise <- function(f) f(runif(1e3))
randomise(mean)
randomise(sum)

This function is not terribly useful, but it illustrates the basic idea: in R, functions are first class objects and there's no difference between calling a function with a vector or function as input. The chances are that you've already used a functional: lapply() is probably the most commonly used functional in R, followed closely by apply() and tapply().

Many functionals (like lapply()) offer alternatives to for loops. For loops have a bad rap in R, and some programmers try to eliminate them at all costs. The real performance story is a little more complicated than what you might have heard (we'll explore their speed in the performance chapter); the real downside of for loops is that they're not very expressive. A for loop conveys that you're iterating over something, but it doesn't communicate the higher-level task you're trying to achieve. Functionals are not as general as for loops, but by being more specific they allow you to communicate more clearly. Functionals allow you to take a step up the ladder of abstraction: instead of expressing solutions using looping constructs, you express them at a higher level of specificity.

As well as more clearly communicating intent (e.g. I want to transform each element of this list, or each row of this array), functionals reduce the chances of bugs, and can be more efficient. Both of these features occur because functionals are used by many people, so they will be well tested, and may have been implemented with special tricks. For example, many functionals in base R are written in C, and often use a few tricks to get extra performance.

Thinking about functionals as replacements for loops is not the only way to think about them. They are also useful tools for encapsulating common data manipulation tasks, the split-apply-combine pattern; for thinking "functionally"; and for working with mathematical functions. In this chapter, you'll learn about:

  • Functionals that encapulsate a common pattern of for-loop use, like lapply, vapply and Map.

  • Functionals for manipulating common R data structures, like apply, split, tapply and the plyr package.

  • Popular functionals from other programming languages, like Map, Reduce and Filter

  • Mathematical functionals, like integrate, uniroot, and optim.

  • how to convert a loop to a functionals, and when you shouldn't do it.

The focus in this chapter is on clear communication with your code, and developing tools to solve wide classes of problems. Once you do have clear, correct code you can focus on optimising it use the techniques in the performance chapter.

For loop functionals: lapply() and friends

You're probably already familiar with lapply(), which applies a function to each element of a list, storing the results in a list. It's informative to look at a pure R implementation:

lapply2 <- function(x, f, ...) {
  out <- vector("list", length(x))
  for (i in seq_along(x)) {
    out[[i]] <- f(x[[i]], ...)
  }
  out
}

lapply() is just a wrapper around a common for loop pattern. The art of using functionals is to recognise what common looping patterns are implemented in existing base functionals, and then use them instead of loops.

Once you've mastered the existing functionals, the next step is to start writing your own: if you discover you're duplicating the same looping pattern in many places, you should extract it out into its own function. Once we've talked about the most important R functionals, we'll introduce some other loop patterns and start writing our own.

The following sections discuss:

  • patterns of looping, for both for loops and lapply(). The same ideas also apply to pretty much every for-loop-functional in R.

  • sapply() and vapply(), variants of lapply() that produce vectors, matrices and arrays as output, instead of lists.

  • Map() and mapply() which iterate over multiple input data structures in parallel.

  • Parallel versions of lapply() and Map()

  • Rolling computations, showing how a new problem can be solved with for loops, or by building on top of lapply().

Looping patterns

When using functionals that encapsulate for loops, it's useful to remember that there's usually three ways to loop over an vector:

  1. loop over the elements of the vector: for(x in xs)
  2. loop over the numeric indices of the vector: for(i in seq_along(xs))
  3. loop over the names of the vector: for(nm in names(xs)) {}

If you're saving the results from a for loop, you usually can't use the first form because it makes very inefficient code: if you're extending an existing data structure, the existing data gets copied every time you extend it:

xs <- runif(1e3)
res <- c()
for(x in xs) {
  res <- c(res, sqrt(x))
}

It's much better to create enough space for the output and then fill it in:

res <- numeric(length(xs))
for(i in seq_along(xs)) {
  res[i] <- sqrt(xs[i])
}

Corresponding to the three ways to use a for loop there are three ways to use lapply() with an object:

lapply(xs, function(x) {})
lapply(seq_along(xs), function(i) {})
lapply(names(xs), function(nm) {})

Typically you use the first form because lapply() takes care of saving the output for your. However, if you need to know the position or the name of the element you're working with, you'll need to use the second or third form; they give you both the position of the object (i, nm) and its value (x[[i]], x[[nm]]). If you're struggling to solve a problem using one form, you might find it easier with a different way.

