Sequence Mapping Functions（序列映射函数）

最终，序列函数的最后是通用映射函数。MAP 和序列谓词函数一样，接受一个带有 n 个参数函数和 n 个序列。但并非返回布尔值，MAP 返回一个新序列，它由那些将函数应用在序列的相继元素上所得到的结果组成。与 CONCATENATE 和 MERGE 相似，MAP 需要被告知其所创建序列的类型。

MAP-INTO is like MAP except instead of producing a new sequence of a given type, it places the results into a sequence passed as the first argument. This sequence can be the same as one of the sequences providing values for the function. For instance, to sum several vectors—, b, and c--into one, you could write this:

If the sequences are different lengths, MAP-INTO affects only as many elements as are present in the shortest sequence, including the sequence being mapped into. However, if the sequence being mapped into is a vector with a fill pointer, the number of elements affected isn’t limited by the fill pointer but rather by the actual size of the vector. After a call to MAP-INTO, the fill pointer will be set to the number of elements mapped. MAP-INTO won’t, however, extend an adjustable vector.

如果这些序列的长度不同，那么 MAP-INTO 将只影响那些与最短序列元素数量相当元素，其中也包括那个将被映射到的序列。不过，如果序列被映射到一个带有填充指针的向量里，受影响元素的数量将不限于填充指针而是该向量的实际大小。在一个对 MAP-INTO 的调用之后，填充指针将被设置成被映射元素的数量。尽管如此，MAP-INTO 将不会扩展一个可调整大小的向量。

最后一个序列函数是 REDUCE，它可以做另一种类型的映射：映射在单个序列上，先将一个两参数函数应用到序列的最初两个元素上，再将函数返回的值和序列的后续元素继续用于该函数。这样，下面的表达式将对从 1 到 10 的整数求和：

REDUCE is a surprisingly useful function—whenever you need to distill a sequence down to a single value, chances are you can write it with REDUCE, and it will often be quite a concise way to express what you want. For instance, to find the maximum value in a sequence of numbers, you can write . REDUCE also takes a full complement of keyword arguments (:key, :from-end, :start, and :end) and one unique to REDUCE (:initial-value). The latter specifies a value that’s logically placed before the first element of the sequence (or after the last if you also specify a true argument).