The Common Lisp Cookbook – Data structures

Table of Contents

The Common Lisp Cookbook – Data structures

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We hope to give here a clear reference of the common data structures. To really learn the language, you should take the time to read other resources. The following resources, which we relied upon, also have many more details:

Don’t miss the appendix and when you need more data structures, have a look at the awesome-cl list and Quickdocs.


Building lists. Cons cells, lists.

A list is also a sequence, so we can use the functions shown below.

The list basic element is the cons cell. We build lists by assembling cons cells.

(cons 1 2)
;; => (1 . 2) ;; representation with a point, a dotted pair.

It looks like this:

[o|o]--- 2

If the cdr of the first cell is another cons cell, and if the cdr of this last one is nil, we build a list:

(cons 1 (cons 2 nil))
;; => (1 2)

It looks like this:

 |       |
 1       2

(ascii art by draw-cons-tree).

See that the representation is not a dotted pair ? The Lisp printer understands the convention.

Finally we can simply build a literal list with list:

(list 1 2)
;; => (1 2)

or by calling quote:

'(1 2)
;; => (1 2)

which is shorthand notation for the function call (quote (1 2)).

Circular lists

A cons cell car or cdr can refer to other objects, including itself or other cells in the same list. They can therefore be used to define self-referential structures such as circular lists.

Before working with circular lists, tell the printer to recognise them and not try to print the whole list by setting *print-circle* to T:

(setf *print-circle* t)

A function which modifies a list, so that the last cdr points to the start of the list is:

(defun circular! (items)
  "Modifies the last cdr of list ITEMS, returning a circular list"
  (setf (cdr (last items)) items))

(circular! (list 1 2 3))
;; => #1=(1 2 3 . #1#)

(fifth (circular! (list 1 2 3)))
;; => 2

The list-length function recognises circular lists, returning nil.

The reader can also create circular lists, using Sharpsign Equal-Sign notation. An object (like a list) can be prefixed with #n= where n is an unsigned decimal integer (one or more digits). The label #n# can be used to refer to the object later in the expression:

'#42=(1 2 3 . #42#)
;; => #1=(1 2 3 . #1#)

Note that the label given to the reader (n=42) is discarded after reading, and the printer defines a new label (n=1).

Further reading

car/cdr or first/rest (and second… to tenth)

(car (cons 1 2)) ;; => 1
(cdr (cons 1 2)) ;; => 2
(first (cons 1 2)) ;; => 1
(first '(1 2 3)) ;; => 1
(rest '(1 2 3)) ;; => (2 3)

We can assign any new value with setf.

last, butlast, nbutlast (&optional n)

return the last cons cell in a list (or the nth last cons cells).

(last '(1 2 3))
;; => (3)
(car (last '(1 2 3)) ) ;; or (first (last …))
;; => 3
(butlast '(1 2 3))
;; => (1 2)

In Alexandria, lastcar is equivalent of (first (last …)):

(alexandria:lastcar '(1 2 3))
;; => 3

reverse, nreverse

reverse and nreverse return a new sequence.

nreverse is destructive. The N stands for non-consing, meaning it doesn’t need to allocate any new cons cells. It might (but in practice, does) reuse and modify the original sequence:

(defparameter mylist '(1 2 3))
;; => (1 2 3)
(reverse mylist)
;; => (3 2 1)
;; => (1 2 3)
(nreverse mylist)
;; => (3 2 1)
;; => (1) in SBCL but implementation dependent.


append takes any number of list arguments and returns a new list containing the elements of all its arguments:

(append (list 1 2) (list 3 4))
;; => (1 2 3 4)

The new list shares some cons cells with the (3 4):

nconc is the recycling equivalent.

push, pushnew (item, place)

push prepends item to the list that is stored in place, stores the resulting list in place, and returns the list.

pushnew is similar, but it does nothing if the element already exists in the place.

See also adjoin below that doesn’t modify the target list.

(defparameter mylist '(1 2 3))
(push 0 mylist)
;; => (0 1 2 3)
(defparameter x ’(a (b c) d))
;; => (A (B C) D)
(push 5 (cadr x))
;; => (5 B C)
;; => (A (5 B C) D)

push is equivalent to (setf place (cons item place )) except that the subforms of place are evaluated only once, and item is evaluated before place.

There is no built-in function to add to the end of a list. It is a more costly operation (have to traverse the whole list). So if you need to do this: either consider using another data structure, either just reverse your list when needed.

pushnew accepts key arguments: :key, :test, :test-not.


a destructive operation.

nthcdr (index, list)

Use this if first, second and the rest up to tenth are not enough.

car/cdr and composites (cadr, caadr…) - accessing lists inside lists

They make sense when applied to lists containing other lists.

(caar (list 1 2 3))                  ==> error
(caar (list (list 1 2) 3))           ==> 1
(cadr (list (list 1 2) (list 3 4)))  ==> (3 4)
(caadr (list (list 1 2) (list 3 4))) ==> 3

destructuring-bind (parameter*, list)

It binds the parameter values to the list elements. We can destructure trees, plists and even provide defaults.

Simple matching:

(destructuring-bind (x y z) (list 1 2 3)
  (list :x x :y y :z z))
;; => (:X 1 :Y 2 :Z 3)

Matching inside sublists:

(destructuring-bind (x (y1 y2) z) (list 1 (list 2 20) 3)
  (list :x x :y1 y1 :y2 y2 :z z))
;; => (:X 1 :Y1 2 :Y2 20 :Z 3)

The parameter list can use the usual &optional, &rest and &key parameters.

