Generating a bitvector in scheme - vector

I am trying to implement a spellchecker that takes a hash function and a dictionary, and then map the hash values of the words to a bitvector. More specifically, I am trying to write a function called gen-checker that takes as input a list of hash functions and a dictionary of words and returns a spellchecker. The spellchecker must generate a bitvector representation for the input of the dictionary, which contains #t or #f indicating the correct or incorrect spelling of the word.
I have already defined the has functions and have a dictionary to use, but I can't seem to get the bit vector setup
I have tried implementing (make-bitvector 8 #f) found here:
http://www.gnu.org/software/guile/manual/html_node/Bit-Vectors.html
But for some reason drracket does not recognize it. What am I doing wrong? How to implement the bitvector representation?

It may seem like this answer is joking, but it is not:
(define make-bitvector make-vector)
(define bitvector-ref vector-ref)
;; ...
After everything is working, and only then, would one need to optimize storage by bit packing.

Related

What are the typical use-cases of (defun (setf …)) defsetf and define-setf-expander

When developing with Common Lisp, we have three possibilities to define new setf-forms:
We can define a function whose name is a list of two symbols, the first one being setf, e.g. (defun (setf some-observable) (…)).
We can use the short form of defsetf.
We can use the long form of defsetf.
We can use define-setf-expander.
I am not sure what is the right or intended use-case for each of these possibilities.
A response to this question could hint at the most generic solution and outline contexts where other solutions are superior.
define-setf-expander is the most general of these. All of setf's functionality is encompassed by it.
Defining a setf function works fine for most accessors. It is also valid to use a generic function, so polymorphism is insufficient to require using something else. Controlling evaluation either for correctness or performance is the main reason to not use a setf function.
For correctness, many forms of destructuring are not possible to do with a setf function (e.g. (setf (values ...) ...)). Similarly I've seen an example that makes functional data structures behave locally like a mutable one by changing (setf (dict-get key some-dict) 2) to assign a new dictionary to some-dict.
For performance, consider the silly case of (incf (nth 10000 list)) which if the nth writer were implemented as a function would require traversing 10k list nodes twice, but in a setf expander can be done with a single traversal.

String Comparison in Common Lisp

I am new to Common Lisp and Functional programming in general. I have a function lets call it "wordToNumber", I want it to check if the input string is "one" "two" "three".. etc (0-9) only. and I want to return 1 2 3 etc. so (wordToNumber "one") should output the number 1. I'm having some trouble with string comparison, tried using eq and eql, but its not working, from what I read it is comparing memory location not actual string. Is there an easier way to go about this or is there someway to compare strings. I need any examples to be purely functional programming, no loops and stuff. This is a small portion of a project I'm working on for school.
Oh, for string comparison im just using a simple function at the moment like this:
(defun wordToNumber(x)
(if(eq 'x "one")(return-from wordToNumber 1)))
and calling it with this : (wordToNumber "one")
keep getting Nil returned
Thanks for any help
The functions to compare strings are string= and string-equal, depending on whether you want the comparison to be case-sensitive.
And when you want to compare the value of a variable, you mustn't quote it, since the purpose of quoting is to prevent evaluation.
(defun word-to-number (x)
(cond ((string-equal x "one") 1)
((string-equal x "two") 2)
((string-equal x "three") 3)
...
))
As a practical matter, before you make a ten-branch conditional, consider this: you can pass string= and string-equal (and any other binary function) as an :test argument to most of the sequence functions. Look though the sequence functions and see if there's something that seems relevant to this problem. http://l1sp.org/cl/17.3 (There totally is!)
One nice thing about Lisp is the apropos function. Lisp is a big language and usually has what you want, and (apropos "string") would prolly have worked for you. I recommend also the Lisp Hyperpec: http://www.lispworks.com/documentation/HyperSpec/Front/
eq is good for symbols, CLOS objects, and even cons cells but be careful: (eq (list 1) (list 1)) is false because each list form returns a different cons pointing to the same number.
eql is fine for numbers and characters and anything eq can handle. One nice thing is that (eql x 42) works even if x is not a number, in which case (= x 42) would not go well.
You need equal for lists and arrays, and strings are arrays so you could use that. Then there is equalp, which I will leave as an exercise.

