Loading CSVs in Clojure

Ivar Thorson bio photo By Ivar Thorson

Loading CSVs in Clojure is really easy using clojure.data.csv. Depending on what you will do with the data, you can either represent it in tabular form (as a vector of vectors), or as a list of hashmaps. Both approaches have their advantages and disadvantages, and here are some very stock functions for how to achieve that:

(ns net.roboloco.csvs
  "Functions for loading and saving CSVs."
  (:require [clojure.data.csv :as csv]))

(defn empty-string-to-nil
  "Returns a nil if given an empty string S, otherwise returns S."
  (if (and (string? s) (empty? s))

(defn dissoc-nils
  "Drops keys with nil values, or nil keys, from the hashmap H."
  (into {} (filter (fn [[k v]] (and v k)) h)))

(defn load-csv
  "Returns a data structure loaded from a CSV file at FILEPATH."
  (with-open [reader (clojure.java.io/reader filepath)]
    (->> (csv/read-csv reader)
         (map (fn [row] (map empty-string-to-nil row)))

(defn save-csv
  "Saves a vector of vectors DATA (i.e. a CSV) to disk at FILEPATH. "
  [vec-of-vecs filepath]
  (with-open [writer (clojure.java.io/writer filepath)]
    (csv/write-csv writer vec-of-vecs)))

(defn tabular->maps
  "Converts a vector of vectors into a vector of maps. Assumes that the
  first row of the CSV is a header that contains column names."
  (let [header (first tabular)]
    (-> (map zipmap (repeat header) (rest tabular))
        (mapv dissoc-nils))))

(defn maps->tabular
  "Converts a vector of vectors into a vector of maps."
  (let [columns (vec (sort (into #{} (map name (flatten (map keys rowmaps))))))]
    (vec (conj (for [row rowmaps]
              (vec (for [col columns]
                     (str (get row col "")))))


  (def data (tabular->maps (load-csv "/path/to/mycsv.csv")))
  (save-csv! (maps->tabular data) "/some/other/path.csv")


Note that the above functions are not lazy, which is a good choice for CSVs 90% of the time. If you find yourself working with very large datasets that cannot be loaded all at once, you might want to adjust load-csv, tabular->maps, and maps->tabular to work lazily and incrementally.