Common <pre>
LISP Hints
Geoffrey J. Gordon
<[e-mail]>
Friday, February 5, 1993
Modified by
Bruno Haible
<[e-mail]>
Note: This tutorial introduction to Common Lisp was written for the
CMU environment, so some of the details of running lisp toward the end
may differ from site to site.
Further Information
The best LISP textbook I know of is
Guy L. Steele Jr. _Common LISP: the Language_. Digital Press. 1984.
The first edition is easier to read; the second describes a more recent
standard. (The differences between the two standards shouldn't affect
casual programmers.)
A book by Dave Touretsky has also been recommended to me, although I
haven't read it, so I can't say anything about it.
Symbols
A symbol is just a string of characters. There are restrictions on what
you can include in a symbol and what the first character can be, but as
long as you stick to letters, digits, and hyphens, you'll be safe.
(Except that if you use only digits and possibly an initial hyphen,
LISP will think you typed an integer rather than a symbol.) Some
examples of symbols:
a
b
c1
foo
bar
baaz-quux-garply
Some things you can do with symbols follow. (Things after a ">" prompt
are what you type to the LISP interpreter, while other things are what
the LISP interpreter prints back to you. The ";" is LISP's comment
character: everything from a ";" to the end of line is ignored.)
> (setq a 5) ;store a number as the value of a symbol
5
> a ;take the value of a symbol
5
> (let ((a 6)) a) ;bind the value of a symbol temporarily to 6
6
> a ;the value returns to 5 once the let is finished
5
> (+ a 6) ;use the value of a symbol as an argument to a function
11
> b ;try to take the value of a symbol which has no value
Error: Attempt to take the value of the unbound symbol B
There are two special symbols, t and nil. The value of t is defined
always to be t, and the value of nil is defined always to be nil. LISP
uses t and nil to represent true and false. An example of this use is
in the if statement, described more fully later:
> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5
The last example is odd but correct: nil means false, and anything else
means true. (Unless we have a reason to do otherwise, we use t to mean
true, just for the sake of clarity.)
Symbols like t and nil are called self-evaluating symbols, because
they evaluate to themselves. There is a whole class of self-evaluating
symbols called keywords; any symbol whose name starts with a colon is a
keyword. (See below for some uses for keywords.) Some examples:
> :this-is-a-keyword
:THIS-IS-A-KEYWORD
> :so-is-this
:SO-IS-THIS
> :me-too
:ME-TOO
Numbers
An integer is a string of digits optionally preceded by + or -. A real
number looks like an integer, except that it has a decimal point and
optionally can be written in scientific notation. A rational looks like
two integers with a / between them. LISP supports complex numbers,
which are written #c(r i) (where r is the real part and i is the
imaginary part). A number is any of the above. Here are some numbers:
5
17
-34
+6
3.1415
1.722e-15
#c(1.722e-15 0.75)
The standard arithmetic functions are all available: +, -, *, /, floor,
ceiling, mod, sin, cos, tan, sqrt, exp, expt, and so forth. All of them
accept any kind of number as an argument. +, -, *, and / return a
number according to type contagion: an integer plus a rational is a
rational, a rational plus a real is a real, and a real plus a complex
is a complex. Here are some examples:
> (+ 3 3/4) ;type contagion
15/4
> (exp 1) ;e
2.7182817
> (exp 3) ;e*e*e
20.085537
> (expt 3 4.2) ;exponent with a base other than e
100.90418
> (+ 5 6 7 (* 8 9 10)) ;the fns +-*/ all accept multiple arguments
There is no limit to the absolute value of an integer except the memory
size of your computer. Be warned that computations with bignums (as
large integers are called) can be slow. (So can computations with
rationals, especially compared to the corresponding computations with
small integers or floats.)
Conses
A cons is just a two-field record. The fields are called "car" and
"cdr", for historical reasons. (On the first machine where LISP was
implemented, there were two instructions CAR and CDR which stood for
"contents of address register" and "contents of decrement register".
Conses were implemented using these two registers.)
Conses are easy to use:
> (cons 4 5) ;Allocate a cons. Set the car to 4 and the cdr to 5.
