Chapter 1. Introduction

This chapter describes the wide range of people this book was written for, in terms of R and programming experience, and how you can get the most out of it. Anyone setting out to improve efficiency should have an understanding of precisely what they mean by the term, and this is discussed with reference to algorithmic and programmer efficiency in “What Is Efficiency?”, and with reference to R in particular in “What Is Efficient R Programming?” on the same page. It may seem obvious, but it’s also worth thinking about why anyone would bother with efficient code now that powerful computers are cheap and accessible. This is covered in “Why Efficiency?”.

This book happily is not completely R-specific. Non R–programming skills that are needed for efficient R programming, which you will develop during the course of following this book, are covered in “Cross-Transferable Skills for Efficiency”. Atypically for a book about programming, this section introduces touch typing and consistency, cross-transferable skills that should improve your efficiency beyond programming. However, this is first and foremost a book about programming and it wouldn’t be so without code examples in every chapter. Despite being more conceptual and discursive, this opening chapter is no exception: its penultimate section (“Benchmarking and Profiling”) describes two essential tools in the efficient R programmer’s toolbox and how to use them with a couple of illustrative examples. The final thing to say at the outset is how to use this book in conjunction with the book’s associated package and its source code. This is covered in “Book Resources”.

Prerequisites

As emphasized in the next section, it’s useful to run code and experiment as you read. This section, found at the beginning of each chapter, ensures that you have the necessary packages for each chapter. The prerequisites for this chapter are:

  • A working installation of R on your computer (see “Installing and Updating RStudio”).

  • Install and load the microbenchmark, profvis, and ggplot2 packages (see “Installing R Packages” for tips on installing packages and keeping them up-to-date). You can ensure that these packages are installed by loading them as follows:

    library("microbenchmark")
    library("profvis")
    library("ggplot2")

The prerequisites needed to run the code contained in the entire book are covered in “Book Resources” at the end of this chapter.

Who This Book Is for and How to Use It

This book is for anyone who wants to make their R code faster to type, faster to run, and more scalable. These considerations generally come after learning the very basics of R for data analysis; we assume you are either accustomed to R or proficient at programming in other languages, although this book could still be of use for beginners. Thus the book should be useful to people with a range of skill levels, who can broadly be divided into three groups:

For programmers with little experience with R

This book will help you navigate the quirks of R to make it work efficiently: it is easy to write slow R code if you treat it as if it were another language.

For R users with little experience in programming

This book will show you many concepts and tricks of the trade, some of which are borrowed from computer science, that will make your work more time effective.

For R beginners with little experience in programming

This book can steer you to get things right (or at least less wrong) at the outset. Bad habits are easy to gain but hard to lose. Reading this book at the outset of your programming career could save you many hours in the future searching the web for issues covered in this book.

Identifying which group you best fit into will help you get the most out of it. For everyone, we recommend reading Efficient R Programming while you have an active R project on the go, whether it’s a collaborative task at work or simply a personal project at home. Why? The scope of this book is wider than most programming textbooks (Chapter 4 covers project management, for example) and working on a project outside the confines of it will help put the concepts, recommendations, and code into practice. Going directly from words into action in this way will help ensure that the information is consolidated: learn by doing.

If you’re an R novice and fit into the final category, we recommend that this active R project not be an important deliverable, but another R resource. Though this book is generic, it is likely that your usage of R will be largely domain-specific. For this reason, we recommend reading it alongside teaching material in your chosen area. Furthermore, we advocate that all readers use this book alongside other R resources such as the numerous vignettes, tutorials, and online articles that the R community has produced (described in the following tip). At a bare minimum, you should be familiar with data frames, looping, and simple plots, which you will learn from these resources.

What Is Efficiency?