It's also useful to remember that while lapply usually varies the first argument of the function, if one of the additional arguments matches the name of the first argument then R's usually matching rules with take over so effectively the second argument is varied over:

# These two calls do the same thing:
lm(mpg ~ disp + wt, data = mtcars)
lm(mtcars, formula = mpg ~ disp + wt)

# So if we want to fit a list of different formulas to the same 
# dataset, we do:
formulas <- list(mpg ~ disp, mpg ~ wt, mpg ~ disp + wt)
models1 <- lapply(formulas, lm, data = mtcars)

# If we want to fit the same formula to different datasets
bootstraps <- lapply(1:10, function(i) {
  rows <- sample(1:nrow(mtcars), rep = TRUE)
  mtcars[rows, ]
})
models2 <- lapply(bootstraps, lm, formula = mpg ~ disp + wt)

Finally if you're working with a list of functions, remember to use call_fun:

call_fun <- function(f, ...) f(...)
f <- list(sum, mean, median, sd)
lapply(f, call_fun, x = runif(1e3))

Or you create a variant of fapply() specifically for working with lists of functions:

fapply <- function(fs, ...) {
  out <- vector("list", length(fs))
  for (i in seq_along(fs)) {
    out[[i]] <- fs[[i]](...)
  }
  out
}
fapply(f, x = runif(1e3))

Vector output: sapply and vapply

sapply() and vapply() are very similar to lapply() except they will simplify their output to produce an atomic vector. sapply() guesses, while you have to be specific with vapply(). sapply() is useful for interactive use because it saves typing, but if you use it inside your functions you will get weird errors if you supply the wrong type of input. vapply() is more verbose, but gives more informative errors messages and never fails silently, so is better suited for programming with.

The following example illustrates these differences. When given a data frame sapply() and vapply() give the same results, but if you pass in an empty list, sapply() has no basis to guess the correct type of output.

sapply(mtcars, is.numeric)
vapply(mtcars, is.numeric, logical(1))
sapply(list(), is.numeric)
vapply(list(), is.numeric, logical(1))

Similarly, if the function returns different types or lengths of results, sapply() will silently coerce, while vapply() will throw an error. sapply() is fine for interactive use because you'll normally notice if something went wrong, but it's dangerous when writing functions. The following example illustrates a possible problem when extracting the class of columns in data frame: if you falsely assume that class only has one value and use sapply() you won't find out about the problem until some future function call gets a list instead of a character vector.

df <- data.frame(x = 1:10, y = letters[1:10])
sapply(df, class)
vapply(df, class, character(1))

df2 <- data.frame(x = 1:10, y = Sys.time() + 1:10)
sapply(df2, class)
vapply(df2, class, character(1))

sapply() is a thin wrapper around lapply(), transforming a list into a vector in the final step; vapply() reimplements lapply() but assigns results into a vector (or matrix) of the appropriate type instead of into a list. The following code shows pure R implementation of the essence of sapply() and vapply(); the real functions have better error handling and preserve names, among other things.

sapply2 <- function(x, f, ...) {
  res <- lapply2(x, f, ...)
  simplify2array(res)
}

vapply2 <- function(x, f, f.value, ...) {
  out <- matrix(rep(f.value, length(x)), nrow = length(x))
  for (i in seq_along(x)) {
    res <- f(x[i], ...)
    stopifnot(
      length(res) == length(f.value), 
      typeof(res) == typeof(f.value)
    )
    out[i, ] <- res
  }
  out
}
vapply2(1:10, f, logical(1))

vapply() and sapply() are like lapply(), but with different inputs; the following section discusses Map(), which is like lapply() but with different inputs.

Multiple inputs: Map (and mapply)

With lapply(), only one argument to the function varies; the others are fixed. With Map, all arguments vary:

a <- c(1, 2, 3)
b <- c("a", "b", "c")

str(lapply(FUN = list, a, b))
str(Map(f = list, a, b))

Map() iterates over all of its arguments, not just the first one, so that Map(f, x, y, z) is equivalent to:

for(i in seq_along(x)) {
  output[[i]] <- f(x[[i]], y[[i]], z[[i]])
}

What if you have arguments that need to be fixed, not varying? Use an anonymous function!