(destructuring-bind (x (y1 &optional y2) z) (list 1 (list 2) 3)
  (list :x x :y1 y1 :y2 y2 :z z))
;; => (:X 1 :Y1 2 :Y2 NIL :Z 3)
(destructuring-bind (&key x y z) (list :z 1 :y 2 :x 3)
  (list :x x :y y :z z))
;; => (:X 3 :Y 2 :Z 1)

The &whole parameter is bound to the whole list. It must be the first one and others can follow.

(destructuring-bind (&whole whole-list &key x y z)
    (list :z 1 :y 2 :x 3)
  (list :x x :y y :z z :whole whole-list))
;; => (:X 3 :Y 2 :Z 1 :WHOLE-LIST (:Z 1 :Y 2 :X 3))

Destructuring a plist, giving defaults:

(example from Common Lisp Recipes, by E. Weitz, Apress, 2016)

(destructuring-bind (&key a (b :not-found) c
    ’(:c 23 :d "D" :a #\A :foo :whatever)
  (list a b c))
;; => (#\A :NOT-FOUND 23)

If this gives you the will to do pattern matching, see pattern matching.

Predicates: null, listp

null is equivalent to not, but considered better style.

listp tests whether an object is a cons cell or nil.

and sequences’ predicates.

ldiff, tailp, list*, make-list, fill, revappend, nreconc, consp, atom

(make-list 3 :initial-element "ta")
;; => ("ta" "ta" "ta")
(make-list 3)
;; => (NIL NIL NIL)
(fill * "hello")
;; => ("hello" "hello" "hello")

member (elt, list)

Returns the tail of list beginning with the first element satisfying eqlity.

Accepts :test, :test-not, :key (functions or symbols).

(member 2 '(1 2 3))
;; (2 3)

Replacing objects in a tree: subst, sublis

subst and subst-if search and replace occurences of an element or subexpression in a tree (when it satisfies the optional test):

(subst 'one 1 '(1 2 3))
;; => (ONE 2 3)

(subst  '(1 . one) '(1 . 1) '((1 . 1) (2 . 2)) :test #'equal)
;; ((1 . ONE) (2 . 2))

sublis allows to replace many objects at once. It substitutes the objects given in alist and found in tree with their new values given in the alist:

(sublis '((x . 10) (y . 20))
        '(* x (+ x y) (* y y)))
;; (* 10 (+ 10 20) (* 20 20))

sublis accepts the :test and :key arguments. :test is a function that takes two arguments, the key and the subtree.

(sublis '((t . "foo"))
        '("one" 2 ("three" (4 5)))
        :key #'stringp)
;; ("foo" 2 ("foo" (4 5)))


lists and vectors (and thus strings) are sequences.

Note: see also the strings page.

Many of the sequence functions take keyword arguments. All keyword arguments are optional and, if specified, may appear in any order.

Pay attention to the :test argument. It defaults to eql (for strings, use :equal).

The :key argument should be passed either nil, or a function of one argument. This key function is used as a filter through which the elements of the sequence are seen. For instance, this:

(find x y :key 'car)

is similar to (assoc* x y): It searches for an element of the list whose car equals x, rather than for an element which equals x itself. If :key is omitted or nil, the filter is effectively the identity function.

Example with an alist (see definition below):

(defparameter my-alist (list (cons 'foo "foo")
                             (cons 'bar "bar")))
;; => ((FOO . "foo") (BAR . "bar"))
(find 'bar my-alist)
;; => NIL
(find 'bar my-alist :key 'car)
;; => (BAR . "bar")

For more, use a lambda that takes one parameter.

(find 'bar my-alist :key (lambda (it) (car it)))
(find 'bar my-alist :key ^(car %))
(find 'bar my-alist :key (lm (it) (car it)))

Predicates: every, some,…

every, notevery (test, sequence): return nil or t, respectively, as soon as one test on any set of the corresponding elements of sequences returns nil.

(defparameter foo '(1 2 3))
(every #'evenp foo)
;; => NIL
(some #'evenp foo)
;; => T

with a list of strings:

(defparameter str '("foo" "bar" "team"))
(every #'stringp str)
;; => T
(some (lambda (it) (= 3 (length it))) str)
;; => T

some, notany (test, sequence): return either the value of the test, or nil.


See also sequence functions defined in Alexandria: starts-with, ends-with, ends-with-subseq, length=, emptyp,…

length (sequence)

elt (sequence, index) - find by index

beware, here the sequence comes first.

count (foo sequence)

Return the number of elements in sequence that match foo.

Additional paramaters: :from-end, :start, :end.

See also count-if, count-not (test-function sequence).

subseq (sequence start, [end])

(subseq (list 1 2 3) 0)
;; (1 2 3)
(subseq (list 1 2 3) 1 2)
;; (2)

However, watch out if the end is larger than the list:

(subseq (list 1 2 3) 0 99)
;; => Error: the bounding indices 0 and 99
;; are bad for a sequence of length 3.

To this end, use alexandria-2:subseq*:

(alexandria-2:subseq* (list 1 2 3) 0 99)
;; (1 2 3)

subseq is “setf”able, but only works if the new sequence has the same length of the one to replace.

sort, stable-sort (sequence, test [, key function])

These sort functions are destructive, so one may prefer to copy the sequence with copy-seq before sorting:

(sort (copy-seq seq) :test #'string<)

Unlike sort, stable-sort guarantees to keep the order of the argument. In theory, the result of this:

(sort '((1 :a) (1 :b)) #'< :key #'first)

could be either ((1 :A) (1 :B)), either ((1 :B) (1 :A)). On my tests, the order is preserved, but the standard does not guarantee it.

find, position (foo, sequence) - get index

also find-if, find-if-not, position-if, position-if-not (test sequence). See :key and :test parameters.

