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  • (instead of printing)
Print a literal string on standard output
(println "Hello World")
Loop to execute some code a constant number of times
(dotimes [_ 10]
  (println "Hello"))
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
(defn main- [& args]
  (println "I got all this arguments " args " and now I'm returning nil. Peace out!"))
Create a function which returns the square of an integer
(defn square [x]
  (* x x))
Declare a container type for two floating-point numbers x and y
[x y]
Alternative implementation:
(vector-of :double x y)
Do something with each item x of the list (or array) items, regardless indexes.
(map do-something items)
Print each index i with its value x from an array-like collection items
(doseq [[i x] (map-indexed vector items)]
  (println i ":" x))
Create a new map object x, and provide some (key, value) pairs as initial content.
(def x {"One" 1
	"Two" 2
	"Three" 3})
Generate a random permutation of the elements of list x
(shuffle x)
The list x must be non-empty.
(rand-nth x)
Check if the list contains the value x.
list is an iterable finite container.
(some (partial = x) list)
Alternative implementation:
(some #{x} list)
Access each key k with its value x from an associative array mymap, and print them.
(doseq [[k x] mymap]
  (println k ":" x))
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
  (defn rand-between [a b]
    (+ a (* (- b a) (rand))))
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
(+ a (rand-int (- b a)))
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
(reverse x)
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
  (defn find-in-2d-matrix [m x]
    (for [i (range (count m))
          j (range (count (first m)))
          :when (= x (-> m (nth i) (nth j)))]
      [i j]))
Swap the values of the variables a and b
(defn swap [a b]
  [b a])
Extract the integer value i from its string representation s (in radix 10)
(def i (Integer/parseInt s))
Alternative implementation:
(def i (Integer. s))
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
(def s "ネコ")
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
(def c (chan 1))

(go (println "Hello," (<! c)))

(>!! c "Alan")
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
(for [i (range 1 (inc m))]
  (for [j (range 1 (inc n))]
    (* i j)))
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
(sort-by :p items)
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
(defn remove-idx [i items]
    (keep-indexed #(when-not (= i %1) %2) items))
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
(dorun (pmap f (range 1 1001)))
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
(defn f [i]
  (loop [cnt i, acc 1N]
    (if (zero? cnt) acc
      (recur (dec cnt) (* cnt acc)))))
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
(defn exp [x n]
  (let [x-squared (* x x)]
      (= n 0) 1
      (= n 1) x
      (= (mod n 2) 0) (exp x-squared (/ n 2))
      :else (* x (exp x-squared (/ (- n 1) 2))))))
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
(def x (atom 0))
(def f inc)
(swap! x f)
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
(defn compose [f g]
   (comp g f))
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
(comp g f)
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
(def add5 (partial + 5))
Alternative implementation:
(def rev-key #(update %2 %1 reverse))

(def rev-a (partial rev-key :a))
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
(def t (subs s i j))
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise.
(def ok (string/includes? word s))
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
(let [s "hello"
      t (apply str (reverse s))]
Alternative implementation:
(def t (str/reverse s))
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
(loop [rows m]
  (if-let [v (some #(when (neg? %) %) (first rows))]
    (recur (rest rows))))
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
(Thread/sleep 5000)
Alternative implementation:
(<! (timeout 5000))
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled.
(def t (apply str (take 5 s)))
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
(let [t (clojure.string/join (take-last 5 s))])
Alternative implementation:
(def t 
  (when (string? s)
    (let [from (-> s (count) (- 5) (max 0))]
      (subs s from))))
Assign to variable s a string literal consisting in several lines of text, including newlines.
(def s "Murs, ville,
Et port,
De mort,
Mer grise
Où brise
La brise,
Tout dort.")
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
(def chunks (split s #"\s+"))
Write a loop that has no end clause.
(loop []
  ;; do something
Determine whether the map m contains an entry for the key k
(contains? m k)
Determine whether the map m contains an entry with the value v, for some key.
(boolean (some #{v} (vals m)))
Concatenate elements of string list x joined by the separator ", " to create a single string y.
(clojure.string/join "," '("abc" "def" "ghi") )
Calculate the sum s of the integer list or array x.
(defn sum [x] (reduce + x))