(4 . 5)
> (cons (cons 4 5) 6)
((4 . 5) . 6)
> (car (cons 4 5))
4
> (cdr (cons 4 5))
5
Lists
You can build many structures out of conses. Perhaps the simplest is a
linked list: the car of each cons points to one of the elements of the
list, and the cdr points either to another cons or to nil. You can
create such a linked list with the list fuction:
> (list 4 5 6)
(4 5 6)
Notice that LISP prints linked lists a special way: it omits some of
the periods and parentheses. The rule is: if the cdr of a cons is nil,
LISP doesn't bother to print the period or the nil; and if the cdr of
cons A is cons B, then LISP doesn't bother to print the period for cons
A or the parentheses for cons B. So:
> (cons 4 nil)
(4)
> (cons 4 (cons 5 6))
(4 5 . 6)
> (cons 4 (cons 5 (cons 6 nil)))
(4 5 6)
The last example is exactly equivalent to the call (list 4 5 6). Note
that nil now means the list with no elements: the cdr of (a b), a list
with 2 elements, is (b), a list with 1 element; and the cdr of (b), a
list with 1 element, is nil, which therefore must be a list with no
elements.
The car and cdr of nil are defined to be nil.
If you store your list in a variable, you can make it act like a stack:
> (setq a nil)
NIL
> (push 4 a)
(4)
> (push 5 a)
(5 4)
> (pop a)
5
> a
(4)
> (pop a)
4
> (pop a)
NIL
> a
NIL
Functions
You saw one example of a function above. Here are some more:
> (+ 3 4 5 6) ;this function takes any number of arguments
18
> (+ (+ 3 4) (+ (+ 4 5) 6)) ;isn't prefix notation fun?
22
> (defun foo (x y) (+ x y 5)) ;defining a function
FOO
> (foo 5 0) ;calling a function
10
> (defun fact (x) ;a recursive function
(if (> x 0)
(* x (fact (- x 1)))
1
) )
FACT
> (fact 5)
120
> (defun a (x) (if (= x 0) t (b (- x)))) ;mutually recursive functions
A
> (defun b (x) (if (> x 0) (a (- x 1)) (a (+ x 1))))
B
> (a 5)
T
> (defun bar (x) ;a function with multiple statements in
(setq x (* x 3)) ;its body -- it will return the value
(setq x (/ x 2)) ;returned by its final statement
(+ x 4)
)
BAR
> (bar 6)
13
When we defined foo, we gave it two arguments, x and y. Now when we
call foo, we are required to provide exactly two arguments: the first
will become the value of x for the duration of the call to foo, and the
second will become the value of y for the duration of the call. In
LISP, most variables are lexically scoped; that is, if foo calls bar
and bar tries to reference x, bar will not get foo's value for x.
The process of assigning a symbol a value for the duration of some
lexical scope is called binding.
You can specify optional arguments for your functions. Any argument
after the symbol &optional is optional:
> (defun bar (x &optional y) (if y x 0))
BAR
> (defun baaz (&optional (x 3) (z 10)) (+ x z))
BAAZ
> (bar 5)
0
> (bar 5 t)
5
> (baaz 5)
15
> (baaz 5 6)
11
> (baaz)
13
It is legal to call the function bar with either one or two arguments.
If it is called with one argument, x will be bound to the value of that
argument and y will be bound to nil; if it is called with two
arguments, x and y will be bound to the values of the first and second
argument, respectively.
The function baaz has two optional arguments. It specifies a default
value for each of them: if the caller specifies only one argument, z
will be bound to 10 instead of to nil, and if the caller specifies no
arguments, x will be bound to 3 and z to 10.
You can make your function accept any number of arguments by ending its
argument list with an &rest parameter. LISP will collect all arguments
not otherwise accounted for into a list and bind the &rest parameter to
that list. So:
> (defun foo (x &rest y) y)
FOO
> (foo 3)
NIL
> (foo 4 5 6)
(5 6)
Finally, you can give your function another kind of optional argument
called a keyword argument. The caller can give these arguments in any
order, because they're labelled with keywords.
> (defun foo (&key x y) (cons x y))
FOO
> (foo :x 5 :y 3)
(5 . 3)
> (foo :y 3 :x 5)
(5 . 3)
> (foo :y 3)
(NIL . 3)
> (foo)
(NIL)
An &key parameter can have a default value too:
> (defun foo (&key (x 5)) x)
FOO
> (foo :x 7)
7
> (foo)
5
Printing
Some functions can cause output. The simplest one is print, which
prints its argument and then returns it.
> (print 3)
3
3
The first 3 above was printed, the second was returned.
If you want more complicated output, you will need to use format.
Here's an example:
> (format t "An atom: ~S~%and a list: ~S~%and an integer: ~D~%"
nil (list 5) 6)
An atom: NIL
and a list: (5)
and an integer: 6
The first argument to format is either t, nil, or a stream. T specifies
output to the terminal. Nil means not to print anything but to return a
string containing the output instead. Streams are general places for
output to go: they can specify a file, or the terminal, or another
program. This handout will not describe streams in any further detail.