(find 20 '(10 20 30))
;; 20
(position 20 '(10 20 30))
;; 1

search and mismatch (sequence-a, sequence-b)

search searches in sequence-b for a subsequence that matches sequence-a. It returns the position in sequence-b, or NIL. It has the from-end, end1, end2 and the usual test and key parameters.

(search '(20 30) '(10 20 30 40))
;; 1
(search '("b" "c") '("a" "b" "c"))
;; NIL
(search '("b" "c") '("a" "b" "c") :test #'equal)
;; 1
(search "bc" "abc")
;; 1

mismatch returns the position where the two sequences start to differ:

(mismatch '(10 20 99) '(10 20 30))
;; 2
(mismatch "hellolisper" "helloworld")
;; 5
(mismatch "same" "same")
;; NIL
(mismatch "foo" "bar")
;; 0

substitute, nsubstitute[if,if-not]

Return a sequence of the same kind as sequence with the same elements, except that all elements equal to old are replaced with new.

(substitute #\o #\x "hellx") ;; => "hello"
(substitute :a :x '(:a :x :x)) ;; => (:A :A :A)
(substitute "a" "x" '("a" "x" "x") :test #'string=)
;; => ("a" "a" "a")

sort, stable-sort, merge

(see above)

replace (sequence-a, sequence-b, &key start1, end1)

Destructively replace elements of sequence-a with elements of sequence-b.

The full signature is:

(replace sequence1 sequence2
      &rest args
      &key (start1 0) (end1 nil) (start2 0) (end2 nil))

Elements are copied to the subseqeuence bounded by START1 and END1, from the subsequence bounded by START2 and END2. If these subsequences are not of the same length, then the shorter length determines how many elements are copied.

(replace "xxx" "foo")

(replace "xxx" "foo" :start1 1)

(replace "xxx" "foo" :start1 1 :start2 1)

(replace "xxx" "foo" :start1 1 :start2 1 :end2 2)

remove, delete (foo sequence)

Make a copy of sequence without elements matching foo. Has :start/end, :key and :count parameters.

delete is the recycling version of remove.

(remove "foo" '("foo" "bar" "foo") :test 'equal)
;; => ("bar")

see also remove-if[-not] below.

remove-duplicates, delete-duplicates (sequence)

remove-duplicates returns a new sequence with uniq elements. delete-duplicates may modify the original sequence.

remove-duplicates accepts the following, usual arguments: from-end test test-not start end key.

(remove-duplicates '(:foo :foo :bar))

(remove-duplicates '("foo" "foo" "bar"))
("foo" "foo" "bar")

(remove-duplicates '("foo" "foo" "bar") :test #'string-equal)
("foo" "bar")

mapping (map, mapcar, remove-if[-not],…)

If you’re used to map and filter in other languages, you probably want mapcar. But it only works on lists, so to iterate on vectors (and produce either a vector or a list, use (map 'list function vector).

mapcar also accepts multiple lists with &rest more-seqs. The mapping stops as soon as the shortest sequence runs out.

map takes the output-type as first argument ('list, 'vector or 'string):

(defparameter foo '(1 2 3))
(map 'list (lambda (it) (* 10 it)) foo)

reduce (function, sequence). Special parameter: :initial-value.

(reduce '- '(1 2 3 4))
;; => -8
(reduce '- '(1 2 3 4) :initial-value 100)
;; => 90

Filter is here called remove-if-not.

Flatten a list (Alexandria)

With Alexandria, we have the flatten function.

Creating lists with variables

That’s one use of the backquote:

(defparameter *var* "bar")
;; First try:
'("foo" *var* "baz") ;; no backquote
;; => ("foo" *VAR* "baz") ;; nope

Second try, with backquote interpolation:

`("foo" ,*var* "baz")     ;; backquote, comma
;; => ("foo" "bar" "baz") ;; good

The backquote first warns we’ll do interpolation, the comma introduces the value of the variable.

If our variable is a list:

(defparameter *var* '("bar" "baz"))
;; First try:
`("foo" ,*var*)
;; => ("foo" ("bar" "baz")) ;; nested list
`("foo" ,@*var*)            ;; backquote, comma-@ to
;; => ("foo" "bar" "baz")

E. Weitz warns that “objects generated this way will very likely share structure (see Recipe 2-7)”.

Comparing lists

We can use sets functions.


We show below how to use set operations on lists.

A set doesn’t contain twice the same element and is unordered.

Most of these functions have recycling (modifying) counterparts, starting with “n”: nintersection,… They all accept the usual :key and :test arguments, so use the test #'string= or #'equal if you are working with strings.

For more, see functions in Alexandria: setp, set-equal,… and the FSet library, shown in the next section.

intersection of lists

What elements are both in list-a and list-b ?

(defparameter list-a '(0 1 2 3))
(defparameter list-b '(0 2 4))
(intersection list-a list-b)
;; => (2 0)

Remove the elements of list-b from list-a (set-difference)

(set-difference list-a list-b)
;; => (3 1)
(set-difference list-b list-a)
;; => (4)

Join two lists with uniq elements (union)

(union list-a list-b)
;; => (3 1 0 2 4) ;; order can be different in your lisp

Remove elements that are in both lists (set-exclusive-or)

(set-exclusive-or list-a list-b)
;; => (4 3 1)

Add an element to a set (adjoin)

A new set is returned, the original set is not modified.

