Книга: Practical Common Lisp

Subtypes of Vector

Subtypes of Vector

All the vectors you've dealt with so far have been general vectors that can hold any type of object. It's also possible to create specialized vectors that are restricted to holding certain types of elements. One reason to use specialized vectors is they may be stored more compactly and can provide slightly faster access to their elements than general vectors. However, for the moment let's focus on a couple kinds of specialized vectors that are important data types in their own right.

One of these you've seen already—strings are vectors specialized to hold characters. Strings are important enough to get their own read/print syntax (double quotes) and the set of string-specific functions I discussed in the previous chapter. But because they're also vectors, all the functions I'll discuss in the next few sections that take vector arguments can also be used with strings. These functions will fill out the string library with functions for things such as searching a string for a substring, finding occurrences of a character within a string, and more.

Literal strings, such as "foo", are like literal vectors written with the #() syntax—their size is fixed, and they must not be modified. However, you can use MAKE-ARRAY to make resizable strings by adding another keyword argument, :element-type. This argument takes a type descriptor. I won't discuss all the possible type descriptors you can use here; for now it's enough to know you can create a string by passing the symbol CHARACTER as the :element-type argument. Note that you need to quote the symbol to prevent it from being treated as a variable name. For example, to make an initially empty but resizable string, you can write this:

(make-array 5 :fill-pointer 0 :adjustable t :element-type 'character) ""

Bit vectors—vectors whose elements are all zeros or ones—also get some special treatment. They have a special read/print syntax that looks like #*00001111 and a fairly large library of functions, which I won't discuss, for performing bit-twiddling operations such as "anding" together two bit arrays. The type descriptor to pass as the :element-type to create a bit vector is the symbol BIT.

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