Map(function(x, y) f(x, y, zs), xs, ys)

You may be more familiar with mapply() than Map(). I prefer Map() because:

  • it is equivalent to mapply with simplify = FALSE, which is almost always what you want.

  • Instead of using an anonymous function to provide constant inputs, mapply has the MoreArgs argument which takes a list of extra arguments that will be supplied, as is, to each call. This breaks R's usual lazy evaluation semantics, and is inconsistent with other functions.

In brief, mapply() is much more complicated for little gain.

Rolling computations

What if you need a for-loop replacement that doesn't exist in base R? You can create your own by recognising common looping structures and implementing your own wrapper.

For example, you might be interested in smoothing your data using a rolling (or running) mean function:

rollmean <- function(x, n) {
  out <- rep(NA, length(x))

  offset <- trunc(n / 2)
  for (i in (offset + 1):(length(x) - n + offset - 1)) {
    out[i] <- mean(x[(i - offset):(i + offset - 1)])
  }
  out
}
x <- seq(1, 3, length = 1e2) + runif(1e2)
plot(x)
lines(rollmean(x, 5))
lines(rollmean(x, 10), col = "red")

But if your noise was more variable (i.e. it had a longer-tail) you might worry that your rolling mean was too sensitive to the occassional outlier and instead implement a rolling median. To modify rollmean() to rollmedian() all you need to do is replace mean with median inside the loop, but instead of copying and pasting to create a new function, you might think about abstracting out the notion of computing a rolling summary.

x <- seq(1, 3, length = 1e2) + rt(1e2, df = 2) / 3
plot(x)
lines(rollmean(x, 5))

rollapply <- function(x, n, f, ...) {
  out <- rep(NA, length(x))

  offset <- trunc(n / 2)
  for (i in (offset + 1):(length(x) - n + offset - 1)) {
    out[i] <- f(x[(i - offset):(i + offset - 1)], ...)
  }
  out
}
lines(rollapply(x, 5, median), col = "red")

You might notice that this is pretty similar to what vapply does, and in fact we could rewrite it as

which is effectively how zoo::rollapply implements it, albeit with many more features and much more error checking.

Parallelisation

One thing that's interesting about our defintions of lapply() and and variants is that because each iteration is isolated from all others, it doesn't matter in which order they are computed. Even though lapply3(), defined below, scrambles the order in which computation occurs, the results are always the same:

lapply3 <- function(x, f, ...) {
  out <- vector("list", length(x))
  for (i in sample(seq_along(x))) {
    out[[i]] <- f(x[[i]], ...)
  }
  out
}
lapply3(1:10, sqrt)
lapply3(1:10, sqrt)
lapply3(1:10, sqrt)

This has a very important consequence: since we can compute each element in any order, it's easy to dispatch the tasks to different cores, and compute in parallel. This is what mclapply() (and mcMap) in the parallel package do:

library(parallel)
mclapply(1:10, sqrt, mc.cores = 4)

In this case mclapply() is actually slower than lapply(), becuase the cost of the individual computations is low, and some additional work is needed to send the computation to the different cores and then collect the results together. If we take a more realistic example, generating bootstrap replicates of a linear model, we see the advantage of parallel computation:

boot_df <- function(x) x[sample(nrow(x), rep = T), ]
rsquared <- function(mod) summary(mod)$r.square
boot_lm <- function(i) {
  rsquared(lm(mpg ~ wt + disp, data = boot_df(mtcars)))
}

system.time(lapply(1:100, boot_lm))
system.time(mclapply(1:100, boot_lm, mc.cores = 2))

Exercises

  • Implement a combination of Map() and vapply() to create an lapply() variant that iterates in parallel over all of its inputs and stores its outputs in a vector (or a matrix).

  • What does replicate() do? What sort of for loop does it eliminate? Why do its arguments differ from lapply() and friends?

  • Implement mcsapply(), a multicore version of sapply()

Data structure

As well as functionals that exist to eliminate common looping constructs, anotherfamily of functionals works to eliminate loops for common data manipulation tasks.

