I recently started reading Paul Grahams 'On Lisp', and learning learning clojure along with it, so there's probably some really obvious error in here, but I can't see it: (its a project euler problem, obviously)
(ns net.projecteuler.problem31)
(def paths (ref #{}))
; apply fun to all elements of coll for which pred-fun returns true
(defn apply-if [pred-fun fun coll]
(apply fun (filter pred-fun coll)))
(defn make-combination-counter [coin-values]
(fn recurse
([sum] (recurse sum 0 '()))
([max-sum current-sum coin-path]
(if (= max-sum current-sum)
; if we've recursed to the bottom, add current path to paths
(dosync (ref-set paths (conj #paths (sort coin-path))))
; else go on recursing
(apply-if (fn [x] (<= (+ current-sum x) max-sum))
(fn [x] (recurse max-sum (+ x current-sum) (cons x coin-path)))
coin-values)))))
(def count-currency-combinations (make-combination-counter '(1 2 5 10 20 50 100 200)))
(count-currency-combinations 200)
When I run the last line in the REPL, i get the error:
<#CompilerException java.lang.IllegalArgumentException: Wrong number of args passed to: problem31$eval--25$make-combination-counter--27$recurse--29$fn (NO_SOURCE_FILE:0)>
Apart from the question where the error is, the more interesting question would be: How would one debug this? The error message isn't very helpful, and I haven't found a good way to single-step clojure code, and I can't really ask on stack overflow every time I have a problem.
Three tips that might make your life easier here:
Wrong number of args passed to: problem31$eval--25$make-combination-counter--27$recurse--29$fn (NO_SOURCE_FILE:0)>
Tells you roughly where the error occurred: $fn at the end there means anonymous function and it tells you it was declared inside recurse, which was declared inside make-combination-counter. There are two anonymous functions to choose from.
If you save your source-code in a file and execute it as a script it will give you a full stack trace with the line numbers in the file.
at net.projecteuler.problem31$apply_if__9.invoke(problem31.clj:7)
Note you can also examine the last exception and stack trace from within the REPL by examining *e eg: (.stackTrace *e) The stack trace is at first quite daunting because it throws up all the Java internals. You need to learn to ignore those and just look for the lines that refer to your code. This is pretty easy in your case as they all start with net.projecteuler
You can name your anonymous functions to help more quickly identify them:
(fn check-max [x] (<= (+ current-sum x) max-sum))
In your case using all this info you can see that apply-if is being passed a single argument function as fun. Apply does this (f [1 2 3]) -> (f 1 2 3). From your comment what you want is map. (map f [1 2 3]) -> (list (f 1) (f 2) (f 3)). When I replace apply with map the program seems to work.
Finally, if you want to examine values you might want to look into clojure-contrib.logging which has some helpers to this effect. There is a spy macro which allows you to wrap an expression, it will return exactly the same expression so it does not affect the result of your function but will print out EXPR = VALUE, which can be handy. Also on the group various people have posted full tracing solutions. And there is always the trusty println. But the key skill here is being able to identify precisely what blew up. Once you know that it is usually clear why, but sometimes printouts are needed when you can't tell what the inputs are.
dont have a REPL on me though it looks like:
(defn apply-if [pred-fun fun coll]
(apply fun (filter pred-fun coll)))
takes a list like '(1 2 3 4 5) filters some of them out '(1 3 5)
and then creates a function call like (fun 1 3 5)
and it looks like it is being called (apply-if (fn [x] with a function that wants to receive a list of numbers as a single argument.
you could change the apply-if function to just pass call to the fun (with out the apply) or you could change the call to it to take a function that takes an arbitrary number of arguments.
Related
So, this is what I want to do
(def body `(prn sth))
(defn f [sth] body)
(f "hello")
; can it prn hello here?
Is this possible?
If you want to "take a data structure and embed it in code to be executed", then you can do something like this.
