DOLIST and DOTIMES（DOLIST 和 DOTIMES 宏）

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

先从易于使用的 DOLIST 和 DOTIMES 宏开始。

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

DOLIST 在一个列表的元素上循环操作，使用一个依次持有列表中所有后继元素的变量来执行循环体。下面是其基本形式（去掉了一些比较难懂的选项）：

  1. (dolist (var list-form)
  2.   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:

当循环开始时，list-form 被求值一次以产生一个列表。然后循环体在列表的每一项上求值一次，同时用变量 var 保存当前项的值。例如：

  1. CL-USER> (dolist (x '(1 2 3)) (print x))
  2. 1
  3. 2
  4. 3
  5. NIL

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

在这种方式下，DOLIST 这种形式本身求值为 NIL。

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

如果想在列表结束之前中断一个 DOLIST 循环，则可以使用 RETURN。

  1. CL-USER> (dolist (x '(1 2 3)) (print x) (if (evenp x) (return)))
  2. 1
  3. 2
  4. NIL

DOTIMES is the high-level looping construct for counting loops. The basic template is much the same as DOLIST‘s.

DOTIMES 是用于循环计数的高级循环构造。其基本模板和 DOLIST 非常相似。

  1. (dotimes (var count-form)
  2.   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:

其中的 count-form 必须要能求值为一个整数。通过每次循环，var 所持有的整数依次为从 0 到比那个数小 1 的每一个后继整数。例如：

  1. CL-USER> (dotimes (i 4) (print i))
  2. 0
  3. 1
  4. 2
  5. 3
  6. NIL

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

和 DOLIST 一样，也可以使用 RETURN 来提前中断循环。

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:

由于 DOLIST 和 DOTIMES 的循环体中可以包含任何类型的表达式，因此也可以使用嵌套循环。例如，为了打印出从 1 × 1 = 1 到 20 × 20 = 400 的乘法表，可以写出下面这对嵌套的 DOTIMES 循环：

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