In this section, we'll give a brief overview of the available options, not spending too much time on each one. The focus is to show you some of the options that are available, hint at how they can help you, and point you in the right direction to learn more:

  • base matrix functions apply(), sweep() and outer()

  • the tapply() function for summarising a vector divided into groups by another vector

  • the plyr package, which generalises the ideas of tapply() to work with inputs of data frames, lists and arrays, and outputs of data frames, lists, arrays and nothing

Matrix and array operations

So far, all the functionals we've seen work with 1d input structures. The three functionals in this section provide useful tools for working with high-dimensional data strucures.

apply() is like a variant of sapply() that works with matrices and arrays, but it's easier to think of it as an operation than summarises a matrix or array, collapsing a row or column to a single number. It has four arguments:

  • X, the matrix or array to summarise
  • MARGIN, an integer vector giving the dimensions to summarise over, 1 = rows, 2 = columns, etc
  • FUN, a summary function
  • ... other arguments passed on to FUN

A typical example of apply() looks like this

a <- matrix(1:20, nrow = 5)
apply(a, 1, mean)
apply(a, 2, mean)

There are a few caveats apply to using apply(): it does not have a simplify argument, so you can never be completely sure what type of output you will get. This generally means that apply() is not safe to program with, unless you very carefully check the inputs. apply() is also not idempotent in the sense that if the summary function is the identity operator, the output is not always the same as the input:

a1 <- apply(a, 1, identity)
identical(a, a1)
identical(a, t(a1))

sweep() is a function that allows you to "sweep" out the values of a summary statistic. It is most often useful in conjunction with apply() and it often used to standardise arrays in some way.

x <- matrix(runif(20), nrow = 4)
x1 <- sweep(x, 1, apply(x, 1, min))
x2 <- sweep(x1, 1, apply(x1, 1, max), "/")

The final matrix functional is outer(). It's a little different in that it takes multiple vector inputs and creates a matrix or array output where the input function is run over every combination of the inputs:

# Create a times table
outer(1:10, 1:10, "*")

Good places to learn more about apply() are:

Group apply

In some sense, tapply() is a generalisation to apply() that allows for "ragged" arrays, where each row can have different numbers of rows. This often comes about when you're trying to summarise a data set. For example, imagine you've collected some pulse rate from a medical trial, and you want to compare the two groups:

pulse <- round(rnorm(22, 70, 10 / 3)) + rep(c(0, 5), c(10, 12))
group <- rep(c("A", "B"), c(10, 12))

tapply(pulse, group, length)
tapply(pulse, group, mean)

It's easiest to understand how tapply() works by first creating a "ragged" data structure from the inputs. This is the job of the split() function, which takes two inputs, and returns a list, where all the elements in the first vector with equal entries in the second vector get put in the same element of the list:

split(pulse, group)

Then it's obvious that tapply() is just the combination of split() and sapply():

tapply2 <- function(x, group, f, ..., simplify = TRUE) {
  pieces <- split(x, group)
  sapply(pieces, f, simplify = simplify)
}
tapply2(pulse, group, length)
tapply2(pulse, group, mean)

This is a common pattern: if you have a good foundational set of functionals, you can solve many problems by combining then with new tools.

The plyr package

One challenge with using the functionals provided by the base package is that they have grown organically over time, and have been written by multiple authors. This means that they are not very consistent:

  • The simplify argument is called simplify in tapply() and sapply(), but SIMPLIFY for mapply(), and apply() doesn't have any way to turn off simplification.

  • vapply() provides a variant of sapply() where you describe what the output should be, but there are not corresponding variants for tapply(), or apply().

This makes learning these operators challenging, as you have to memorise all of the variations. Additionally, if you think about the combination of input and output types, base R only provides a partial set of functions:

list data frame array
list lapply sapply
data frame by
array apply

This was one of the driving forces behind the creation of the plyr package, which provides consistently named functions with consistently named arguments, that implement all combinations of input and output data structures:

list data frame array
list llply ldply laply
data frame dlply ddply daply
array alply adply aaply

Each of these functions processes breaks up a data structure in some way, applies a function to each piece and then joins the results back together.

You can read more about these plyr function in The Split-Apply-Combine Strategy for Data Analysis, an open-access article published in the Journal of Statistical Software.