You will want to tweak the body to embed like this:
(def body `(prn ~'sth))
that is, prefixing the sth local variable with ~' so that it will not be namespaced. Then you need a macro that will embed the code for you:
(defmacro insert-body [body]
(eval body))
Using this macro inside the f function to embed the body and putting things together, you get this code:
(defmacro insert-body [body]
(eval body))
(def body `(prn ~'sth))
(defn f [sth] (insert-body body))
You can now call f with an argument and it will work as expected:
> (f "hello")
"hello"
nil
The function macroexpand comes in handy to test that the macro does what it is supposed to be doing:
(macroexpand `(insert-body body))
;; => (clojure.core/prn sth)
But it is not clear to me what you are trying to accomplish or what you would gain from writing your code this way. Whatever you want to accomplish eventually, there is most likely a better way to accomplish it than what I suggest here. I am just providing a specific answer to your question, nothing more, nothing less. So if you clarify your question and give more details, it will also be possible to provide a better answer that addresses you actual problem.
I'm currently using Leiningen to learn Clojure and I'm confused about the requirement of doall for running this function:
;; take a function (for my purposes, println for output formatting) and repeat it n times
(defn repeat [f n] (for [i (range n)] (f)))
(repeat println 2) works just fine in an REPL session, but not when running with lein run unless I include the doall wrapper. (doall (repeat println 2)) works and I'm curious why. Without it, lein run doesn't show the two blank lines in the output.
I also have:
(defmacro newPrint1 [& args] `'(println ~args))
(defmacro newPrint2 [& args] `'(println ~#args))
The first function I thought of myself. The next two macros are examples from a tutorial video I'm following on Udemy. Even if I wrap the macro calls with doall, such as (doall (newPrint1 1 2 3)), lein run produces no output, but (newPrint1 1 2 3) in a terminal REPL session produces the desired output of (clojure.core/println (1 2 3)) as it does in the video tutorial. Why doesn't doall work here?
for creates a lazy sequence. This lazy sequence is returned. The P in REPL (read eval Print loop) prints the sequence, thus realizing it. For realizing, the code to produce each element is run.
If you do not use the sequence, it is not realized, so the code is never run. In non-interactive use, this is likely the case. As noted, doall forces realization.
If you want to do side-effects, doseq is often better suited than for.
Here's some code I wrote, using clojure.core.match, which performs a pretty common programmng task. A function takes some "commands" (or "objects", "records", or whatever you prefer to call them), has to do something different with each type, and has to destructure them to figure out exactly what to do, and different command types might have to be destructured differently:
(defn action->edits [g action]
"Returns vector of edits needed to perform action in graph g."
(match action
[:boost from to]
[[:add-edge from to 1.0]]
[:retract from to]
[[:remove-edge from to]]
[:normalize from to] ; a change has just been made to from->to
(map (fn [that] [:remove-edge from that])
(successors-except g from to))
[:recip-normalize to from] ; a change has just been made to from->to
[]
[:reduce-to-unofficial from to competitor]
[[:remove-edge from to] (make-competitive-edge from competitor]))
I'm mostly imitating the way people commonly use the pmatch macro in Scheme. I'd like to know what's the idiomatic way to do this in Clojure.
Here's what I like about the above code:
It's very readable.
It was effortless to write.
Here's what I don't like:
Accessing the from and to fields from anywhere but inside a match macro is extremely unreadable and error-prone. For example, to extract the from element from most of the action vectors, you write (action 1). That code will break if I ever add a new action, and it breaks right now on :recip-normalize.
The code generated by match is inefficient: it searches by repeatedly throwing and catching exceptions. It doesn't just generate a big nested if.
I experimented a little with representing the commands as maps, but it seemed to get verbose, and the name of the command doesn't stand out well, greatly reducing readability:
(match action
{:action :boost :from from :to to}
[{:edit :add-edge :from from :to to :weight 1.0}]
{:action :retract :from from :to to}
[{:edit :remove-edge :from from :to to}]
. . .)
