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Split-apply-combine in Scheme

August 18, 2022

I’ve been a devotee of the “split-apply-combine” way, popularized in the plyr (and legacy) package, of performing data operations in R. In its simplest form, and using base functions only, it amounts to:

data(crabs, package = "MASS")
spl = with(crabs, split(BD, sp))
apl = lapply(spl, mean)
cbn = rbind(x = apl)
cbn

Using plyr, this would simply be ddply(data = crabs, .(sp), summarize, x = mean(BD)). Related tools include subset and transform, and higher-order variations thereof (Filter, Reduce, and Map). I’ve seen few people using the later, but it really was not so common back in the days. Maybe David Springate talked about those functional recipes in one of his talks or tutorials, I don’t remember quite well, although I’m sure he blogged about functional programming in R. Thomas Mailund did this too. If I were to resume the above aspects of data processing using keywords that bear some resemblance with functional PLs, I would say this all amounts to filter, apply (or map), reduce and collect (the later is specific to Common Lisp, though).

On a related point, Eli Bendersky cites Ben Vandgrift and Alex Miller (Clojure Applied) in one of his Clojure-related blog post. Look at the following piece of code:

(defn revenue-by-department [carts]
  (->> (filter :settled? carts)
       (mapcat :line-items)
       (map line-summary)
       (group-by :dept)
       (reduce-kv dept-total)))

Doesn’t it look like what we discussed above?1 The point of Eli Bendersky is that soon or later, when processing large amount of data, we need the power and expressiveness of SQL-inspired queries:

Applications that process and extract useful bits of information from large datasets all look alike in many programming languages, at least to some extent. What we really want in many cases is SQL-like primitives built into our languages, but this is often challenging (.NET’s LINQ is one example of a successful approach).

Of course, filtering rows or columns, or aggregating numerical quantities with or without grouping first are all builtin in DSLs for statistical computing (R, Stata, LispStat, to name a few). And there have been some attempt at producing reliable Lisp or Scheme packages for data munging. This will probably de dealt with in a future post, especially for Common Lisp and Racket.

In the mean time, since I’m reading a lot of Scheme books at the moment, I found a close analogy to this approach in chapter 8 (Higher-order functions) of Simply Scheme, where the author explains and illustrates the behavior of three procedures:

Function Purpose First argument
everytransformone-arg transforming function
keepselectone-arg predicate function
accumulatecombinetwo-arg combining function

In this case, every will usually play the role of the R transform function, or Map; keep acts as a filter and does the job of (part of) subset or Filter; finally, accumulate resembles to many of R’s builtin functions (e.g., cumsum) and more generally acts as a reducer (Reduce). It is the combination of those higher-order functions that makes Scheme so powerful, as in the following examples:

(every square (keep even? '(87 4 7 12 0 5)))
(accumulate word (keep vowel? (every first '(and i love her))))

♪ Keith Jarrett & Charlie Haden • Goodbye


  1. Clojure has been on my radar for 8 years now. Long time ago, I bought dozens of books, from which I barely scratched the surface. I never got into it really, except for writing toy examples based on my knowledge of Common Lisp and Scheme, but in a few months I think it will become my Lisp of choice for more serious development. ↩︎

See Also

» Scheming with Vim » Welch t-test in Scheme » 20 years with R: A retrospective » Syracuse numbers » Fisher-Yates shuffling