Exercises

  • There's no equivalent to split() + vapply(). Should there be? When would it be useful? Implement it yourself.

  • Implement a pure R version of split(). (Hint: use unique and subseting)

  • What other types of input and output are missing? Brainstorm before you look up in the answers in the plyr paper

Functional programming

The three most important functionals, implemented in almost every other functional programming language are Map(), Reduce(), and Filter(). We've seen Map() already, Reduce() is a powerful tool for extending two-argument functions, and Filter() is a member of an important class of functions that work with predicate functions, functions that return a single boolean.

Reduce()

Reduce(): recursively reduces a vector to a single value by first calling f with the first two elements, then the result of f and the second element and so on.

out <- x[[1]]
for(i in seq(2, length(x)) {
  out <- f(out, x[[i]])
}

Reduce is useful for implementing many types of recursive operations: merges, finding smallest values, intersections, unions.

Reduce is an elegant way of turning binary functions into functions that can deal with any number of arguments, but it is generally of limited use in R because it will produce functions that are much slower than equivalent hand-vectorised code.

Can you implement unlist with reduce? + c?

Predicates

A predicate is a function that returns TRUE or FALSE.

Predicate functions are more useful in non-vectorised languages: you don't need them in R, because many useful predictates are already vectorised: is.na, comparisons, boolean operators

  • Filter: returns a new vector containing only elements where the predicate is TRUE.

One function that I find very helpful is compact.

As well as filter, two other functions are useful when you have logical predicates

  • Find(): return the first element that matches the predicate (or the last element if right = TRUE).

    for(i in seq_along(x)) {
  if (f(x[[i]])) return(x[[i]])
}

  • Position(): return the position of the first element that matches the predicate (or the last element if right = TRUE).

    for(i in seq_along(x)) {
  if (f(x[[i]])) return(i)
}

Other languages like Haskell and clojure provide more functions:

take_while <- function(x, f) {
  x[1:Position(x, f, nomatch = length(x))]
}
drop_while <- function(x, f) {
  x[1:Position(x, f, nomatch = length(x))]
}
break <- function(x, f) {
  list(take_while(x, f), drop_while(x, f))
}

Another useful function is the ability to apply a non-vectorised predicate to a vectorised predicate:

where <- function(x, f) {
  vapply(x, f, logical(1))
}
where(mtcars, is.character)

Once we have that function, Filter is particularly easy to implement:

Filter <- function(f, x) x[where(x, f)]

But where makes for a better low level function because there are many ways to combine logical vectors.

  • STL: find_if, count_if, replace_if, remove_if, none_of, any_of, stable_partition

Exercises

  • Implement Any which takes a list and a predicate function, and returns TRUE if the predicate function returns TRUE for any of the inputs. Implement a similar All function.

  • Implement a more efficient version of break that avoids finding the location of the true value twice.

  • Implement the span function from Haskell, which given a list x and a predicate function f, returns the longest sequential run of elements where the predicate is true.

Mathematical functionals

Functionals are very common in mathematics: the limit, the maximum, the roots (the set of points where f(x) = 0), and the definite integral are all functionals; given a function, they return a single number (or a vector of numbers). At first glance, these functions don't seem to fit in with the theme of eliminating loops, but if you dig deeper you'll see all of them are implemented using an algorithm that involves iteration.

In this section we'll explore some of R's built-in mathematical functionals. There are three functions that work with a 1d numeric function:

  • integrate: integrate it over a given range
  • uniroot: find where it hits zero over a given range
  • optimise: find location of minima (or maxima)

Let's explore how these are used with a simple function:

integrate(sin, 0, pi)
uniroot(sin, pi * c(1 / 2, 3 / 2))
optimise(sin, c(0, 2 * pi))
optimise(sin, c(0, pi), maximum = TRUE)

In statistics, optimisation is often used for maximum likelihood estimation. Maximum likelihood estimation (MLE) is a natural fit for functional programming. We have a problem and a general technique to solve it. MLE also works well with closures because the arguments to a likelihood fall into two groups: the data, which is fixed for a given problem, and the parameters, which will vary as we try to find a maximum numerically. This naturally gives rise to an approach like the following.