(adjoin 3 list-a)
;; => (0 1 2 3)   ;; <-- nothing was changed, 3 was already there.

(adjoin 5 list-a)
;; => (5 0 1 2 3) ;; <-- element added in front.

;; => (0 1 2 3)  ;; <-- original list unmodified.

You can also use pushnew, that modifies the list (see above).

Check if this is a subset (subsetp)

(subsetp '(1 2 3) list-a)
;; => T

(subsetp '(1 1 1) list-a)
;; => T

(subsetp '(3 2 1) list-a)
;; => T

(subsetp '(0 3) list-a)
;; => T

Fset - immutable data structure

You may want to have a look at the FSet library (in Quicklisp).

Arrays and vectors

Arrays have constant-time access characteristics.

They can be fixed or adjustable. A simple array is neither displaced (using :displaced-to, to point to another array) nor adjustable (:adjust-array), nor does it have a fill pointer (fill-pointer, that moves when we add or remove elements).

A vector is an array with rank 1 (of one dimension). It is also a sequence (see above).

A simple vector is a simple array that is also not specialized (it doesn’t use :element-type to set the types of the elements).

Create an array, one or many dimensions

make-array (sizes-list :adjustable bool)

adjust-array (array, sizes-list, :element-type, :initial-element)

Access: aref (array i [j …])

aref (array i j k …) or row-major-aref (array i) equivalent to (aref i i i …).

The result is setfable.

(defparameter myarray (make-array '(2 2 2) :initial-element 1))
;; => #3A(((1 1) (1 1)) ((1 1) (1 1)))
(aref myarray 0 0 0)
;; => 1
(setf (aref myarray 0 0 0) 9)
;; => 9
(row-major-aref myarray 0)
;; => 9


array-total-size (array): how many elements will fit in the array ?

array-dimensions (array): list containing the length of the array’s dimensions.

array-dimension (array i): length of the ith dimension.

array-rank number of dimensions of the array.

(defparameter myarray (make-array '(2 2 2)))
;; => #3A(((0 0) (0 0)) ((0 0) (0 0)))
(array-rank myarray)
;; => 3
(array-dimensions myarray)
;; => (2 2 2)
(array-dimension myarray 0)
;; => 2
(array-total-size myarray)
;; => 8


Create with vector or the reader macro #(). It returns a simple vector.

(vector 1 2 3)
;; => #(1 2 3)
#(1 2 3)
;; => #(1 2 3)

The following interface is available for vectors (or vector-like arrays):

and see also the sequence functions.

The following shows how to create an array that can be pushed to and popped from arbitrarily, growing its storage capacity as needed. This is roughly equivalent to a list in Python, an ArrayList in Java, or a vector<T> in C++ – though note that elements are not erased when they’re popped.

CL-USER> (defparameter *v* (make-array 0 :fill-pointer t :adjustable t))
CL-USER> *v*
CL-USER> (vector-push-extend 42 *v*)
CL-USER> (vector-push-extend 43 *v*)
CL-USER> (vector-pop *v*)
CL-USER> *v*
CL-USER> (aref *v* 1) ; beware, the element is still there!
CL-USER> (setf (aref *v* 1) nil) ; manually erase elements if necessary

Transforming a vector to a list.

If you’re mapping over it, see the map function whose first parameter is the result type.

Or use (coerce vector 'list).

Hash Table

Hash Tables are a powerful data structure, associating keys with values in a very efficient way. Hash Tables are often preferred over association lists whenever performance is an issue, but they introduce a little overhead that makes assoc lists better if there are only a few key-value pairs to maintain.

Alists can be used sometimes differently though:

Creating a Hash Table

Hash Tables are created using the function make-hash-table. It has no required argument. Its most used optional keyword argument is :test, specifying the function used to test the equality of keys.

Note: see shorter notations in the Serapeum or Rutils libraries. For example, Serapeum has dict, and Rutils a #h reader macro.

Adding an Element to a Hash Table

If you want to add an element to a hash table, you can use gethash, the function to retrieve elements from the hash table, in conjunction with setf.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (setf (gethash 'one-entry *my-hash*) "one")
CL-USER> (setf (gethash 'another-entry *my-hash*) 2/4)
CL-USER> (gethash 'one-entry *my-hash*)
CL-USER> (gethash 'another-entry *my-hash*)

With Serapeum’s dict, we can create a hash-table and add elements to it in one go:

(defparameter *my-hash* (dict :one-entry "one"
                              :another-entry 2/4))
;; =>
  :ONE-ENTRY "one"

Getting a value from a Hash Table

The function gethash takes two required arguments: a key and a hash table. It returns two values: the value corresponding to the key in the hash table (or nil if not found), and a boolean indicating whether the key was found in the table. That second value is necessary since nil is a valid value in a key-value pair, so getting nil as first value from gethash does not necessarily mean that the key was not found in the table.

Getting a key that does not exist with a default value

gethash has an optional third argument:

(gethash 'bar *my-hash* "default-bar")
;; => "default-bar"
;;     NIL

Getting all keys or all values of a hash table

The Alexandria library (in Quicklisp) has the functions hash-table-keys and hash-table-values for that.