(sum [1 20 300])
;;=> 321
Create the string representation s (in radix 10) of the integer value i.
(let [s (str i)])
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
(def y (filter p x))
Create the string lines from the content of the file with filename f.
(def lines (slurp f))
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
(binding [*out* *err*]
  (println (str x " is negative")))
Assign to the variable d the current date/time value, in the most standard type.
(def d #(java.util.Date.))
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
(defn binom [n k]
  (let [fact #(apply * (range 1 (inc %)))]
    (/ (fact n)
       (* (fact k) (fact (- n k))))))
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
(println (clojure.string/join " " *command-line-args*))
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
(defn gcd [a b]
  (if (zero? b)
    (recur b (mod a b))))
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
(defn s
  (pprint/cl-format nil "~b" x))
Execute a block once, then execute it again as long as boolean condition c is true.
(loop []
  (do something)
  (when c
Declare the floating point number y and initialize it with the value of the integer x .
(def y (float x))
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
(defn rnd [y] (int (+ y 0.5)))
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
(count (re-seq t s))
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
(def r #"htt+p")
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
(def c (count (re-seq #"1" (Integer/toBinaryString i))))
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
(System/exit 0)
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
(def x (read-value (file "data.json")))
Write the contents of the object x into the file data.json.
(write-value (file "data.json") x)
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
(type x)
Set the boolean b to true if string s starts with prefix prefix, false otherwise.
(def b (starts-with? s prefix))
Set boolean b to true if string s ends with string suffix, false otherwise.
(let [b (str/ends-with? s suffix)]
   ; ...  
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
(def x (.format (java.text.SimpleDateFormat. "yyyy-MM-dd") d))
Sort elements of array-like collection items, using a comparator c.
(sort c items)
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s.
(def s (slurp u))
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
(blank? s)
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types.
(def b (= x y))
Alternative implementation:
(def b (identical? x y))
Remove all occurrences of string w from string s1, and store the result in s2.
(def s2 (clojure.string/replace s1 w ""))
Set n to the number of elements of the list x.
(def n (count x))
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
(def y (set x))
Remove duplicates from the list x.
Explain if the original order is preserved.
(distinct x)
Read an integer value from the standard input into the variable n
(def n (Integer/parseInt (read-line)))
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
Alternative implementation:
(def Suit #{"Spades" "Hearts"  "Diamonds" "Clubs"})
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
(assert (isConsistent))
Alternative implementation:
(assert (true? (isConsistent)))
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
(time (foo))
Write a function foo that returns a string and a boolean value.
(defn foo
  ["bar" true])
Import the source code for the function foo body from a file "foobody.txt".
(def foo (load-file "foobody.txt"))
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
    c1 (f1)
    c2 (f2)
    c3 (f3))
Run the procedure f, and return the duration of the execution of f.
(time (f))
Declare and initialize a new list items, containing 3 elements a, b, c.
(def items (list a b c))
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
(let [[n m]
      (split-with (partial not= x) items)]
  (concat n (rest m)))
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
(remove #{x} items)
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
(def b (every? #(Character/isDigit %) s))
Create a new temporary file on the filesystem.
(File/createTempFile "prefix" "suffix")
Alternative implementation:
(File/createTempFile "prefix" "suffix" (new File "/tmp"))
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
(dissoc m k)
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
;; for side effects
(doseq [x (concat items1 items2)]
   (println x))
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
(doseq [i (interleave items1 items2)]
  (println i))
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
(.exists (io/file fp))
Extract floating point value f from its string representation s
(Float/parseFloat s)
Alternative implementation:
(read-string s)
Alternative implementation:
(parse-double s)
Create string t from string s, keeping only ASCII characters
(def t (clojure.string/replace s #"[^\x00-\x7F]" ""))
Remove the last character from the string p, if this character is a forward slash /
(defn remove-ending-slash
  (if (clojure.string/ends-with? p "/")
    (subs p 0 (dec (count p)))
Alternative implementation:
(clojure.string/replace p #"/$" "")
Create string s containing only the character c.
(def s (str c))
Create the string t as the concatenation of the string s and the integer i.
(def t (str s i))
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
(def s (format "%03d" i))
Initialize a constant planet with string value "Earth".
(def planet "Earth")
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
(def y (->> x shuffle (take k)))
Multiply all the elements of the list elements by a constant c
(map (partial * c) elements)
execute bat if b is a program option and fox if f is a program option.
(defn -main [arg & _]
  (case arg
    "b" (bat)
    "f" (fox)
Print all the list elements, two by two, assuming list length is even.
(doseq [xs (partition 2 list)]
  (apply println xs))
Assign to the variable x the last element of the list items.
(def x (last items))
Alternative implementation:
(def x (peek items))
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
(def ab (concat a b))
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
(def n (.codePointCount s 0 (count s)))
Alternative implementation:
(defn n[s] (.codePointCount s 0 (count s)))
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
(def n (count mymap))
Append the element x to the list s.
(conj s x)
Insert value v for key k in map m.
(assoc m k v)
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
(defn c 
  "calculates the center c of a rectangle of coordinates(x1, y1, x2, y2)" 
  [x1 y1 x2 y2]
  {:x (/ (+ x1 x2) 2)
   :y (/ (+ y1 y2) 2)})
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing.
(def x ((comp file-seq d))
Schedule the execution of f(42) in 30 seconds.
(do (Thread/sleep 30000)
    (f 42))
Perform matrix multiplication of a real matrix a with nx rows and ny columns, a real matrix b with ny rows and nz columns and assign the value to a real matrix c with nx rows and nz columns.
(defn t [x] (apply (partial mapv vector) x))
(defn v* [u v] (reduce + (mapv * u v)))
(defn shape [a] (comp (partition-all (count a)) (map vec)))