First, we create a function that computes the negative log likelihood (NLL) for a given dataset. In in R, it's common to use the negative since optimise() defaults to findinging the minimum.

poisson_nll <- function(x) {
  n <- length(x)
  function(lambda) {
    n * lambda - sum(x) * log(lambda) # + terms not involving lambda
  }
}

With the general NLL in hand, we create two specific NLL functions for two datasets, and use optimise() to find the best values, given a generous starting range.

nll1 <- poisson_nll(c(41, 30, 31, 38, 29, 24, 30, 29, 31, 38)) 
nll2 <- poisson_nll(c(6, 4, 7, 3, 3, 7, 5, 2, 2, 7, 5, 4, 12, 6, 9)) 

optimise(nll1, c(0, 100))
optimise(nll2, c(0, 100))

Another important mathmatical functional is optim(). It is a generalisation of optim() to more than one direction. If you're interested in how optim() works, you might want to explore the Rvmmin package, which provides a pure-R implementation of R. Interestingly Rvmmin is no slower than optim(), even though it is written in R, not C: for this problem, the bottleneck is evaluating the function multiple times, not controlling the optimisation.

Exercises

  • Implement the arg_max function. It should take a function, and a vector of inputs, returning the elements of the input where the function returns the highest number. For example, arg_max(-10:5, function(x) x ^ 2) should return -10. arg_max(-5:5, function(x) x ^ 2) should return c(-5, 5). Also implement the matching arg_min.

  • Read about the fixed point algorithm in http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#%_sec_1.3. Complete the exercises using R.

Converting loops to functionals, and when it's not possible

That there are wide class of for loops that can not be simplified to a single existing function call in R. It is not always a good idea to eliminate a for-loop: for loops are verbose and not very expressive, but all R programmers are familiar with them. For example, it's sometimes possible to work around the limitations of lapply by using more estoeric language features like <<-

trans <- list(
  disp = function(x) x * 0.0163871,
  am = function(x) factor(x, levels = c("auto", "manual"))
)
for(var in names(trans)) {
  mtcars[[var]] <- trans[[var]](mtcars[[var]])
}

You could rewrite this as

lapply(names(trans), function(var) {
  mtcars[[var]] <<- trans[[var]](mtcars[[var]])
})

We've eliminated the obvious for loop, but our code is longer, and we've had to use a language feature that few people are familiar with <<-. And to really understand what mtcars[[var]] <<- is doing, you need to have a good mental model of how replacement functions really work. So we've taken something simple and made more complicated, for effectively no gain.

There is Good stackoverflow discussions on converting loops to more efficient/expressive code:

exps <- function(x, alpha) {
  s <- numeric(length(x) + 1)
  for (i in seq_along(s)) {
    if (i == 1) {
      s[i] <- x[i]
    } else {
      s[i] <- alpha * x[i - 1] + (1 - alpha) * s[i - 1]
    }
  }
}
exps(x, 0.5)

(What's the key feature? Dependening on previously calculated values? Or is it just a cumulative weighted sum?)

lbapply <- function(x, f, init = x[1], ...) {
  out <- numeric(length(x))
  out[1] <- init
  for(i in seq(2:length(x))) {
    out[i] <- f(x[i - 1], out[i - 1], ...)
  }  
}

f <- function(x, out, alpha) alpha * x + (1 - alpha) * out
lbapply(x, f, alpha = 0.5)

lbapply(x, function(x, out) x + y, 0)

Closures vs .... ... usually requires less typing, but there can be confusion about which function arguments belong to. If you're passing in more than one function, definitely go with closures.

Sometimes it's possible to solve the recurrence relation. In this case, it's possible to rewrite in terms of i:

exps1 <- function(x, alpha) {
  function(t) {
    c(rep(alpha, t), 1) * x[-t] * (1 - alpha)^(rev(seq_along(head)))
  }
}
lapply(seq_along(x), expsm1(x, alpha = 0.5))

We'll see another example of a function defined recursively, the Fibonacci series, in the SoftwareSystems chapter.