(ql:quickload "alexandria")
;; […]
(alexandria:hash-table-keys *my-hash*)
;; => (BAR)

Testing for the Presence of a Key in a Hash Table

The first value returned by gethash is the object in the hash table that’s associated with the key you provided as an argument to gethash or nil if no value exists for this key. This value can act as a generalized boolean if you want to test for the presence of keys.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (setf (gethash 'one-entry *my-hash*) "one")
CL-USER> (if (gethash 'one-entry *my-hash*)
           "Key exists"
           "Key does not exist")
"Key exists"
CL-USER> (if (gethash 'another-entry *my-hash*)
           "Key exists"
           "Key does not exist")
"Key does not exist"

But note that this does not work if nil is amongst the values that you want to store in the hash.

CL-USER> (setf (gethash 'another-entry *my-hash*) nil)
CL-USER> (if (gethash 'another-entry *my-hash*)
           "Key exists"
           "Key does not exist")
"Key does not exist"

In this case you’ll have to check the second return value of gethash which will always return nil if no value is found and T otherwise.

CL-USER> (if (nth-value 1 (gethash 'another-entry *my-hash*))
           "Key exists"
           "Key does not exist")
"Key exists"
CL-USER> (if (nth-value 1 (gethash 'no-entry *my-hash*))
           "Key exists"
           "Key does not exist")
"Key does not exist"

Deleting from a Hash Table

Use remhash to delete a hash entry. Both the key and its associated value will be removed from the hash table. remhash returns T if there was such an entry, nil otherwise.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (setf (gethash 'first-key *my-hash*) 'one)
CL-USER> (gethash 'first-key *my-hash*)
CL-USER> (remhash 'first-key *my-hash*)
CL-USER> (gethash 'first-key *my-hash*)
CL-USER> (gethash 'no-entry *my-hash*)
CL-USER> (remhash 'no-entry *my-hash*)
CL-USER> (gethash 'no-entry *my-hash*)

Deleting a Hash Table

Use clrhash to delete a hash table. This will remove all of the data from the hash table and return the deleted table.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (setf (gethash 'first-key *my-hash*) 'one)
CL-USER> (setf (gethash 'second-key *my-hash*) 'two)
CL-USER> *my-hash*
#<hash-table :TEST eql :COUNT 2 {10097BF4E3}>
CL-USER> (clrhash *my-hash*)
#<hash-table :TEST eql :COUNT 0 {10097BF4E3}>
CL-USER> (gethash 'first-key *my-hash*)
CL-USER> (gethash 'second-key *my-hash*)

Traversing a Hash Table

If you want to perform an action on each entry (i.e., each key-value pair) in a hash table, you have several options:

You can use maphash which iterates over all entries in the hash table. Its first argument must be a function which accepts two arguments, the key and the value of each entry. Note that due to the nature of hash tables you can’t control the order in which the entries are provided by maphash (or other traversing constructs). maphash always returns nil.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (setf (gethash 'first-key *my-hash*) 'one)
CL-USER> (setf (gethash 'second-key *my-hash*) 'two)
CL-USER> (setf (gethash 'third-key *my-hash*) nil)
CL-USER> (setf (gethash nil *my-hash*) 'nil-value)
CL-USER> (defun print-hash-entry (key value)
    (format t "The value associated with the key ~S is ~S~%"
            key value))
CL-USER> (maphash #'print-hash-entry *my-hash*)
The value associated with the key FIRST-KEY is ONE
The value associated with the key SECOND-KEY is TWO
The value associated with the key THIRD-KEY is NIL
The value associated with the key NIL is NIL-VALUE

You can also use with-hash-table-iterator, a macro which turns (via macrolet) its first argument into an iterator that on each invocation returns three values per hash table entry - a generalized boolean that’s true if an entry is returned, the key of the entry, and the value of the entry. If there are no more entries, only one value is returned - nil.

;;; same hash-table as above
CL-USER> (with-hash-table-iterator (my-iterator *my-hash*)
              (multiple-value-bind (entry-p key value)
                (if entry-p
                    (print-hash-entry key value)
The value associated with the key FIRST-KEY is ONE
The value associated with the key SECOND-KEY is TWO
The value associated with the key THIRD-KEY is NIL
The value associated with the key NIL is NIL-VALUE

Note the following caveat from the HyperSpec: “It is unspecified what happens if any of the implicit interior state of an iteration is returned outside the dynamic extent of the with-hash-table-iterator form such as by returning some closure over the invocation form.”

And there’s always loop:

;;; same hash-table as above
CL-USER> (loop for key being the hash-keys of *my-hash*
           do (print key))
CL-USER> (loop for key being the hash-keys of *my-hash*
           using (hash-value value)
           do (format t "The value associated with the key ~S is ~S~%"
                      key value))
The value associated with the key FIRST-KEY is ONE
The value associated with the key SECOND-KEY is TWO
The value associated with the key THIRD-KEY is NIL
The value associated with the key NIL is NIL-VALUE
CL-USER> (loop for value being the hash-values of *my-hash*
           do (print value))
CL-USER> (loop for value being the hash-values of *my-hash*
           using (hash-key key)
           do (format t "~&~A -> ~A" key value))

Traversing keys or values

To map over keys or values we can again rely on Alexandria with maphash-keys and maphash-values.

Counting the Entries in a Hash Table

No need to use your fingers - Common Lisp has a built-in function to do it for you: hash-table-count.

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (hash-table-count *my-hash*)
CL-USER> (setf (gethash 'first *my-hash*) 1)
CL-USER> (setf (gethash 'second *my-hash*) 2)
CL-USER> (setf (gethash 'third *my-hash*) 3)
CL-USER> (hash-table-count *my-hash*)
CL-USER> (setf (gethash 'second *my-hash*) 'two)
CL-USER> (hash-table-count *my-hash*)
CL-USER> (clrhash *my-hash*)
#<EQL hash table, 0 entries {48205F35}>
CL-USER> (hash-table-count *my-hash*)

Printing a Hash Table readably

With print-object (non portable)

It is very tempting to use print-object. It works under several implementations, but this method is actually not portable. The standard doesn’t permit to do so, so this is undefined behaviour.

