I'm trying to use Haskell and I am new to this programming language. I was running this code which was intended to print Greater when the function had an integer greater than 50 and Less when the function was run with an integer less than 50.
printLessorGreater :: Int -> String
if a > 50
then return ("Greater")
else return ("Less")
main = do
print(printLessorGreater 10)
However, when I ran the code, it gave me this error:
main.hs:2:5: error: parse error on input ‘if’
I went to line 5 and there was nothing in the line. Does anyone know how to solve this error at this point? I would appreciate it!
your function clause has no "head". You need to specify the name of the function and with optional patterns:
printLessorGreater :: Int -> String
printLessorGreater a = if a > 50 then return ("Greater") else return ("Less")
but this will still not work. Thre return is not equivalent to the return statement in imperative languages. return :: Monad m => a -> m a injects a value in a monadic type. While a list is a monadic type, if you use the list monad, you can only use return with a Character in that case.
You thus should rewrite this to:
printLessorGreater :: Int -> String
printLessorGreater a = if a > 50 then "Greater" else "Less"
or with a guard:
printLessorGreater :: Int -> String
printLessorGreater a
| a > 50 = "Greater"
| otherwise = "Less"
You probably want something like this:
printLessorGreater :: Int -> String
printLessorGreater a = if a > 50
then "Greater"
else "Less"
Note that this does not actually print anything, but only returns a string.
Using an if is fine for this, but note that guards are also a common alternative.
printLessorGreater :: Int -> String
printLessorGreater a | a > 50 = "Greater"
| otherwise = "Less"
Related
type Googol = {
number : float
power : float
result : float
}
let generatePowers (n:float) : list<Googol> =
let rec powerInner (n:float) (p:float) (acc : list<Googol>) =
match n with
| p when p <= 1.0 -> acc
| p when p > 1.0 -> powerInner n (p-1.0) ([{ number=n; power=p; result=n**p}]#acc)
let rec numberInner (n:float) (acc : list<Googol>) =
match n with
| n when n <=1.0 -> acc
| n when n >1.0 -> numberInner (n-1.0) ((powerInner n [])#acc)
numberInner n []
ProjectEuler.fsx(311,50): error FS0001: This expression was expected to have type
'Googol list'
but here has type
'Googol list -> Googol list'
I am trying to solve this problem -> https://projecteuler.net/problem=56 | but for this I need to generate powers below n < 100. When I try to concatenate [{ number=n; power=p; result=n**p}]#acc
these lists I get the error above. Explain please why error says 'Googol list -> Googol list' is in the function, does I plug a function as a parameter to the function or I plug the actual list when just after concatenation. Is # a function?
This looks like homework or practice, so first I'll give some hints to move on. Finally I'll show a version that seems to work, and then tell how I would approach the problem.
The task is to find the number a ** b, for a and b less than 100, that has the highest sum of its own digits.
The first problem is that float won't give us all the digits of a ** b, so that type is useless to solve the problem. To fix that, we turn to the BigInteger type, and the BigInteger.Pow function. Then we get a 1 followed by 200 zeroes if we run the following snippet, just like it says in the problem description.
let x: bigint = BigInteger.Pow (100I, 100)
let x: string = string x
printfn "s=%s" x
To get useful results, change the Googol type so that it uses bigint, except for power that should be an int.
Why are the functions powerInner and numberInner inside the function generatePowers? This doesn't seem to have a specific purpose, so I suggest moving them out to make this clearer.
The function powerInner do a match on n, but then goes on to name the results p, which shadows the p parameter so that it is unused. Ok, the intention here is probably to match on p rather than n, so just fix that, and then the shadowing of the p parameter is perfectly fine.
The tests first on <= 1 and then on > 1 causes incomplete matches. If the first line checks that the number is less or equal to one, then it must the greater than one in the next line. So just use n -> without the when to fix that. I also suspect you want to test <= 0 instead of 1.
This
[{ number=n; power=p; result=n**p}]#acc
can be just
{ number=n; power=p; result=n**p } :: acc
and here
(powerInner n [])
I suspect you just need a starting value for the power, which would be 99
(powerInner n 99 [])
SPOILER WARNING
After a bit of tinkering, this is what I ended up with, and it seems to print out a useful list of numbers. Note that in order to not run through all 99 by 99 results with printouts, I've used low starting numbers 3 and 5 for the countdowns here, so we get some simple printout we can study for analysis.
type Googol = { number: bigint; power: int; result: bigint }
let rec powerInner (n: bigint) (p: int) (acc: Googol list) =
match p with
| p when p <= 0 -> acc
| p ->
let newNumber = { number = n; power = p; result = n ** p }
printfn "newNumber=%0A" newNumber
powerInner n (p - 1) (newNumber :: acc)
let rec numberInner (n: bigint) (acc: Googol list) =
match n with
| n when n <= 0I -> acc
| n -> numberInner (n - 1I) ((powerInner n 5 []) # acc)
let generatePowers (n: bigint) : Googol list =
numberInner n []
let powers = generatePowers 3I
I'm not sure if this solution is correct. I'd do it differently anyway.