Recursively run through a vector in Clojure

I'm just starting to play with Clojure.
How do I run through a vector of items?
My naive recursive function would have a form like the classic map eg.
(defn map [f xs] (
(if (= xs [])
[]
(cons (f (first xs)) (map f (rest xs))
)
))
The thing is I can't find any examples of this kind of code on the web. I find a lot of examples using built-in sequence traversing functions like for, map and loop. But no-one doing the raw recursive version.
Is that because you SHOULDN'T do this kind of thing in Clojure? (eg. because it uses lower-level Java primitives that don't have tail-call optimisation or something?)?
When you say "run through a vector" this is quite vague; as Clojure is a lisp and thus specializes in sequence analysis and manipulation, the beauty of using this language is that you don't think in terms "run through a vector and then do something with each element," instead you'd more idiomatically say "pull this out of a vector" or "transform this vector into X" or "I want this vector to give me X".
It is because of this type of perspective in lisp languages that you will see so many examples and production code that doesn't just loop/recur through a vector but rather specifically goes after what is wanted in a short, idiomatic way. Using simple functions like reduce map filter for into and others allow you to elegantly move over a sequence such as a vector while simultaneously doing what you want with the contents. In most other languages, this would be at least 2 different parts: the loop, and then the actual logic to do what you want.
You'll often find that if you think about sequences using the more imperative idea you get with languages like C, C++, Java, etc, that your code is about 4x longer (at least) than it would otherwise be if you first thought about your plan in a more functional approach.
Clojure re-uses stack frames only with tail-recurstion and only when you use the explicit recur call. Everything else will be stack consuming. The above map example is not tail recursive because the cons happens after the recursive call so it can't be TCO'd in any language. If you switch it to use the continuation passing style and use an explicit call to recur instead of map then you should be good to go.

Map to accept singular or collection

Is there a better way to do this in Clojure?
(if (coll? coll)
(map my-fn coll)
(my-fn coll)
my-fn is to be applied to input coll. coll can be either singular or a collection.
If I don't check for coll?, using map alone would throw an IllegalArgumentException for don't know how to create an ISeq from xxx.
Your code is fine (although I'd rename the variable coll since you don't actually know if it is a collection and this might confuse readers).
However I'd suggest this whole chunk of code looks suspiciously like a code smell - it's taking dynamic typing a bit too far / trying to be a bit too clever in my opinion: in the sense of "cleverness considered harmful".
Alternative ideas to consider:
If you actually want to treat everything like a collection, then wrap singular input values when they are first obtained in a list/vector of length 1. Then the rest of your code can safely assume collections throughout.
Write separate functions to deal with collections and single values. The rationale is that they are conceptually different data types, so deserve different treatment.
If coll doesn't contain any nested sequences:
(map my-fn (flatten (list coll)))
No general solution can exist, because my-fn may be a function that takes lists and returns lists. Then you can't somehow inspect the input and decide whether to map over it or not.
Better is to not get yourself into the scenario where you don't know what type of data you have, but I can't give any specific advice on this without knowing more about your program.

Mapping over sequence with a constant

If I need to provide a constant value to a function which I am mapping to the items of a sequence, is there a better way than what I'm doing at present:
(map my-function my-sequence (cycle [my-constant-value]))
where my-constant-value is a constant in the sense that it's going to be the same for the mappings over my-sequence, although it may be itself a result of some function further out. I get the feeling that later I'll look at what I'm asking here and think it's a silly question because if I structured my code differently it wouldn't be a problem, but well there it is!
In your case I would use an anonymous function:
(map #(my-function % my-constant-value) my-sequence)
Using a partially applied function is another option, but it doesn't make much sense in this particular scenario:
(map (partial my-function my-constant-value) my-sequence)
You would (maybe?) need to redefine my-function to take the constant value as the first argument, and you don't have any need to accept a variable number of arguments so using partial doesn't buy you anything.
I'd tend to use partial or an anonymous function as dbyrne suggests, but another tool to be aware of is repeat, which returns an infinite sequence of whatever value you want:
(map + (range 4) (repeat 10))
=> (10 11 12 13)
Yet another way that I find sometimes more readable than map is the for list comprehension macro:
(for [x my-sequence]
(my-function x my-constant-value))
yep :) a little gem from the "other useful functions" section of the api constantly
(map my-function my-sequence (constantly my-constant-value))
the pattern of (map compines-data something-new a-constant) is rather common in idomatic clojure. its relativly fast also with chunked sequences and such.
EDIT: this answer is wrong, but constantly and the rest of the "other useful functions" api are so cool i would like to leave the reference here to them anyway.

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