(defmethod print-object ((object hash-table) stream)
  (format stream "#HASH{~{~{(~a : ~a)~}~^ ~}}"
          (loop for key being the hash-keys of object
                using (hash-value value)
                collect (list key value))))


;;                            #<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD

and let’s try it:

(let ((ht (make-hash-table)))
  (setf (gethash :foo ht) :bar)
;; #HASH{(FOO : BAR)}

With a custom function (portable way)

Here’s a portable way.

This snippets prints the keys, values and the test function of a hash-table, and uses alexandria:alist-hash-table to read it back in:

(defun print-hash-table-readably (hash-table
                                  (stream *standard-output*))
  "Prints a hash table readably using ALEXANDRIA:ALIST-HASH-TABLE."
  (let ((test (hash-table-test hash-table))
        (*print-circle* t)
        (*print-readably* t))
    (format stream "#.(ALEXANDRIA:ALIST-HASH-TABLE '(~%")
    (maphash (lambda (k v) (format stream "   (~S . ~S)~%" k v)) hash-table)
    (format stream "   ) :TEST '~A)" test)

Example output:

'((ONE . 1))
#<HASH-TABLE :TEST EQL :COUNT 1 {10046D4863}>

This output can be read back in to create a hash-table:

 (with-output-to-string (s)
     '((a . 1) (b . 2) (c . 3))) s)))
;; #<HASH-TABLE :TEST EQL :COUNT 3 {1009592E23}>
;; 83

With Serapeum (readable and portable)

The Serapeum library has the dict constructor, the function pretty-print-hash-table and the toggle-pretty-print-hash-table switch, all which do not use print-object under the hood.

CL-USER> (serapeum:toggle-pretty-print-hash-table)
CL-USER> (serapeum:dict :a 1 :b 2 :c 3)
  :A 1
  :B 2
  :C 3

This printed representation can be read back in.

Thread-safe Hash Tables

The standard hash-table in Common Lisp is not thread-safe. That means that simple access operations can be interrupted in the middle and return a wrong result.

Implementations offer different solutions.

With SBCL, we can create thread-safe hash tables with the :synchronized keyword to make-hash-table:

If nil (the default), the hash-table may have multiple concurrent readers, but results are undefined if a thread writes to the hash-table concurrently with another reader or writer. If t, all concurrent accesses are safe, but note that clhs 3.6 (Traversal Rules and Side Effects) remains in force. See also: sb-ext:with-locked-hash-table.

(defparameter *my-hash* (make-hash-table :synchronized t))

But, operations that expand to two accesses, like the modify macros (incf) or this:

(setf (gethash :a *my-hash*) :new-value)

need to be wrapped around sb-ext:with-locked-hash-table:

Limits concurrent accesses to HASH-TABLE for the duration of BODY. If HASH-TABLE is synchronized, BODY will execute with exclusive ownership of the table. If HASH-TABLE is not synchronized, BODY will execute with other WITH-LOCKED-HASH-TABLE bodies excluded – exclusion of hash-table accesses not surrounded by WITH-LOCKED-HASH-TABLE is unspecified.

(sb-ext:with-locked-hash-table (*my-hash*)
  (setf (gethash :a *my-hash*) :new-value))

In LispWorks, hash-tables are thread-safe by default. But likewise, there is no guarantee of atomicity between access operations, so we can use with-hash-table-locked.

Ultimately, you might like what the cl-gserver library proposes. It offers helper functions around hash-tables and its actors/agent system to allow thread-safety. They also maintain the order of updates and reads.

Performance Issues: The Size of your Hash Table

The make-hash-table function has a couple of optional parameters which control the initial size of your hash table and how it’ll grow if it needs to grow. This can be an important performance issue if you’re working with large hash tables. Here’s an (admittedly not very scientific) example with CMUCL pre-18d on Linux:

CL-USER> (defparameter *my-hash* (make-hash-table))
CL-USER> (hash-table-size *my-hash*)
CL-USER> (hash-table-rehash-size *my-hash*)
CL-USER> (time (dotimes (n 100000)
                 (setf (gethash n *my-hash*) n)))
Compiling LAMBDA NIL:
Compiling Top-Level Form:

Evaluation took:
  0.27 seconds of real time
  0.25 seconds of user run time
  0.02 seconds of system run time
  0 page faults and
  8754768 bytes consed.
CL-USER> (time (dotimes (n 100000)
                 (setf (gethash n *my-hash*) n)))
Compiling LAMBDA NIL:
Compiling Top-Level Form:

Evaluation took:
  0.05 seconds of real time
  0.05 seconds of user run time
  0.0 seconds of system run time
  0 page faults and
  0 bytes consed.