The second argument is a formatting template, which is a string
optionally containing formatting directives.
All remaining arguments may be referred to by the formatting
directives. LISP will replace the directives with some appropriate
characters based on the arguments to which they refer and then print
the resulting string.
Format always returns nil unless its first argument is nil, in which
case it prints nothing and returns a string.
There are three different directives in the above example: ~S, ~D, and
~%. The first one accepts any LISP object and is replaced by a printed
representation of that object (the same representation which is
produced by print). The second one accepts only integers. The third one
doesn't refer to an argument; it is always replaced by a carriage
return.
Another useful directive is ~~, which is replaced by a single ~.
Refer to a LISP manual for (many, many) additional formatting
directives.
Forms and the Top-Level Loop
The things which you type to the LISP interpreter are called forms; the
LISP interpreter repeatedly reads a form, evaluates it, and prints the
result. This procedure is called the read-eval-print loop.
Some forms will cause errors. After an error, LISP will put you into
the debugger so you can try to figure out what caused the error. LISP
debuggers are all different; but most will respond to the command
"help" or ":help" by giving some form of help.
In general, a form is either an atom (for example, a symbol, an
integer, or a string) or a list. If the form is an atom, LISP evaluates
it immediately. Symbols evaluate to their value; integers and strings
evaluate to themselves. If the form is a list, LISP treats its first
element as the name of a function; it evaluates the remaining elements
recursively, and then calls the function with the values of the
remaining elements as arguments.
For example, if LISP sees the form (+ 3 4), it treats + as the name of
a function. It then evaluates 3 to get 3 and 4 to get 4; finally it
calls + with 3 and 4 as the arguments. The + function returns 7, which
LISP prints.
The top-level loop provides some other conveniences; one particularly
convenient convenience is the ability to talk about the results of
previously typed forms. LISP always saves its most recent three
results; it stores them as the values of the symbols *, **, and ***.
For example:
> 3
3
> 4
4
> 5
5
> ***
3
> ***
4
> ***
5
> **
4
> *
4
Special forms
There are a number of special forms which look like function calls but
aren't. These include control constructs such as if statements and do
loops; assignments like setq, setf, push, and pop; definitions such as
defun and defstruct; and binding constructs such as let. (Not all of
these special forms have been mentioned yet. See below.)
One useful special form is the quote form: quote prevents its argument
from being evaluated. For example:
> (setq a 3)
3
> a
3
> (quote a)
A
> 'a ;'a is an abbreviation for (quote a)
A
Another similar special form is the function form: function causes its
argument to be interpreted as a function rather than being evaluated.
For example:
> (setq + 3)
3
> +
3
> '+
+
> (function +)
#<Function + @ #x-fbef9de>
> #'+ ;#'+ is an abbreviation for (function +)
#<Function + @ #x-fbef9de>
The function special form is useful when you want to pass a function as
an argument to another function. See below for some examples of
functions which take functions as arguments.
Binding
Binding is lexically scoped assignment. It happens to the variables in
a function's parameter list whenever the function is called: the formal
parameters are bound to the actual parameters for the duration of the
function call. You can bind variables anywhere in a program with the
let special form, which looks like this:
(let ((var1 val1)
(var2 val2)
...
)
body
)
Let binds var1 to val1, var2 to val2, and so forth; then it executes
the statements in its body. The body of a let follows exactly the same
rules that a function body does. Some examples:
> (let ((a 3)) (+ a 1))
4
> (let ((a 2)
(b 3)
(c 0))
(setq c (+ a b))
c
)
5
> (setq c 4)
4
> (let ((c 5)) c)
5
> c
4
Instead of (let ((a nil) (b nil)) ...), you can write (let (a b) ...).
The val1, val2, etc. inside a let cannot reference the variables var1,
var2, etc. that the let is binding. For example,
> (let ((x 1)
(y (+ x 1)))
y
)
Error: Attempt to take the value of the unbound symbol X
If the symbol x already has a global value, stranger happenings will
result:
> (setq x 7)
7
> (let ((x 1)
(y (+ x 1)))
y
)
8
The let* special form is just like let except that it allows values to
reference variables defined earlier in the let*. For example,
> (setq x 7)
7
> (let* ((x 1)
(y (+ x 1)))
y
)
2
The form
(let* ((x a)
(y b))
...