In everyday life, efficiency roughly means working well. An efficient vehicle goes far without guzzling gas. An efficient worker gets the job done fast without stress. And an efficient light shines brightly with a minimum of energy consumption. In this final sense, efficiency (η) has a formal definition as the ratio of work done (W, light output) per unit effort (Q, energy consumption in this case):

How does this translate into programming? Efficient code can be defined narrowly or broadly. The first, more narrow definition is algorithmic efficiency: how fast the computer can undertake a piece of work given a particular piece of code. This concept dates back to the very origins of computing, as illustrated by the following quote by Ada Lovelace (1842) in her notes on the work of Charles Babbage:

In almost every computation a great variety of arrangements for the succession of the processes is possible, and various considerations must influence the selections amongst them for the purposes of a calculating engine. One essential object is to choose that arrangement which shall tend to reduce to a minimum the time necessary for completing the calculation.

The second, broader definition of efficient computing is programmer productivity. This is the amount of useful work a person (not a computer) can do per unit time. It may be possible to rewrite your code base in C to make it 100 times faster. But if this takes 100 human hours, it may not be worth it. Computers can chug away day and night. People cannot. Human productivity is the subject of Chapter 4.

By the end of this book, you should know how to write code that is efficient from both algorithmic and productivity perspectives. Efficient code is also concise, elegant, and easy to maintain, which is vital when working on large projects. But this raises the wider question: what is different about efficient R code compared with efficient code in any other language?

What Is Efficient R Programming?

The issue flagged by Ada of having a great variety of ways to solve a problem is key to understanding how efficient R programming differs from efficient programming in other languages. R is notorious for allowing users to solve problems in many ways. This is due to R’s inherent flexibility, in which almost “anything can be modified after it is created” (Wickham 2014). R’s inventors, Ross Ihaka and Robert Gentleman, designed it to be this way: a cell in a data frame can be selected in multiple ways in base R alone (three of which are illustrated later in this chapter, in “Benchmarking Example”). This is useful because it allows programmers to use the language as best suits their needs, but it can be confusing for people looking for the right way of doing things and can cause inefficiencies if you don’t fully understand the language.

R’s notoriety for being able to solve a problem in multiple ways has grown with the proliferation of community-contributed packages. In this book, we focus on the best way of solving problems from an efficiency perspective. Often it is instructive to discover why a certain way of doing things is faster than other ways. However, if your aim is simply to get stuff done, you only need to know what is likely to be the most efficient way. In this way, R’s flexibility can be inefficient: although it may be easier to find a way of solving any given problem in R than other languages, solving the problem with R may make it harder to find the best way to solve that problem, as there are so many. This book tackles this issue head on by recommending what we believe are the most efficient approaches. We hope you trust our views, based on years of using and teaching R, but we also hope that you challenge them at times and test them with benchmarks if you suspect there’s a better way of doing things (thanks to R’s flexibility and ability to interface with other languages, there may well be).

It is well known that R code can lack algorithmic efficiency compared with low-level languages for certain tasks, especially if it was written by someone who doesn’t fully understand the language. But it is worth highlighting the numerous ways that R encourages and guides efficiency, especially programmer efficiency:

  • R is not compiled, but it calls compiled code. This means that you get the best of both worlds: thankfully, R removes the laborious stage of compiling your code before being able to run it, but provides impressive speed gains by calling compiled C, FORTRAN, and other language behind the scenes.

  • R is a functional and object-orientated language (Wickham 2014). This means that it is possible to write complex and flexible functions in R that get a huge amount of work done with a single line of code.

  • R uses RAM for memory. This may seem obvious, but it’s worth saying: RAM is much faster than any hard disk system. Compared with databases, R is therefore very fast at common data manipulation, processing, and modeling operations. RAM is now cheaper than ever, meaning the potential downsides of this feature are further away than ever.

  • R is supported by excellent integrated development environments (IDEs). The environment in which you program can have a huge impact on programmer efficiency as it can provide help quickly, allow for interactive plotting, and allow your R projects to be tightly integrated with other aspects of your project such as file management, version management, and interactive visualization systems, as discussed in “RStudio”.

