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Lisp
1
Print a literal string on standard output
(princ "Hello, world!")
Alternative implementation:
(print "Hello World")
Alternative implementation:
(format T "Hello World")
2
Loop to execute some code a constant number of times
(loop repeat 10 do (write-line "Hello"))
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
(defun show (message)
  (format t "Hello: ~a" message)
  (values))
4
Create a function which returns the square of an integer
(defun square (x)
  (* x x))
5
Declare a container type for two floating-point numbers x and y
(defstruct point
  (x 0.0d0 :type double-float)
  (y 0.0d0 :type double-float))
6
Do something with each item x of an array-like collection items, regardless indexes.
(map nil #'something items)
7
Print each index i with its value x from an array-like collection items
(loop for i from 0 and x across items
      do (format t "~a = ~a~%" i x))
8
Create a new map object x, and provide some (key, value) pairs as initial content.
(let ((table (make-hash-table)))
  (setf (gethash 'one table) 1)
  (setf	(gethash 'two table) 2))
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
(defclass node ()
  ((value 
      :initarg :value :initform nil 
      :accessor node-value)
   (left-child 
      :initarg :left-child :initform nil 
      :accessor node-left-child)
   (right-child 
      :initarg :right-child :initform nil 
      :accessor node-right-child)))
10
Generate a random permutation of the elements of list x
(let ((end (length x)))
 (loop for i from 0 below end
       do (rotatef (aref x i)
                   (aref x (+ i (random (- end i)))))))
11
The list x must be non-empty.
(nth (random (length x)) x)
12
Check if the list contains the value x.
list is an iterable finite container.
(member x list)
Alternative implementation:
(find x list :test #'equal)
13
Access each key k with its value x from an associative array mymap, and print them.
(loop for k being each hash-key of mymap
      for x being each hash-value of mymap                                                                            
      do (format t "~A ~A~%" k x)))
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
(+ a (random (float (- b a))))
19
Reverse the order of the elements of list x.
This may reverse "in-place" and destroy the original ordering.
(reverse x)
20
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.
(defun mysearch (x m)
  (loop for i below (array-dimension m 0)
        do (loop for j below (array-dimension m 1)
                 when (eql x (aref m i j))
                 do (return-from my-search
                                 (values i j)))))
21
Swap the values of the variables a and b
(rotatef a b)
22
Extract the integer value i from its string representation s (in radix 10)
(setf i (parse-integer s))
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
(make-array (list m n)
   :element-type 'double-float
   :initial-element 1.0d0)
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
(defparameter *x*
  (make-array (list m n p)
              :element-type 'double-float
              :initial-element 0.0d0))
28
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 #'< items :key #'p)
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
(defun f (i)
  (if (< i 2)
    1
    (* i (f (- i 1)))))
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
(loop
  for (power rem) = (list p 0) then (multiple-value-list (floor power 2))
  for multiple = 1 then (if (zerop rem) multiple (* multiple base))
  for base = x then (* base base)
  when (= power 0) return 1
  when (= power 1) return (* multiple base)))
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
(defun compose (f g)
  (lambda (x) (g (f x))))
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
(defun curry (fn &rest args)
  (lambda (&rest remaining-args)
    (apply fn (append args remaining-args))))

(defun add (a b)
  (+ a b))

(funcall (curry add 2) 1) 
  
