I'm teaching myself OCaml, and the main resources I'm using for practice are some problem sets Cornell has made available from their 3110 class. One of the problems is to write a function to reverse an int (i.e: 1234 -> 4321, -1234 -> -4321, 2 -> 2, -10 -> -1 etc).
I have a working solution, but I'm concerned that it isn't exactly idiomatic OCaml:
let rev_int (i : int) : int =
let rec power cnt value =
if value / 10 = 0 then cnt
else power (10 * cnt) (value/10) in
let rec aux pow temp value =
if value <> 0 then aux (pow/10) (temp + (value mod 10 * pow)) (value / 10)
else temp in
aux (power 1 i) 0 i
It works properly in all cases as far as I can tell, but it just seems seriously "un-OCaml" to me, particularly because I'm running through the length of the int twice with two inner-functions. So I'm just wondering whether there's a more "OCaml" way to do this.
I would say, that the following is idiomatic enough.
(* [rev x] returns such value [y] that its decimal representation
is a reverse of decimal representation of [x], e.g.,
[rev 12345 = 54321] *)
let rev n =
let rec loop acc n =
if n = 0 then acc
else loop (acc * 10 + n mod 10) (n / 10) in
loop 0 n
But as Jeffrey said in a comment, your solution is quite idiomatic, although not the nicest one.
Btw, my own style, would be to write like this:
let rev n =
let rec loop acc = function
| 0 -> acc
| n -> loop (acc * 10 + n mod 10) (n / 10) in
loop 0 n
As I prefer pattern matching to if/then/else. But this is a matter of mine personal taste.
I can propose you some way of doing it:
let decompose_int i =
let r = i / 10 in
i - (r * 10) , r
This function allows me to decompose the integer as if I had a list.
For instance 1234 is decomposed into 4 and 123.
Then we reverse it.
let rec rev_int i = match decompose_int i with
| x , 0 -> 10 , x
| h , t ->
let (m,r) = rev_int t in
(10 * m, h * m + r)
The idea here is to return 10, 100, 1000... and so on to know where to place the last digit.
What I wanted to do here is to treat them as I would treat lists, decompose_int being a List.hd and List.tl equivalent.
Related
needing some help (if possible) in how to count the amount of times a recursive function executes itself.
I don't know how to make some sort of counter in OCaml.
Thanks!
Let's consider a very simple recursive function (not Schroder as I don't want to do homework for you) to calculate Fibonacci numbers.
let rec fib n =
match n with
| 0 | 1 -> 1
| _ when n > 0 -> fib (n - 2) + fib (n - 1)
| _ -> raise (Invalid_argument "Negative values not supported")
Now, if we want to know how many times it's been passed in, we can have it take a call number and return a tuple with that call number updated.
To get each updated call count and pass it along, we explicitly call fib in let bindings. Each time c shadows its previous binding, as we don't need that information.
let rec fib n c =
match n with
| 0 | 1 -> (1, c + 1)
| _ when n > 0 ->
let n', c = fib (n - 1) (c + 1) in
let n'', c = fib (n - 2) (c + 1) in
(n' + n'', c)
| _ -> raise (Invalid_argument "Negative values not supported")
And we can shadow that to not have to explicitly pass 0 on the first call.
let fib n = fib n 0
Now:
utop # fib 5;;
- : int * int = (8, 22)
The same pattern can be applied to the Schroder function you're trying to write.
You can create a reference in any higher scope like so
let counter = ref 0 in
let rec f ... =
counter := !counter + 1;
... (* Function body *)
If the higher scope happens to be the module scope (or file top-level scope) you should omit the in
You can return a tuple (x,y) where y you increment by one for each recursive call. It can be useful if your doing for example a Schroder sequence ;)
I am trying to solve the coin change problem with tail recursion. The recursive solutions I come across are usually something like this
let rec combinations (amount:int) (coins:list<int>) =
if amount = 0 then
1
elif coins.IsEmpty || amount < 0 then
0
else
combinations (amount - coins.Head) coins + combinations amount coins.Tail
clearly inefficient and non tail recursive. I tried to make the solution tail recursive myself:
let combinationsTail (amount:int) (coins:list<int>) : int =
let rec go (amount:int) (sum:int) (coins:list<int>) =
match amount,sum ,coins with
| _,_, [] -> 0
| n,s,_ when n = 0 -> s
| n,_,cs when n < 0 || cs.IsEmpty -> 0
| n,s,h::t -> go (n - h) (n + s) t
go amount 0 coins
But it doesn't work. Does anyone know how to implement a tail recursive solution to this problem? is it even possible?