The values for hash-table-size and hash-table-rehash-size are implementation-dependent. In our case, CMUCL chooses and initial size of 65, and it will increase the size of the hash by 50 percent whenever it needs to grow. Let’s see how often we have to re-size the hash until we reach the final size…

CL-USER> (log (/ 100000 65) 1.5)
CL-USER> (let ((size 65))
           (dotimes (n 20)
             (print (list n size))
             (setq size (* 1.5 size))))
(0 65)
(1 97.5)
(2 146.25)
(3 219.375)
(4 329.0625)
(5 493.59375)
(6 740.3906)
(7 1110.5859)
(8 1665.8789)
(9 2498.8184)
(10 3748.2275)
(11 5622.3413)
(12 8433.512)
(13 12650.268)
(14 18975.402)
(15 28463.104)
(16 42694.656)
(17 64041.984)
(18 96062.98)
(19 144094.47)

The hash has to be re-sized 19 times until it’s big enough to hold 100,000 entries. That explains why we saw a lot of consing and why it took rather long to fill the hash table. It also explains why the second run was much faster - the hash table already had the correct size.

Here’s a faster way to do it: If we know in advance how big our hash will be, we can start with the right size:

CL-USER> (defparameter *my-hash* (make-hash-table :size 100000))
CL-USER> (hash-table-size *my-hash*)
CL-USER> (time (dotimes (n 100000)
                 (setf (gethash n *my-hash*) n)))
Compiling LAMBDA NIL:
Compiling Top-Level Form:

Evaluation took:
  0.04 seconds of real time
  0.04 seconds of user run time
  0.0 seconds of system run time
  0 page faults and
  0 bytes consed.

That’s obviously much faster. And there was no consing involved because we didn’t have to re-size at all. If we don’t know the final size in advance but can guess the growth behaviour of our hash table we can also provide this value to make-hash-table. We can provide an integer to specify absolute growth or a float to specify relative growth.

CL-USER> (defparameter *my-hash* (make-hash-table :rehash-size 100000))
CL-USER> (hash-table-size *my-hash*)
CL-USER> (hash-table-rehash-size *my-hash*)
CL-USER> (time (dotimes (n 100000)
                 (setf (gethash n *my-hash*) n)))
Compiling LAMBDA NIL:
Compiling Top-Level Form:

Evaluation took:
  0.07 seconds of real time
  0.05 seconds of user run time
  0.01 seconds of system run time
  0 page faults and
  2001360 bytes consed.

Also rather fast (we only needed one re-size) but much more consing because almost the whole hash table (minus 65 initial elements) had to be built during the loop.

Note that you can also specify the rehash-threshold while creating a new hash table. One final remark: Your implementation is allowed to completely ignore the values provided for rehash-size and rehash-threshold



An association list is a list of cons cells.

This simple example:

(defparameter *my-alist* (list (cons 'foo "foo")
                             (cons 'bar "bar")))
;; => ((FOO . "foo") (BAR . "bar"))

looks like this:

 |       |
 |      [o|o]---"bar"
 |       |
 |      BAR


We can construct an alist like its representation:

(setf *my-alist* '((:foo . "foo")
                 (:bar . "bar")))

The constructor pairlis associates a list of keys and a list of values:

(pairlis '(:foo :bar)
         '("foo" "bar"))
;; => ((:BAR . "bar") (:FOO . "foo"))

Alists are just lists, so you can have the same key multiple times in the same alist:

(setf *alist-with-duplicate-keys*
  '((:a . 1)
    (:a . 2)
    (:b . 3)
    (:a . 4)
    (:c . 5)))


To get a key, we have assoc (use :test 'equal when your keys are strings, as usual). It returns the whole cons cell, so you may want to use cdr or second to get the value, or even assoc-value list key from Alexandria.

(assoc :foo *my-alist*)
;; (:FOO . "foo")
(cdr *)
;; "foo"
(alexandria:assoc-value *my-alist* :foo)
;; "foo"
;; (:FOO . "FOO")
;; It actually returned 2 values.

There is assoc-if, and rassoc to get a cons cell by its value:

(rassoc "foo" *my-alist*)
;; NIL
;; bummer! The value "foo" is a string, so use:
(rassoc "foo" *my-alist* :test #'equal)
;; (:FOO . "foo")

If the alist has repeating (duplicate) keys, you can use remove-if-not, for example, to retrieve all of them.

  (lambda (entry)
      (eq :a entry))
  :key #'car)

Insert and remove entries

To add a key, we push another cons cell:

(push (cons 'team "team") *my-alist*)
;; => ((TEAM . "team") (FOO . "foo") (BAR . "bar"))

We can use pop and other functions that operate on lists, like remove:

(remove :team *my-alist*)
;; ((:TEAM . "team") (FOO . "foo") (BAR . "bar"))
;; => didn't remove anything
(remove :team *my-alist* :key 'car)
;; ((FOO . "foo") (BAR . "bar"))
;; => returns a copy

Remove only one element with :count:

(push (cons 'bar "bar2") *my-alist*)
;; ((BAR . "bar2") (TEAM . "team") (FOO . "foo") (BAR . "bar"))
;; => twice the 'bar key

(remove 'bar *my-alist* :key 'car :count 1)
;; ((TEAM . "team") (FOO . "foo") (BAR . "bar"))

;; because otherwise:
(remove 'bar *my-alist* :key 'car)
;; ((TEAM . "team") (FOO . "foo"))
;; => no more 'bar

Update entries

Replace a value:

;; => '((:FOO . "foo") (:BAR . "bar"))
(assoc :foo *my-alist*)
;; => (:FOO . "foo")
(setf (cdr (assoc :foo *my-alist*)) "new-value")
;; => "new-value"
;; => '((:foo . "new-value") (:BAR . "bar"))

Replace a key:

;; => '((:FOO . "foo") (:BAR . "bar")))
(setf (car (assoc :bar *my-alist*)) :new-key)
;; => :NEW-KEY
;; => '((:FOO . "foo") (:NEW-KEY . "bar")))

In the Alexandria library, see more functions like hash-table-alist, alist-plist,…


A property list is simply a list that alternates a key, a value, and so on, where its keys are symbols (we can not set its :test). More precisely, it first has a cons cell whose car is the key, whose cdr points to the following cons cell whose car is the value.