38
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.
(setf u (subseq s i j))
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
(setf ok (search word s))
41
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.
(reverse s)
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
(loop named outer
      for i below (array-dimension m 0)
      do (loop for j below (array-dimension m 1)
               for v = (aref m i j)
               when (minusp v)
               do (progn
                    (print v)
                    (return-from outer))))
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
(sleep 5)
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
(setf *t* (subseq s 0 5))
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
(setf *t* (subseq s (- (length s) 5)))
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
(setf s "a
b
c
d")
50
Write a loop that has no end clause.
(loop)
Alternative implementation:
(defun infinite-loop (x)
  (apply x (infinite-loop x)))
51
Determine whether the map m contains an entry for the key k
(nth-value 1 (gethash k m))
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
(defvar y (format nil "~{~A~^, ~}" x))
Alternative implementation:
(setf y (format nil "~{~a~^,~}" x))
Alternative implementation:
(setf y (reduce (lambda (a b)
                   (concatenate 'string a ", " b))
                 x))
54
Calculate the sum s of the integer list or array x.
(reduce #'+ x)
55
Create the string representation s (in radix 10) of the integer value i.
(setf s (princ-to-string i))
56
Fork-join : 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.
Wait for the completion of the 1000 tasks and then print "Finished".
(lambda (fn)
  (pdotimes (i 1000)
    (funcall fn i)))
57
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.
(setf y (remove-if-not p x))
58
Create string lines from the content of the file with filename f.
(with-open-file (stream f)
  (uiop:slurp-stream-string stream))
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
(format *error-output*
        "~a is negative"
        x)
64
Assign to x the value 3^247
(setf x (expt 3 247))
68
Create an object x to store n bits (n being potentially large).
(let ((x (make-array n :element-type 'bit))))
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
call random_seed (put=s)
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
(setf *random-state* (make-random-state t))
71
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.
(format t "~{~A~^ ~}~%" *args*)
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
(setf x (lcm a b))
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
(let ((x #c(-2 3)))
  (* x #c(0 1)))
78
Execute a block once, then execute it again as long as boolean condition c is true.
(loop do (something)
      while c)
79
Declare the floating point number y and initialize it with the value of the integer x .
(defparameter *y* (float x))
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
(setf c (logcount i))
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
(exit)
92
Write the contents of the object x into the file data.json.
(with-open-file (out "data.json" :direction :output)
  (yason:encode-plist x out))
93
Implement procedure control which receives one parameter f, and runs f.
(defun control (f)
   (funcall f))
101
Make an HTTP request with method GET to URL u, then store the body of the response in string s.
(defvar *s* (string (dex:get u)))
102
Make an HTTP request with method GET to URL u, then store the body of the response in file result.txt. Try to save the data as it arrives if possible, without having all its content in memory at once.
(serapeum:write-stream-into-file
   (dex:get u :want-stream t)
              #P"result.txt"
              :direction :output
              :if-exists :supersede
              :if-does-not-exist :create)
103
Read from the file data.xml and write its contents into the object x.
Assume the XML data is suitable for the type of x.
(let ((x (plump:parse #p"data.xml")))
  ...)
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
(setf blank (not (find #\space s :test-not #'eql)))
117
Set n to the number of elements of the list x.
(setf n (length x))
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
(setf y (remove-duplicates x))
119
Remove duplicates from the list x.
Explain if the original order is preserved.
(remove-duplicates x)
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
(assert (isConsistent))
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
(time (foo))
131
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.
(cond (c1 (f1))
      (c2 (f2))
      (c3 (f3)))
134
Declare and initialize a new list items, containing 3 elements a, b, c.
(defparameter *items* (list a b c))
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
(setf b (every #'digit-char-p s))
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
(map nil #'print (append items1 items))
147
Create string t from string s, keeping only ASCII characters
(remove-if-not (lambda (i) (<= 0 i 127))
               s
               :key #'char-code)
148
Read a list of integer numbers from the standard input, until EOF.
(read)
149
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152
Create string s containing only the character c.
(defparameter *s* (string c))
Alternative implementation:
(defparameter *s* (string c))
153
Create the string t as the concatenation of the string s and the integer i.
(format nil "~a~a" s i)
157
Initialize a constant planet with string value "Earth".
(defparameter +planet+ "Earth")
161
Multiply all the elements of the list elements by a constant c
(mapcar (lambda (n) (* n c))
        elements)
165
Assign to variable x the last element of list items.
(setf x (car (last items)))
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
(setf ab (append a b))
169
Assign to integer n the number of characters of string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
(setf n (length s))
Alternative implementation:
(length s)
174
Make a HTTP request with method POST to URL u
(drakma:http-request u :method :post)
Alternative implementation:
(dex:post u)
175
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
(lambda (bytes &aux (size (* 2 (length bytes))))
  (let ((hex (make-array size :element-type 'character :fill-pointer 0)))
    (prog1 hex
      (with-output-to-string (o hex)
        (map () (lambda (byte) (format o "~2,'0x" byte)) bytes)))))
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
(setf b (and (< x1 x x2)
             (< y1 y y2)))
184
Assign to variable t a string representing the day, month and year of the day after the current date.
(setf _t (chronicity:parse "tomorrow"))
223
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.
(loop for item in items
      when (check-match item) return (do-something item)
      finally (return (do-something-else))