Another family of looping constructs in R is the while loop: this runs code until a condition is met. while loops are more general than for loops because every for loop than rewriting into a while loop:

for (i in 1:10) print(i)

i <- 1
while(i <= 10) {
  print(i)
  i <- i + 1
}

Not every while loop can be turned into a for loop, because for many while loops you don't know in advance how many times it will be run:

i <- 0
while(TRUE) {
  if (runif(1) > 0.9) break
  i <- i + 1
}

This is a common situation when you're writing simulations: one of the random parameters in your simulation may be how many times it is run.

In some cases, like above, you may be able to remove the loop by recongnising some special feature of the problem. For example, the above problem is counting how many times a Bernoulli trial with p = 0.1 is run before it is successful: this is a geometric random variable so you could replace the above code with i <- rgeom(1, 0.1). Similar to solving recurrence relations, this is extremely difficult to do in general, but you'll get big gains if you manage to. In most cases it is difficult to write code like that efficiently in R, and if you are calling the code a lot, you may need to convert it to C++.

It's certainly possible to write functions that encapsulate these types of loops, but they are not built in to R. Whether or not it's worth building your own function depends on how often you'll be using, and how much more expressive a better function name would be.

It is also possible to create more sophisticated control flow structures by computing on the language - developing these might be appropriate if you have a special need not otherwise met, e.g. you want to access a variable defined elsewhere, but this come with a high cost of increased complexity, and will be harder for new readers of the code to understand.

Often the trick is to not to solve the problem in complete generality, but identify what patterns common recur in your code, and then develop functions to automate them. Once you have done this a few times, you might start to recognise bigger patterns.

A family of functions

The following case study shows how you can use functionals to start small, with very simple functions, then build them up into more complicated and featureful tools. We'll start with a simple idea, adding two numbers together, and show how we can extending to summing any number inputs, or computing parallel sums, or cumulative sums, and sums for arrays in various structures.

We'll start with addition, and show how we can use exactly the same ideas for multiplication, smallest and largest, and string concatenation to generate a wide family of functions, including over 20 functions provided in base R.

We'll start by defining a very simple plus function, that takes two scalar arguments:

add <- function(x, y) {
  stopifnot(length(x) == 1, length(y) == 1, 
    is.numeric(x), is.numeric(y))
  x + y
}

(We're using R's existing addition operator here, which does much more, but the focus in this section is on how we can take very very simple functions and extend them to do more).

We really should also have some way to deal with missing values. A helper function will make this a bit easier - if x is missing it returns y, if y is missing it returns x, and if both inputs are missing then it returns another argument to the function: identity. (We'll talk a bit later about while we've called it identity later). This function is probably a bit more general than what we need now, but it will come in handy when you implement other binary operators.

rm_na <- function(x, y, identity) {
  if (is.na(x) && is.na(y)) {
    identity
  } else if (is.na(x)) {
    y
  } else {
    x
  }  
}

That allows us to write a version of add that can deal with missing values if needed: (and it often is!)

add <- function(x, y, na.rm = FALSE) {
  if (na.rm && (is.na(x) || is.na(y))) rm_na(x, y, 0) else x + y
}

Why should add(NA, NA, na.rm = TRUE) return 0? Well for every other input it returns a numeric vector of length 1, so it should probably do that too even if both arguments are missing values. There's also something special about add: it's associative, which means if you're adding together multiple numbers, it shouldn't matter in which order you're doing it. In other words, the following two function calls should return the same value:

add(add(3, NA, na.rm = TRUE), NA, na.rm = TRUE)
add(3, add(NA, NA, na.rm = TRUE), na.rm = TRUE)

Which implies that add(NA, NA, na.rm = TRUE) must be 0.

The first way we might want to extend this function is to make it possible to add multiple numbers together. This is a simple application of Reduce: if the input is c(1, 2, 3), then we want to compute add(1, add(2, 3)):

r_add <- function(xs, na.rm = TRUE) {
  Reduce(function(x, y) add(x, y, na.rm = na.rm), xs)
}
r_add(c(1, 4, 10))

This looks good, but we need to test it for a few special cases:

r_add(NA, na.rm = TRUE)
r_add(numeric())

These are incorrect: in the first case we get a missing value even thought we've explicitly asked for them to be ignored, and in the second case we get a null, instead of a length 1 numeric vector (as for every other set of inputs).