For example this plist:

(defparameter my-plist (list 'foo "foo" 'bar "bar"))

looks like this:

 |       |       |       |
FOO     "foo"   BAR     "bar"

We access an element with getf (list elt) (it returns the value) (the list comes as first element),

we remove an element with remf.

(defparameter my-plist (list 'foo "foo" 'bar "bar"))
;; => (FOO "foo" BAR "bar")
(setf (getf my-plist 'foo) "foo!!!")
;; => "foo!!!"


Structures offer a way to store data in named slots. They support single inheritance.

Classes provided by the Common Lisp Object System (CLOS) are more flexible however structures may offer better performance (see for example the SBCL manual).


Use defstruct:

(defstruct person
   id name age)

At creation slots are optional and default to nil.

To set a default value:

(defstruct person
   (name "john doe")

Also specify the type after the default value:

(defstruct person
  (name "john doe" :type string)

We create an instance with the generated constructor make- + <structure-name>, so make-person:

(defparameter *me* (make-person))
#S(PERSON :ID NIL :NAME "john doe" :AGE NIL)

note that printed representations can be read back by the reader.

With a bad name type:

(defparameter *bad-name* (make-person :name 123))
Invalid initialization argument:
in call for class #<STRUCTURE-CLASS PERSON>.
   [Condition of type SB-PCL::INITARG-ERROR]

We can set the structure’s constructor so as to create the structure without using keyword arguments, which can be more convenient sometimes. We give it a name and the order of the arguments:

(defstruct (person (:constructor create-person (id name age)))

Our new constructor is create-person:

(create-person 1 "me" 7)
#S(PERSON :ID 1 :NAME "me" :AGE 7)

However, the default make-person does not work any more:

(make-person :name "me")
;; debugger:
obsolete structure error for a structure of type PERSON
[Condition of type SB-PCL::OBSOLETE-STRUCTURE]

Slot access

We access the slots with accessors created by <name-of-the-struct>- + slot-name:

(person-name *me*)
;; "john doe"

we then also have person-age and person-id.


Slots are setf-able:

(setf (person-name *me*) "Cookbook author")
(person-name *me*)
;; "Cookbook author"


A predicate function is generated:

(person-p *me*)

Single inheritance

Use single inheritance with the :include <struct> argument:

(defstruct (female (:include person))
     (gender "female" :type string))
(make-female :name "Lilie")
;; #S(FEMALE :ID NIL :NAME "Lilie" :AGE NIL :GENDER "female")

Note that the CLOS object system is more powerful.


After a change, instances are not updated.

If we try to add a slot (email below), we have the choice to lose all instances, or to continue using the new definition of person. But the effects of redefining a structure are undefined by the standard, so it is best to re-compile and re-run the changed code.

(defstruct person
       (name "john doe" :type string)

gives an error and we drop in the debugger:

attempt to redefine the STRUCTURE-OBJECT class PERSON
incompatibly with the current definition
   [Condition of type SIMPLE-ERROR]

 0: [CONTINUE] Use the new definition of PERSON, invalidating already-loaded code and instances.
 1: [RECKLESSLY-CONTINUE] Use the new definition of PERSON as if it were compatible, allowing old accessors to use new instances and allowing new accessors to use old instances.
 2: [CLOBBER-IT] (deprecated synonym for RECKLESSLY-CONTINUE)
 3: [RETRY] Retry SLIME REPL evaluation request.
 4: [*ABORT] Return to SLIME's top level.
 5: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {1002A0FFA3}>)

If we choose restart 0, to use the new definition, we lose access to *me*:

obsolete structure error for a structure of type PERSON
   [Condition of type SB-PCL::OBSOLETE-STRUCTURE]

There is also very little introspection. Portable Common Lisp does not define ways of finding out defined super/sub-structures nor what slots a structure has.

The Common Lisp Object System (which came after into the language) doesn’t have such limitations. See the CLOS section.


tree-equal, copy-tree. They descend recursively into the car and the cdr of the cons cells they visit.

Sycamore - purely functional weight-balanced binary trees


Controlling how much of data to print (*print-length*, *print-level*)

Use *print-length* and *print-level*.

They are both nil by default.

If you have a very big list, printing it on the REPL or in a stacktrace can take a long time and bring your editor or even your server down. Use *print-length* to choose the maximum of elements of the list to print, and to show there is a rest with a ... placeholder:

(setf *print-length* 2)
(list :A :B :C :D :E)
;; (:A :B ...)

And if you have a very nested data structure, set *print-level* to choose the depth to print:

(let ((*print-level* 2))
  (print '(:a (:b (:c (:d :e))))))
;; (:A (:B #))             <= *print-level* in action
;; (:A (:B (:C (:D :E))))
;; => the list is returned,
;; the let binding is not in effect anymore.

*print-length* will be applied at each level.

Reference: the HyperSpec.

Appendix A - generic and nested access of alists, plists, hash-tables and CLOS slots

The solutions presented below might help you getting started, but keep in mind that they’ll have a performance impact and that error messages will be less explicit.

Appendix B - accessing nested data structures

Sometimes we work with nested data structures, and we might want an easier way to access a nested element than intricated “getf” and “assoc” and all. Also, we might want to just be returned a nil when an intermediary key doesn’t exist.

The access library given above provides this, with (accesses var key1 key2…).

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