(defn mm [a b]
    (into [] (shape a)
        (for [line a col (t b)]
            (v* line col))))

(def c (mm a b))
Alternative implementation:
(def c (mmul a b))
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P.
(def y 
  (eduction (filter P)
            (map T))
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
(when (seq (filter #(> % x) a)) (f))
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
(def a 1234567890.12345678901M)
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange).
(def a [[1 2 3] [4 5 6] [7 8 9]])
(def b (apply (partial mapv vector) a))
Calculate the sum of squares s of data, an array of floating point values.
(defn square [x] (* x x))

(def s (reduce + (map square data)))
Alternative implementation:
(defn square [x] (* x x))

(def s (->> data (map square) (reduce +)))
Alternative implementation:
(defn square [x] (* x x))

(def s (transduce (map square) + data))
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
(def foo
  (or (System/getenv "FOO") "none"))
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
(some-> str {"foo" foo "bar" bar "baz" baz "barfl" barfl} (.call))
Alternative implementation:
(def whitelist #{#'foo #'bar #'baz #'barfl})
(some-> str symbol resolve whitelist (.call))
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
(def c (clojure.set/intersection (set a) (set b)))
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
(def t (clojure.string/replace s #"\s+" " "))
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
(let [s "1a22b3c4de5f6"
      t (str/replace s #"[^\d]" "")]
  (println t))
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
(defn find-index [x items]
  (or (->> items
           (map-indexed vector)
           (filter #(= x (peek %)))
Alternative implementation:
(defn find-index [x items]
    #(if (= x %3) (reduced %2) %1) -1 items))
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
(if (seq (filter odd? my-col))
  "contains odds"
  "no odds found")
Insert the element x at the beginning of the list items.
(def items2 (conj items x))
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
(defn f 
  ([] (println "Not present"))
  ([x] (println "Present" x)))
Alternative implementation:
(defn f [& [x]]
  (if (integer? x)
    (println "Present" x)
    (println "Not present")))
Remove the last element from the list items.
(drop-last items)
Assign to c the result of (a xor b)
(def c (bit-xor a b))
Lists a and b have the same length. Apply the same permutation to a and b to have them sorted based on the values of a.
(->> (map vector a b)
     (sort-by first)
     (apply (partial mapv vector)))
Call a function f on each element e of a set x.
(doseq [e x]
  (f e))
Print the contents of the list or array a on the standard output.
(print a)
Set c to the number of distinct elements in the list items.
(def items [1 2 3 4 4 5 5 5])

(def c (count (set items)))
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
(def i (read-string (str "2r" s)))
Assign to the variable x the string value "a" if calling the function condition returns true, or the value "b" otherwise.
(def x (if condition
Replace all exact occurrences of "foo" with "bar" in the string list x
(replace {"foo" "bar"} x)
Print the numbers 5, 4, ..., 0 (included), one line per number.
(doseq [n (range 5 0 -1)]
  (println n))
Declare a new list items of string elements, containing zero elements
(def items '())
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
(def s (apply str (repeat n v)))
Alternative implementation:
(def s (clojure.string/join "" (repeat n v)))
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
(defn foo [x]
  (println (if (string? x) x "Nothing.")))

(foo "Hello, world!")
(foo 42)
Remove the element e from the set x.

Explains what happens if e was already absent from x.
(disj x e)
Build the list parts consisting of substrings of input string s, separated by the string sep.
(def parts (clojure.string/split s sep))
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
(def a (repeatedly n #(identity 0)))
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
(dorun (map-indexed (fn [i c] (println "Char" i "is" c)) s))
Create the string s by concatenating the strings a and b.
(def s (str a b))
Write a line of comments.

This line will not be compiled or executed.
;This is a comment.

;;This is a comment.

;;;This is a comment.
Set b to true if the string s is empty, false otherwise
(def b (nil? s))