Книга: Practical Common Lisp

DOLIST and DOTIMES

DOLIST and DOTIMES

I'll start with the easy-to-use DOLIST and DOTIMES macros.

DOLIST loops across the items of a list, executing the loop body with a variable holding the successive items of the list.[89] This is the basic skeleton (leaving out some of the more esoteric options):

(dolist (var list-form)
body-form*)

When the loop starts, the list-form is evaluated once to produce a list. Then the body of the loop is evaluated once for each item in the list with the variable var holding the value of the item. For instance:

CL-USER> (dolist (x '(1 2 3)) (print x))
1
2
3
NIL

Used this way, the DOLIST form as a whole evaluates to NIL.

If you want to break out of a DOLIST loop before the end of the list, you can use RETURN.

CL-USER> (dolist (x '(1 2 3)) (print x) (if (evenp x) (return)))
1
2
NIL
DOTIMES is the high-level looping construct for counting loops. The basic template is much the same as DOLIST's.

(dotimes (var count-form)
body-form*)

The count-form must evaluate to an integer. Each time through the loop var holds successive integers from 0 to one less than that number. For instance:

CL-USER> (dotimes (i 4) (print i))
0
1
2
3
NIL

As with DOLIST, you can use RETURN to break out of the loop early.

Because the body of both DOLIST and DOTIMES loops can contain any kind of expressions, you can also nest loops. For example, to print out the times tables from 1 ? 1 = 1 to 20 ? 20 = 400, you can write this pair of nested DOTIMES loops:

(dotimes (x 20)
(dotimes (y 20)
(format t "~3d " (* (1+ x) (1+ y))))
(format t "~%"))

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