  • R has a strong user community. This boosts efficiency because if you encounter a problem that has not yet been solved, you can simply ask the community. If it is a new, clearly stated, and reproducible question asked on a popular forum such as Stack Overflow or an appropriate R list, you are likely to get a response from an accomplished R programmer within minutes. The obvious benefit of this crowd-sourced support system is that the efficiency benefits of the answer will, from that moment on, be available to everyone.

Efficient R programming is the implementation of efficient programming practices in R. All languages are different, so efficient R code does not look like efficient code in another language. Many packages have been optimized for performance so, for some operations, achieving maximum computational efficiency may simply be a case of selecting the appropriate package and using it correctly. There are many ways to get the same result in R, and some are very slow. Therefore, not writing slow code should be prioritized over writing fast code.

Returning to the analogy of the two cars sketched in the preface, efficient R programming for some use cases can simply mean trading in your old, heavy, gas-guzzling SUV function for a lightweight velomobile. The search for optimal performance often has diminishing returns, so it is important to find bottlenecks in your code to prioritize work for maximum increases in computational efficiency. Linking back to R’s notoriety as a flexible language, efficient R programming can be interpreted as finding a solution that is fast enough in terms of computational efficiency but as fast as possible in terms of programmer efficiency. After all, you and your coworkers probably have better and more valuable things to do outside work, so it is important that you get the job done quickly and take time off for other interesting pursuits.

Why Efficiency?

Computers are always getting more powerful. Does this not reduce the need for efficient computing? The answer is simple: no. In an age of Big Data and stagnating computer clock speeds (see Chapter 8), computational bottlenecks are more likely than ever before to hamper your work. An efficient programmer can “solve more complex tasks, ask more ambitious questions, and include more sophisticated analyses in their research” (Visser et al. 2015).

A concrete example illustrates the importance of efficiency in mission-critical situations. Robin was working on a tight contract for the UK’s Department for Transport to build the Propensity to Cycle Tool, an online application that had to be ready for national deployment in less than four months. For this work, he developed the function line2route() in the stplanr package to generate routes via the (CycleStreets) API. Hundreds of thousands of routes were needed, but, to his dismay, code slowed to a standstill after only a few thousand routes. This endangered the contract. After eliminating other issues and via code profiling (covered in “Code Profiling”), it was found that the slowdown was due to a bug in line2route(): it suffered from the vector growing problem, discussed in “Memory Allocation”.

The solution was simple. A single commit made line2route() more than ten times faster and substantially shorter. This potentially saved the project from failure. The moral of this story is that efficient programming is not merely a desirable skill—it can be essential.

There are many concepts and skills that are language-agnostic. Much of the knowledge imparted in this book should be relevant to programming in other languages (and other technical activities beyond programming). There are strong reasons for focusing on efficiency in one language, however. In R, simply using replacement functions from a different package can greatly improve efficiency, as discussed in relation to reading text files in Chapter 5. This level of detail, with reproducible examples, would not be possible in a general-purpose efficient programming book. Skills for efficient working, which apply beyond R programming, are covered in the next section.

Cross-Transferable Skills for Efficiency

The meaning of efficient R code, as opposed to generic efficient code, should be clear from reading the preceding two sections. However, that does not mean that the skills and concepts covered in this book are not transferable to other languages and non-programming tasks. Likewise, working on these cross-transferable skills will improve your R programming (as well as other aspects of your working life). Two of these skills are especially important: touch typing and use of a consistent style.

Touch Typing

The other side of the efficiency coin is programmer efficiency. There are many things that will help increase the productivity of you and your collaborators, not least following the advice of Philipp Janert to “think more, work less” (Janert 2010). The evidence suggests that good diet, physical activity, plenty of sleep, and a healthy work-life balance can all boost your speed and effectiveness at work (Jensen 2011; Pereira et al. 2015; Grant, Spurgeon, and Wallace 2013).

