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Building a String from Parts

by Ryan Neufeld

Problem

You have multiple strings, values, or collections that you need to combine into one string.

Solution

Use the str function to concatenate strings and/or values:

(str "John" " " "Doe")
;; -> "John Doe"

;; str also works with vars, or any other values
(def first-name "John")
(def last-name "Doe")
(def age 42)

(str last-name ", " first-name " - age: " age)
;; -> "Doe, John - age: 42"

Use apply with str to concatenate a collection of values into a single string:

;; To collapse a sequence of characters back into a string
(apply str "ROT13: " [\W \h \y \v \h \f \  \P \n \r \f \n \e])
;; -> "ROT13: Whyvhf Pnrfne"

;; Or, to reconstitute a file from lines (if they already have newlines...)
(def lines ["#! /bin/bash\n"
            "du -a ./ | sort -n -r\n"])
(apply str lines)
;; -> "#! /bin/bash\ndu -a ./ | sort -n -r\n"

Discussion

Clojure’s str is like a good Unix tool: it has one job, and it does it well. When provided with one or more arguments, str invokes Java’s .toString() method on its argument, tacking each result onto the next. When provided nil or invoked without arguments, str will return the identity value for strings, the empty string.

When it comes to string concatenation, Clojure takes a fairly hands-off approach. There is nothing string-specific about (apply str …​). It is merely the higher-order function apply being used to emulate calling str with a variable number of arguments.

This apply:

(apply str ["a" "b" "c"])

is functionally equivalent to:

(str "a" "b" "c")

Since Clojure injects little opinion into joining strings, you’re free to inject your own with the plethora of manipulating functions Clojure provides. For example, take constructing a comma-separated value (CSV) from a header and a number of rows. This example is particularly well suited for apply, as you can prefix the header without having to insert it onto the front of your rows collection:

;; Constructing a CSV from a header string and vector of rows
(def header "first_name,last_name,employee_number\n")
(def rows ["luke,vanderhart,1"
           "ryan,neufeld,2"])

(apply str header (interpose "\n" rows))
;; -> "first_name,last_name,employee_number\nluke,vanderhart,1\nryan,neufeld,2"

apply and interpose can be a lot of ceremony when you’re not doing anything too fancy. It is often easier to use the clojure.string/join function for simple string joins. The join function takes a collection and an optional separator. With a separator, join returns a string with each item of the collection separated by the provided separator. Without, it returns each item squashed together, similar to what (apply str coll) would return:

(def food-items ["milk" "butter" "flour" "eggs"])
(clojure.string/join ", " food-items)
;; -> "milk, butter, flour, eggs"

(clojure.string/join [1 2 3 4])
;; -> "1234"

See Also