)
is equivalent to
(let ((x a))
(let ((y b))
...
) )
Dynamic Scoping
The let and let* forms provide lexical scoping, which is what you
expect if you're used to programming in C or Pascal. Dynamic scoping is
what you get in BASIC: if you assign a value to a dynamically every scoped
variable, mention of that variable returns that value until you
assign another value to the same variable.
In LISP, dynamically scoped variables are called special variables. You
can declare a special variable with the defvar special form. Here are
some examples of lexically and dynamically scoped variables.
In this example, the function check-regular references a regular (ie,
lexically scoped) variable. Since check-regular is lexically outside of
the let which binds regular, check-regular returns the variable's
global value.
> (setq regular 5)
5
> (defun check-regular () regular)
CHECK-REGULAR
> (check-regular)
5
> (let ((regular 6)) (check-regular))
5
In this example, the function check-special references a special (ie,
dynamically scoped) variable. Since the call to check-special is
temporally inside of the let which binds special, check-special returns
the variable's local value.
> (defvar *special* 5)
*SPECIAL*
> (defun check-special () *special*)
CHECK-SPECIAL
> (check-special)
5
> (let ((*special* 6)) (check-special))
6
By convention, the name of a special variable begins and ends with a *.
Special variables are chiefly used as global variables, since
programmers usually expect lexical scoping for local variables and
dynamic scoping for global variables.
For more information on the difference between lexical and dynamic
scoping, see _Common LISP: the Language_.
Arrays
The function make-array makes an array. The aref function accesses its
elements. All elements of an array are initially set to nil. For
example:
> (make-array '(3 3))
#2a((NIL NIL NIL) (NIL NIL NIL) (NIL NIL NIL))
> (aref * 1 1)
NIL
> (make-array 4) ;1D arrays don't need the extra parens
#(NIL NIL NIL NIL)
Array indices always start at 0.
See below for how to set the elements of an array.
Strings
A string is a sequence of characters between double quotes. LISP
represents a string as a variable-length array of characters. You can
write a string which contains a double quote by preceding the quote
with a backslash; a double backslash stands for a single backslash. For
example:
"abcd" has 4 characters
"\"" has 1 character
"\\" has 1 character
Here are some functions for dealing with strings:
> (concatenate 'string "abcd" "efg")
"abcdefg"
> (char "abc" 1)
#\b ;LISP writes characters preceded by #\
> (aref "abc" 1)
#\b ;remember, strings are really arrays
The concatenate function can actually work with any type of sequence:
> (concatenate 'string '(#\a #\b) '(#\c))
"abc"
> (concatenate 'list "abc" "de")
(#\a #\b #\c #\d #\e)
> (concatenate 'vector '#(3 3 3) '#(3 3 3))
#(3 3 3 3 3 3)
Structures
LISP structures are analogous to C structs or Pascal records. Here is
an example:
> (defstruct foo
bar
baaz
quux
)
FOO
This example defines a data type called foo which is a structure
containing 3 fields. It also defines 4 functions which operate on this
data type: make-foo, foo-bar, foo-baaz, and foo-quux. The first one
makes a new object of type foo; the others access the fields of an
object of type foo. Here is how to use these functions:
> (make-foo)
#s(FOO :BAR NIL :BAAZ NIL :QUUX NIL)
> (make-foo :baaz 3)
#s(FOO :BAR NIL :BAAZ 3 :QUUX NIL)
> (foo-bar *)
NIL
> (foo-baaz **)
3
The make-foo function can take a keyword argument for each of the
fields a structure of type foo can have. The field access functions
each take one argument, a structure of type foo, and return the
appropriate field.
See below for how to set the fields of a structure.
Setf
Certain forms in LISP naturally define a memory location. For example,
if the value of x is a structure of type foo, then (foo-bar x) defines
the bar field of the value of x. Or, if the value of y is a one-
dimensional array, (aref y 2) defines the third element of y.
The setf special form uses its first argument to define a place in
memory, evaluates its second argument, and stores the resulting value
in the resulting memory location. For example,
> (setq a (make-array 3))
#(NIL NIL NIL)
> (aref a 1)
NIL
> (setf (aref a 1) 3)
3
> a
#(NIL 3 NIL)
> (aref a 1)
3
> (defstruct foo bar)
FOO
> (setq a (make-foo))
#s(FOO :BAR NIL)
> (foo-bar a)
NIL
> (setf (foo-bar a) 3)
3
> a
#s(FOO :BAR 3)
> (foo-bar a)
3
Setf is the only way to set the fields of a structure or the elements
of an array.
