Книга: Practical Common Lisp
Sorting and Merging
Sorting and Merging
STABLE-SORT provide two ways of sorting a sequence. They both take a sequence and a two-argument predicate and return a sorted version of the sequence.
(sort (vector "foo" "bar" "baz") #'string<) ==> #("bar" "baz" "foo")
The difference is that
STABLE-SORT is guaranteed to not reorder any elements considered equivalent by the predicate while
SORT guarantees only that the result is sorted and may reorder equivalent elements.
Both these functions are examples of what are called destructive functions. Destructive functions are allowed—typically for reasons of efficiency—to modify their arguments in more or less arbitrary ways. This has two implications: one, you should always do something with the return value of these functions (such as assign it to a variable or pass it to another function), and, two, unless you're done with the object you're passing to the destructive function, you should pass a copy instead. I'll say more about destructive functions in the next chapter.
Typically you won't care about the unsorted version of a sequence after you've sorted it, so it makes sense to allow
STABLE-SORT to destroy the sequence in the course of sorting it. But it does mean you need to remember to write the following:
(setf my-sequence (sort my-sequence #'string<))
rather than just this:
(sort my-sequence #'string<)
Both these functions also take a keyword argument,
:key, which, like the
:key argument in other sequence functions, should be a function and will be used to extract the values to be passed to the sorting predicate in the place of the actual elements. The extracted keys are used only to determine the ordering of elements; the sequence returned will contain the actual elements of the argument sequence.
MERGE function takes two sequences and a predicate and returns a sequence produced by merging the two sequences, according to the predicate. It's related to the two sorting functions in that if each sequence is already sorted by the same predicate, then the sequence returned by
MERGE will also be sorted. Like the sorting functions,
MERGE takes a
:key argument. Like
CONCATENATE, and for the same reason, the first argument to
MERGE must be a type descriptor specifying the type of sequence to produce.
(merge 'vector #(1 3 5) #(2 4 6) #'<) ==> #(1 2 3 4 5 6)
(merge 'list #(1 3 5) #(2 4 6) #'<) ==> (1 2 3 4 5 6)
- Разработка приложений баз данных InterBase на Borland Delphi
- Open Source Insight and Discussion
- Introduction to Microprocessors and Microcontrollers
- Chapter 6. Traversing of tables and chains
- Chapter 8. Saving and restoring large rule-sets
- Chapter 11. Iptables targets and jumps
- Chapter 5 Installing and Configuring VirtualCenter 2.0
- Chapter 16. Commercial products based on Linux, iptables and netfilter
- Appendix A. Detailed explanations of special commands
- Appendix B. Common problems and questions
- Appendix E. Other resources and links