While we recommend that the reader reflect on this evidence and their own well-being, this is not a self-help book. It is a book about programming. However, there is one nonprogramming skill that can have a huge impact on productivity: touch typing. This skill can be relatively painless to learn, and can have a huge impact on your ability to write, modify, and test R code quickly. Learning to touch type properly will pay off in small increments throughout the rest of your programming life (of course, the benefits are not constrained to R programming).

The key difference between a touch typist and someone who constantly looks down at the keyboard, or who uses only two or three fingers for typing, is hand placement. Touch typing involves positioning your hands on the keyboard with each finger of both hands touching or hovering over a specific letter (Figure 1-1). This takes time and some discipline to learn. Fortunately there are many resources that will help you get in the habit early, including the open source software projects Klavaro and TypeFaster.

The starting position for touch typing, with the fingers over the 'home keys'. Source: https://commons.wikimedia.org/wiki/File:QWERTY-home-keys-position.svg(Wikipedia) under the Creative Commons license.
Figure 1-1. The starting position for touch typing, with the fingers over the home keys. Source: Wikipedia under the Creative Commons license.

Consistent Style and Code Conventions

Getting into the habit of clear and consistent style when writing anything, be it code or poetry, will have benefits in many other projects, programming or non-programming. As outlined in “Coding Style”, style is to some extent a personal preference. However, it is worth noting the conventions we use at the outset of this book, to maximize its readability. Throughout this book we use a consistent set of conventions to refer to code.

  • Package names are in bold, e.g., dplyr.

  • Functions are in a code font, followed by parentheses, like plot() or median().

  • Other R objects, such as data or function arguments, are in a code font without parentheses, like x and name.

  • Occasionally, we’ll highlight the package of the function using two colons, like microbenchmark::microbenchmark(). Note that this notation can be efficient if you only need to use a package’s function once, as it avoids attaching the package.

The concepts of benchmarking and profiling are not R-specific. However, they are done in a particular way in R, as outlined in the next section.

Benchmarking and Profiling

Benchmarking and profiling are key to efficient programming, especially in R. Benchmarking is the process of testing the performance of specific operations repeatedly. Profiling involves running many lines of code to find bottlenecks. Both are vital for understanding efficiency, and we use them throughout the book. Their centrality to efficient programming practice means they must be covered in this introductory chapter, despite being seen by many as an intermediate or advanced R programming topic.

In some ways, benchmarks can be seen as the building blocks of profiles. Profiling can be understood as automatically running many benchmarks for every line in a script and comparing the results line by line. Because benchmarks are smaller, easier, and more modular, we cover them first.

Benchmarking

Modifying elements from one benchmark to the next and recording the results after the modification enables us to determine the fastest piece of code. Benchmarking is important in the efficient programmer’s toolkit: you may think that your code is faster than mine, but benchmarking allows you to prove it. The easiest way to benchmark a function is to use system.time(). However, it is important to remember that we are taking a sample. We wouldn’t expect a single person in London to be representative of the entire UK population; similarly, a single benchmark provides us with a single observation on our function’s behavior. Therefore, we’ll need to repeat the timing many times with a loop.

An alternative way of benchmarking is via the flexible microbenchmark package. This allows us to easily run each function multiple times (by default, 100) in order to detect microsecond differences in code performance. We then get a convenient summary of the results: the minimum/maximum and lower/upper quartiles, and the mean/median times. We suggest focusing on the median time to get a feel for the standard time and the quartiles to understand the variability.