Here are some more examples of setf and related functions.
> (setf a (make-array 1)) ;setf on a variable is equivalent to setq
#(NIL)
> (push 5 (aref a 1)) ;push can act like setf
(5)
> (pop (aref a 1)) ;so can pop
5
> (setf (aref a 1) 5)
5
> (incf (aref a 1)) ;incf reads from a place, increments,
6 ;and writes back
> (aref a 1)
6
Booleans and Conditionals
LISP uses the self-evaluating symbol nil to mean false. Anything other
than nil means true. Unless we have a reason not to, we usually use the
self-evaluating symbol t to stand for true.
LISP provides a standard set of logical functions, for example and, or,
and not. The and and or connectives are short-circuiting: and will not
evaluate any arguments to the right of the first one which evaluates to
nil, while or will not evaluate any arguments to the right of the first
one which evaluates to t.
LISP also provides several special forms for conditional execution. The
simplest of these is if. The first argument of if determines whether
the second or third argument will be executed:
> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5
If you need to put more than one statement in the then or else clause
of an if statement, you can use the progn special form. Progn executes
each statement in its body, then returns the value of the final one.
> (setq a 7)
7
> (setq b 0)
0
> (setq c 5)
5
> (if (> a 5)
(progn
(setq a (+ b 7))
(setq b (+ c 8)))
(setq b 4)
)
13
An if statement which lacks either a then or an else clause can be
written using the when or unless special form:
> (when t 3)
3
> (when nil 3)
NIL
> (unless t 3)
NIL
> (unless nil 3)
3
When and unless, unlike if, allow any number of statements in their
bodies. (Eg, (when x a b c) is equivalent to (if x (progn a b c)).)
> (when t
(setq a 5)
(+ a 6)
)
11
More complicated conditionals can be defined using the cond special
form, which is equivalent to an if ... else if ... fi construction.
A cond consists of the symbol cond followed by a number of cond
clauses, each of which is a list. The first element of a cond clause is
the condition; the remaining elements (if any) are the action. The cond
form finds the first clause whose condition evaluates to true (ie,
doesn't evaluate to nil); it then executes the corresponding action and
returns the resulting value. None of the remaining conditions are
evaluated; nor are any actions except the one corresponding to the
selected condition. For example:
> (setq a 3)
3
> (cond
((evenp a) a) ;if a is even return a
((> a 7) (/ a 2)) ;else if a is bigger than 7 return a/2
((< a 5) (- a 1)) ;else if a is smaller than 5 return a-1
(t 17) ;else return 17
)
2
If the action in the selected cond clause is missing, cond returns what
the condition evaluated to:
> (cond ((+ 3 4)))
7
Here's a clever little recursive function which uses cond. You might be
interested in trying to prove that it terminates for all integers x at
least 1. (If you succeed, please publish the result.)
> (defun hotpo (x steps) ;hotpo stands for Half Or Triple Plus One
(cond
((= x 1) steps)
((oddp x) (hotpo (+ 1 (* x 3)) (+ 1 steps)))
(t (hotpo (/ x 2) (+ 1 steps)))
) )
A
> (hotpo 7 0)
16
The LISP case statement is like a C switch statement:
> (setq x 'b)
B
> (case x
(a 5)
((d e) 7)
((b f) 3)
(otherwise 9)
)
3
The otherwise clause at the end means that if x is not a, b, d, e, or
f, the case statement will return 9.
Iteration
The simplest iteration construct in LISP is loop: a loop construct
repeatedly executes its body until it hits a return special form. For
example,
> (setq a 4)
4
> (loop
(setq a (+ a 1))
(when (> a 7) (return a))
)
8
> (loop
(setq a (- a 1))
(when (< a 3) (return))
)
NIL
The next simplest is dolist: dolist binds a variable to the elements of
a list in order and stops when it hits the end of the list.
> (dolist (x '(a b c)) (print x))
A
B
C
NIL
Dolist always returns nil. Note that the value of x in the above
example was never nil: the NIL below the C was the value that dolist
returned, printed by the read-eval-print loop.
The most complicated iteration primitive is called do. A do statement
looks like this:
> (do ((x 1 (+ x 1))
(y 1 (* y 2)))
((> x 5) y)
(print y)
(print 'working)
)
1
WORKING
2
WORKING
4
WORKING
8
WORKING
16
WORKING
32
The first part of a do specifies what variables to bind, what their
initial values are, and how to update them. The second part specifies a
termination condit...