For achieving tail-recursiveness, you probably want to look into continuation-passing style. Here's an example applied to the Fibonacci sequence, which you could translate verbatim to the coins change problem, since both problems are dealing with aggregation of a recursive tree structure.
It's not the last word on efficiency.
let cc amount coins =
let rec aux k = function
| amount, _ when amount = 0 -> k 1
| amount, _ when amount < 0 -> k 0
| _, [] -> k 0
| amount, hd::tl ->
let k' x =
let k'' y = k (x + y)
aux k'' (amount - hd, hd::tl)
aux k' (amount, tl)
aux id (amount, coins)
I'm working on an implementation of prime decomposition in OCaml. I am not a functional programmer; Below is my code. The prime decomposition happens recursively in the prime_part function. primes is the list of primes from 0 to num. The goal here being that I could type prime_part into the OCaml interpreter and have it spit out when n = 20, k = 1.
2 + 3 + 7
5 + 7
I adapted is_prime and all_primes from an OCaml tutorial. all_primes will need to be called to generate a list of primes up to b prior to prime_part being called.
(* adapted from http://www.ocaml.org/learn/tutorials/99problems.html *)
let is_prime n =
let n = abs n in
let rec is_not_divisor d =
d * d > n || (n mod d <> 0 && is_not_divisor (d+1)) in
n <> 1 && is_not_divisor 2;;
let rec all_primes a b =
if a > b then [] else
let rest = all_primes (a + 1) b in
if is_prime a then a :: rest else rest;;
let f elem =
Printf.printf "%d + " elem
let rec prime_part n k lst primes =
let h elem =
if elem > k then
append_item lst elem;
prime_part (n-elem) elem lst primes in
if n == 0 then begin
List.iter f lst;
Printf.printf "\n";
()
end
else
if n <= k then
()
else
List.iter h primes;
();;
let main num =
prime_part num 1 [] (all_primes 2 num)
I'm largely confused with the reclusive nature with the for loop. I see that List.ittr is the OCaml way, but I lose access to my variables if I define another function for List.ittr. I need access to those variables to recursively call prime_part. What is a better way of doing this?
I can articulate in Ruby what I'm trying to accomplish with OCaml. n = any number, k = 1, lst = [], primes = a list of prime number 0 to n
def prime_part_constructive(n, k, lst, primes)
if n == 0
print(lst.join(' + '))
puts()
end
if n <= k
return
end
primes.each{ |i|
next if i <= k
prime_part_constructive(n - i, i, lst+[i], primes)
}
end
Here are a few comments on your code.
You can define nested functions in OCaml. Nested functions have access to all previously defined names. So you can use List.iter without losing access to your local variables.
I don't see any reason that your function prime_part_constructive returns an integer value. It would be more idiomatic in OCaml for it to return the value (), known as "unit". This is the value returned by functions that are called for their side effects (such as printing values).
The notation a.(i) is for accessing arrays, not lists. Lists and arrays are not the same in OCaml. If you replace your for with List.iter you won't have to worry about this.
To concatenate two lists, use the # operator. The notation lst.concat doesn't make sense in OCaml.
Update
Here's how it looks to have a nested function. This made up function takes a number n and a list of ints, then writes out the value of each element of the list multiplied by n.
let write_mults n lst =
let write1 m = Printf.printf " %d" (m * n) in
List.iter write1 lst
The write1 function is a nested function. Note that it has access to the value of n.
Update 2
Here's what I got when I wrote up the function:
let prime_part n primes =
let rec go residue k lst accum =
if residue < 0 then
accum
else if residue = 0 then
lst :: accum
else
let f a p =
if p <= k then a
else go (residue - p) p (p :: lst) a
in
List.fold_left f accum primes
in
go n 1 [] []
It works for your example:
val prime_part : int -> int list -> int list list = <fun>
# prime_part 12 [2;3;5;7;11];;
- : int list list = [[7; 5]; [7; 3; 2]]
Note that this function returns the list of partitions. This is much more useful (and functional) than writing them out (IMHO).
I'm doing some homework but I've been stuck for hours on something.