The two problems are related: if we give Reduce() a length one vector it doesn't have anything to reduce, so it just returns the same value. And if we give it a length 0 input it returns NULL. There are two ways to fix this: we can add 0 to every input vector, or we can use the init argument to Reduce() which effectively does the same thing:

r_add <- function(xs, na.rm = TRUE) {
  Reduce(function(x, y) add(x, y, na.rm = na.rm), c(0, xs))
}
r_add(c(1, 4, 10))
r_add(NA, na.rm = TRUE)
r_add(numeric())

(There is of course a function in R that already does that: sum)

But it would be nice to have a vectorised version so that we could give it two vectors of numbers and they were added together.

We have two options to implement this, neither of which are perfect. We could use Map, but that will give us a list, or we could use vapply by looping over the indices. That gives us a better output data structure, but a version of Map where we could specify the output type would be even better.

A few test cases makes sure that it behaves as we expect: the output is always the same as the input (we're a bit stricter than base R here because we don't do recyclying - you could add that if you wanted, but I find you get fewer bugs by avoidingin recycling and being specific anyway.)

v_add <- function(x, y, na.rm = TRUE) {
  stopifnot(length(x) == length(x), is.numeric(x), is.numeric(y))
  Map(function(x, y) add(x, y, na.rm = na.rm), x, y)
}

v_add <- function(x, y, na.rm = TRUE) {
  stopifnot(length(x) == length(x), is.numeric(x), is.numeric(y))
  vapply(seq_along(x), function(i) add(x[i], y[i], na.rm = na.rm),
    numeric(1))
}
v_add(1:10, 1:10)
v_add(numeric(), numeric())
v_add(c(1, NA), c(1, NA), na.rm = TRUE)

(This is of course exactly the usual behavior of + in R, although we don't have the same control over missing values - there's no way to tell + to remove missing values)

Another variant of adding is the cumulative sum: it's like the reductive version, but we see every step along the way to the final result. This is easy to implement with Reduce()'s accumuate argument:

c_add <- function(xs, na.rm = FALSE) {
  Reduce(function(x, y) add(x, y, na.rm = na.rm), xs, 
    accumulate = TRUE)
}
c_add(1:10)
c_add(10:1)

(This function also already has an existing R equivalent)

Finally, we might want to define versions for more complicated data structures, like matrices. We could create row and col variants that sum across rows and columns respectively, or we could go the whole hog and define an array version that would sum across any arbitrary dimensions of an array. These are easy to implement: they're a combination of add and apply

row_sum <- function(x, na.rm = TRUE) apply(x, 1, add, na.rm = na.rm)
col_sum <- function(x, na.rm = TRUE) apply(x, 2, add, na.rm = na.rm)
arr_sum <- function(x, dim, na.rm = TRUE) apply(x, dim, add, na.rm = na.rm)

(And again we have the existing rowSums and colSums functions that do the same thing)

So if every function we have created already has an existing equivalent in base R, why did we bother? There are three main reasons:

  • because we've created all our variants from a primitive binary operator (add) and a functional (Reduce, Map and apply), we know all the functions will behave absolutely consistently.

  • we've seen the infrastructure for addition, and now we can adapt it to other operators.

The downside of this approach is that these implementations are unlikely to be very efficient. However, even if they don't turn out to be fast enough for your purposes they are still a good starting point because they are less likely to have bugs - when you create faster versions (maybe using Rcpp), you can compare results to make sure your fast versions are still correct.

Exercises

  • Implement smaller and larger functions that given two inputs return either the small or the large value. Implement na.rm = TRUE: what should the identity be? (Hint: smaller(x, smaller(NA, NA, na.rm = TRUE), na.rm = TRUE) must be x, so smaller(NA, NA, na.rm = TRUE) must be bigger than any other value of x.). Use smaller and larger to implement equivalents of min, max, pmin, pmax, and new functions row_min and row_max

  • Create a table that has:

    • columns: add, multiple, smaller, larger, and, or

    • rows: binary operator, reducing version, vectorised version, array versions

    • Fill in the cells with the names of base R functions that perform each of the roles

    • Compare the names and arguments of the existing R functions. How consistent are they? How could you improve them?

    • Complete the matric by implementing versions

  • How does paste fit into this structure? What is the primitive function that underlies paste? What are the sep and collapse arguments equivalent to? Are there are any components that are missing for paste?

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