Benchmarking Example

A good example is testing different methods to look up a single value in a data frame. Note that each argument in the following benchmark is a term to be evaluated (for multi-line benchmarks, the term to be evaluated can be surrounded by curly brackets, {}).

library("microbenchmark")
df = data.frame(v = 1:4, name = letters[1:4])
microbenchmark(df[3, 2], df[3, "name"], df$name[3])
# Unit: microseconds
#          expr     min    lq  mean median    uq   max neval cld
#      df[3, 2]   17.99 18.96 20.16  19.38 19.77 35.14   100   b
# df[3, "name"]   17.97 19.13 21.45  19.64 20.15 74.00   100   b
#    df$name[3]   12.48 13.81 15.81  14.48 15.14 67.24   100   a

The results summarize how long each query took: the minimum (min); lower and upper quartiles (lq and uq, respectively); and the mean, median, and maximum (max) for each of the number of evaluations (neval, with the default value of 100 used in this case). cld reports the relative rank of each row in the form of compact letter display: in this case, df$name[3] performs best, with a rank of a and a mean time of around 25% lower than the other two functions.

When using microbenchmark(), you should pay careful attention to the units. In the previous example, each function call takes approximately 20 microseconds, implying around 50,000 function calls could be done in a second. When comparing quick functions, the standard units are:

milliseconds (ms)

One thousand functions takes a second;

microseconds (µs)

one million function calls takes a second;

nanoseconds (ns)

one billion calls takes a second.

We can set the units we want to use with the unit argument (e.g., the results are reported in seconds if we set unit = "s").

When thinking about computational efficiency, there are (at least) two in measures:

Relative time

df$name[3] is 25% faster than df[3, "name"];

Absolute time

df$name[3] is five microseconds faster than df[3, "name"].

Both measures are useful, but it is important not to forget the underlying time scale. It makes little sense to optimize a function that takes microseconds if there are operations that take seconds to complete in your code.

Profiling

Benchmarking generally tests the execution time of one function against another. Profiling, on the other hand, is about testing large chunks of code.

It is difficult to overemphasize the importance of profiling for efficient R programming. Without a profile of what took longest, you will have only a vague idea of why your code is taking so long to run. The following example (which generates Figure 1-2, an image of ice-sheet retreat from 1985 to 2015) shows how profiling can be used to identify bottlenecks in your R scripts:

library("profvis")
profvis(expr = {

  # Stage 1: load packages
  # library("rnoaa") # not necessary as data pre-saved
  library("ggplot2")

  # Stage 2: load and process data
  out = readRDS("extdata/out-ice.Rds")
  df = dplyr::rbind_all(out, id = "Year")

  # Stage 3: visualize output
  ggplot(df, aes(long, lat, group = paste(group, Year))) +
    geom_path(aes(colour = Year))
  ggsave("figures/icesheet-test.png")
}, interval = 0.01, prof_output = "ice-prof")

The results of this profiling exercise are displayed in Figure 1-3.

For more information about profiling and benchmarking, please refer to the Optimising code chapter in Advanced R by Hadley Wickham (CRC Press), and “Code Profiling” in this book. We recommend reading these additional resources while performing benchmarks and profiles on your own code, perhaps based on the following exercises.

Visualisation of North Pole icesheet decline, generated using the code profiled using the profvis package.
Figure 1-2. Visualization of North Pole ice-sheet decline, generated using the code profiled using the profvis package
Profiling results of loading and plotting NASA data on icesheet retreat.
Figure 1-3. Profiling results of loading and plotting NASA data on ice-sheet retreat

Exercises

Consider the following benchmark to evaluate different functions for calculating the cumulative sum of all the whole numbers from 1 to 100:

x = 1:100 # initiate vector to cumulatively sum

# Method 1: with a for loop (10 lines)
cs_for = function(x){
  for(i in x){
    if(i == 1){
      xc = x[i]
    } else {
      xc = c(xc, sum(x[1:i]))
    }
  }
  xc
}

# Method 2: with apply (3 lines)
cs_apply = function(x){
  sapply(x, function(x) sum(1:x))
}

# Method 3: cumsum (1 line, not shown)
microbenchmark(cs_for(x), cs_apply(x), cumsum(x))
#> Unit: nanoseconds
#>         expr    min     lq   mean median     uq    max neval
#>    cs_for(x) 248145 316292 386893 370505 436382 697258   100
#>  cs_apply(x) 157610 198157 255241 233324 306013 478394   100
#>    cumsum(x)    561   1131   1796   1422   2075  18284   100
  1. Which method is fastest and how many times faster is it?