I'm sure it's really trivial but I still can't wrap my head around it after digging through the all documentation available.
Can anybody give me a hand?
Basically, the exercise in OCaml programming asks to define the function x^n with the exponentiation by squaring algorithm.
I've looked at the solution:
let rec exp x = function
0 -> 1
| n when n mod 2 = 0 -> let y = exp x (n/2) in y*y
| n when n mod 2 <> 0 -> let y = exp x ((n-1)/2) in y*y*x
;;
What I don't understand in particular is how the parameter n can be omitted from the fun statement and why should it be used as a variable for a match with x, which has no apparent link with the definition of exponentiation by squaring.
Here's how I would do it:
let rec exp x n = match n with
0 -> 1
| n when (n mod 2) = 1 -> (exp x ((n-1)/2)) * (exp x ((n-1)/2)) * x
| n when (n mod 2) = 0 -> (exp x (n/2)) * (exp x (n/2))
;;
Your version is syntaxically correct, yields a good answer, but is long to execute.
In your code, exp is called recursively twice, thus yielding twice as much computation, each call yielding itself twice as much computation, etc. down to n=0. In the solution, exp is called only once, the result is storred in the variable y, then y is squared.
Now, about the syntax,
let f n = match n with
| 0 -> 0
| foo -> foo-1
is equivalent to:
let f = function
| 0 -> 0
| foo -> foo-1
The line let rec exp x = function is the begging of a function that takes two arguments: x, and an unnammed argument used in the pattern matching. In the pattern matching, the line
| n when n mod 2 = 0 ->
names this argument n. Not that a different name could be used in each case of the pattern matching (even if that would be less clear):
| n when n mod 2 = 0 -> let y = exp x (n/2) in y*y
| p when p mod 2 <> 0 -> let y = exp x ((p-1)/2) in y*y*x
The keyword "function" is not a syntaxic sugar for
match x with
but for
fun x -> match x with
thus
let rec exp x = function
could be replaced by
let rec exp x = fun y -> match y with
which is of course equivalent with your solution
let rec exp x y = match y with
Note that i wrote "y" and not "n" to avoid confusion. The n variable introduced after the match is a new variable, which is only related to the function parameter because it match it. For instance, instead of
let y = x in ...
you could write :
match x with y -> ...
In this match expression, the "y" expression is the "pattern" matched. And like any pattern, it binds its variables (here y) with the value matched. (here the value of x) And like any pattern, the variables in the pattern are new variables, which may shadow previously defined variables. In your code :
let rec exp x n = match n with
0 -> 1
| n when (n mod 2) = 1 -> (exp x ((n-1)/2)) * (exp x ((n-1)/2)) * x
| n when (n mod 2) = 0 -> (exp x (n/2)) * (exp x (n/2))
;;
the variable n in the two cases shadow the parameter n. This isn't a problem, though, since the two variable with the same name have the same value.
I'm trying to write a function that accepts an int n and returns a list that runs down from n to 0.
This is what I have
let rec downFrom n =
let m = n+1 in
if m = 0 then
[]
else
(m-1) :: downFrom (m - 1);;
The function compiles ok but when I test it with any int it gives me the error
Stack overflow during evaluation (looping recursion?).
I know it's the local varible that gets in the way but I don't know another way to declare it. Thank you!!!
First, the real thing wrong with your program is that you have an infinite loop. Why, because your inductive base case is 0, but you always stay at n! This is because you recurse on m - 1 which is really n + 1 - 1
I'm surprised as to why this compiles, because it doesn't include the rec keyword, which is necessary on recursive functions. To avoid stack overflows in OCaml, you generally switch to a tail recursive style, such as follows:
let downFrom n =
let rec h n acc =
if n = 0 then List.rev acc else h (n-1) (n::acc)
in
h n []
Someone suggested the following edit:
let downFrom n =
let rec h m acc =
if m > n then acc else h (m + 1) (m::acc)
in
h 0 [];
This saves a call to List.rev, I agree.
The key with recursion is that the recursive call has to be a smaller version of the problem. Your recursive call doesn't create a smaller version of the problem. It just repeats the same problem.
You can try with a filtering parameter
syntax:
let f = function
p1 -> expr1
| p2 -> expr2
| p3 -> ...;;
let rec n_to_one =function
0->[]
|n->n::n_to_one (n-1);;
# n_to_one 3;;
- : int list = [3; 2; 1]