Probably future versions of match will generate better code, but the poor code generated now (and lack of support for records) suggests that in Clojure, people have been handling this kind of thing happily for years without match. So how do you do this kind of thing in Clojure?
I would utilize clojure's build-in destructuring facilities, since I do not see a requirement for core.match here - but I might be missing something.
For example:
(defn action->edits [g [action from to]]
(condp = action
:boost "boosting"
:retract "retracting"
:normalize-ksp-style (recur g [:boost from to])
nil))
(action->edits 2 [:normalize-ksp-style 1 2])
;=> "boosting"
I'm trying to write a procedure Huffman-leaves; the procedure returns a list of pairs from a created huffman tree.
Example on how it runs
(huffman-leaves sample-tree)
->((A . 8) (C . 5) (B . 1) (D . 1))
What I've comed up with but got writers block...
(define (huffman-leaves tree)
(define (huffman-get-pairs current-branch pairs)
(if (or (null? tree) (null? current-branch))
pairs
(let ((next-branch
(get-branch (car current-branch) current-branch)))
(not (member? next-branch pairs)
(if (leaf? next-branch)
(cons (append pairs next-branch)
(display pairs)
(huffman-get-pairs (cadr current-branch) pairs))
(huffman-get-pairs next-branch pairs))))))
(huffman-get-pairs tree '()))
(member? item 'list) #if item in list return #t else #false
I know that I'm doing something wrong but I can't see it.
How can I stop a search in a huffman-tree in scheme? Any tip that I should be doing different?
I recommend:
Write a data definition for Huffman Tree
Make example input huffman trees, encoded according to your data definition from step 1, and expected outputs (lists of leaves, in this case).
Follow the design recipe to build a basic template for the huffman-leaves function.
Fill in the ... in your template according to the examples you built from step 2.
Translate your examples from step 2. into tests, and test your code from step 4.
Sorry if the above seems vague, but it is the best advice I can give with the level of detail (or lack thereof) you have supplied in this question thus far. If you can provide answers for the steps above, and say explicitly which step you are blocked on, then we can help you get over your writers block in a more systematic way.
If you prefer real code, here is one direction you could go in to make a very generic solution for your problem:
;; make-visitor : (X -> Y) (X -> [Listof X]) (Y [Listof Z] -> Z) -> Z
;; Very generic descend+map+recombine routine
;; (note X, Y, Z are implicitly universally quantified above).
(define (make-visitor transform children combine)
(lambda (x)
(let rec ((x x)) ;; rec : X -> Z
(combine (transform x)
(map rec (children x))))))
;; ... the hard bit is coming up with the appropriate lambda
;; expressions for `transform`, `children`, and `combine` above.
(define leaves
(make-visitor (lambda (x) ...)
(lambda (x) ...)
(lambda (y zs) ...)))
I don't actually recommend trying to jump directly to a solution of this form; you will be better off if you try to follow the design recipe and make a direct solution to your problem. But once you have done that, it can be an educational exercise to see if you can retrofit your own solution onto the generic routine above.
I am parsing a big csv file and I am using the first line of it as the keys for the records. So for a csv file like:
header1,header2
foo,bar
zoo,zip
I end up with a lazy seq like:
({:header1 "foo" :header2 "bar"},
{:header1 "zoo" :header2 "zip"})
The code working fine, but I am not sure if in the following function I am holding the head of "lines" or not.
(defn csv-as-seq [file]
(let [rdr (clojure.java.io/reader file)]
(let [lines (line-seq rdr)
headers (parse-headers (first lines))]
(map (row-mapper headers) (rest lines)))))
Can somebody please clarify?
Yes, this expression syntactically says to hold the head
(let [lines (line-seq rdr)
though in this case you should get away with it because their are no references to
lines and headers after the call to map and the Clojure compiler starting with 1.2.x includes a feature called locals clearing: it sets any locals not used after a function call to nil in the preamble to the function call. In this case it will set lines and headers to nil in the local context of the function and they will be GCd as used. This is one of the rare cases where clojure produces bytecode that cannot be expressed in java.