  2. Run the same benchmark, but with the results reported in seconds, on a vector of all the whole numbers from 1 to 50,000. Hint: also use the argument neval = 1 so that each command is only run once to ensure that the results complete (even with a single evaluation, the benchmark may take up to or more than a minute to complete, depending on your system). Does the relative time difference increase or decrease? By how much?

  3. Test how long the different methods for subsetting the data frame df, presented in “Benchmarking Example”, take on your computer. Is it faster or slower at subsetting than the computer on which this book was compiled?

  4. Use system.time() and a for() loop to test how long it takes to perform the subsetting operation 50,000 times. Before testing this, do you think it will be more or less than one second for each subsetting method? Hint: the test for the first method is shown in the following code:

    # Test how long it takes to subset the data frame 50,000 times:
    system.time(
      for(i in 1:50000) {
        df[3, 2]
      }
    )
  5. Bonus exercise: try profiling a section of code you have written using profvis. Where are the bottlenecks? Were they where you expected?

Book Resources

R Package

This book has an associated R package that contains datasets and functions referenced in the book. The package is hosted on GitHub and can be installed using the devtools package:

devtools::install_github("csgillespie/efficient")

The package also contains solutions (as vignettes) to the exercises found in this book. They can be browsed with the following command:

browseVignettes(package = "efficient")

The following command will install all packages used to generate this book:

devtools::install_github("csgillespie/efficientR")

Online Version

We are grateful to O’Reilly for allowing us to develop this book online. The online version constitutes a substantial additional resource to supplement this book, and will continue to evolve in between reprints of the physical book. The book’s code also represents a substantial learning opportunity in itself as it was written using R Markdown and the bookdown package, allowing us to run the R code each time we compile the book to ensure that it works, and allowing others to contribute to its longevity. To edit this chapter, for example, simply navigate to https://github.com/csgillespie/efficientR/edit/master/01-introduction.Rmd while logged into a GitHub account. The full source of the book is available at https://github.com/csgillespie/efficientR where we welcome comments/questions on the Issue Tracker and Pull Requests.

References

Wickham, Hadley. 2014a. Advanced R. CRC Press.

Visser, Marco D., Sean M. McMahon, Cory Merow, Philip M. Dixon, Sydne Record, and Eelke Jongejans. 2015. “Speeding Up Ecological and Evolutionary Computations in R; Essentials of High Performance Computing for Biologists.” Edited by Francis Ouellette. PLOS Computational Biology 11 (3): e1004140. doi:10.1371/journal.pcbi.1004140.

Janert, Philipp K. 2010. Data Analysis with Open Source Tools. O’Reilly Media.

Jensen, Jørgen Dejgård. 2011. “Can Worksite Nutritional Interventions Improve Productivity and Firm Profitability? A Literature Review.” Perspectives in Public Health 131 (4). SAGE Publications: 184–92.

Pereira, Michelle Jessica, Brooke Kaye Coombes, Tracy Anne Comans, and Venerina Johnston. 2015. “The Impact of Onsite Workplace Health-Enhancing Physical Activity Interventions on Worker Productivity: A Systematic Review.” Occupational and Environmental Medicine 72 (6). BMJ Publishing Group Ltd: 401–12.

Grant, Christine A, Louise M Wallace, and Peter C Spurgeon. 2013. “An Exploration of the Psychological Factors Affecting Remote E-Worker’s Job Effectiveness, Well- Being and Work-Life Balance.” Employee Relations 35 (5). Emerald Group Publishing Limited: 527–46.

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