I would simply loop through a and b in two loops, one inside the other. For each a ** b I would convert the result to a string, and then sum the digits of the string. Then I'd simply use a mutable to hold on to whichever result is the highest. The same could be achieved in a more functional way with one of those fancy List functions.
You're missing a parameter here:
| n when n >1.0 -> numberInner (n-1.0) ((powerInner n [])#acc)
^^^^^^^^^^^^^^^
here
powerInner is defined with three parameters, but you're only passing two.
In F# it is not technically illegal to pass fewer parameters than defined. If you do that, the result will be a function that "expects" the remaining parameters. For example:
let f : int -> int -> string
let x = f 42
// Here, x : int -> string
let y = x 5
// Here, y : string
So in your case omitting the last parameter makes the resulting type Googol list -> Googol list, which then turns out to be incompatible with the type Googol list expected by operator #. Which is what the compiler is telling you in the error message.
. Write a function that takes an integer list and return sum of all elements of the list. If the list is empty then return None.
This is my code now:
let rec sum (xs: int list) =
match xs with
| [] -> None
| [x] -> Some x
| hd::tl -> let m = (hd + (sum tl)) in
Some m
;;
The problem is that I can't seem to find a way to add up the last element without getting an error.
This is my error.
Error: This expression has type int but an expression was expected of type 'a option.
Your recursive call to sum does indeed return an int option. You know this because you're the author of the function, and you coded it up to return that type :-) You can either write a helper function that returns an int, or you can extract the int from the return value of sum, something like this:
let tlsum =
match sum tl with
| None -> (* figure this part out *)
| Some n -> (* figure this part out *)
You can define the addition of two int option.
let sum l =
let (+) a b =
match (a,b) with
| (None,x) | (x,None) -> x
| (Some x,Some y) -> Some (x+y)
in
let convert a = Some a in
let opt_l=List.map convert l in
List.fold_left (+) None opt_l
Test
# sum [];;
- : int option = None
# sum [1;2];;
- : int option = Some 3
That looks like an assignment so I'll be vague:
The easiest way to do that is probably to first define a function of type int list -> int that returns the "normal" sum (with 0 for the empty case). That function will be recursive and 0 will correspond to the base case.
Then write another function of type int list -> int option that checks whether its argument is empty or not and does the right thing based on that.
Trying to write the recursion directly probably is not a good idea since there are two cases when you will need to handle []: when it's the only element in the list, and when it's at the end of a nonempty list.
I have the following problem: given a max(max) apacity, and given a list of values(listOfValues) i need to return a list with values from the listOfValues. The sum of the elements must be <= max and i need to prioritize the higher values.
Example: typing solvingProblem 103 [15, 20, 5, 45, 34] i must get: [45, 45, 5, 5]
To solve the problem i create the following code:
solvingProblem max [] = 0
solvingProblem max listOfValues | max == 0 = 0
| otherwise = createList max listOfValues []
createList max [] result = -1
createList max listOfValues result | smaller listOfValues > max = -1
| higher listOfValues > max = createList max (remove (higher listOfValues) listOfValues) result
| otherwise = createList (max - higher listOfValues) listOfValues (insert (higher listOfValues) result)
higher [a] = a
higher (a:b:x) | a > b = higher (a:x)
| otherwise = higher (b:x)
smaller [a] = a
smaller (a:b:x) | a < b = smaller (a:x)
| otherwise = smaller (b:x)
remove x [] = []
remove x (h:t) | x == h = remove x t
| otherwise = h : remove x t
insert x (h:t) = x : h : t
In the two lines where i'll returning "-1" should be the parameter "result", but if i change "-1" to "result" the code don't load on ghci.
Can someone help me?
Thank you and sorry for my bad english.
If I may begin with a bit of a side note, some of your functions already exist in Haskell (now that I come to think of it you might have written them for an exercise, but just in case it wouldn't be the case, let's discuss that): your higher is maximum, your smaller is minimum and your insert is just (:), beacause like you write it yourself insert x list = x:list. Note that your version will fail if you give it the empty list because the pattern matching is non-exhaustive. Also you could write remove in terms of filter: remove x list = filter (== x) list.
Now why doesn't your code load properly? ghci tells you:
• Non type-variable argument in the constraint: Num [a]
(Use FlexibleContexts to permit this)
• When checking the inferred type
solvingProblem :: forall a.
(Ord a, Num [a], Num a) =>
a -> [a] -> [a]
Which I agree is pretty cryptic, but what it's saying is that the return type of solvingProblem is a list of a and for some reason it is also an instance of the Num type class. The reason why it says it's an instance of Num is because one of the return value of solvingProblem is 0 which is a number, which is a bit odd because it is also a list. Changing the 0 with [] makes the code compile and work (if you change insert with (:) otherwise you get the non-exhaustive pattern matching I was talking about earlier).
λ> solvingProblem 103 [15,20, 5, 45, 34]
[5,5,45,45]
it :: (Ord t, Num t) => [t]
The problem is with the last guard clause in createList.
The type you intended for createList seems to be:
createList :: Int -> [Int] -> Int -> Int
but if you look at the last guard clause you have:
| otherwise = createList (max - ...) listOfValues (insert ...)
^^^^^^^^^^^ ^^^^^^^^^^^^ ^^^^^^^^^^^^
Int [Int] [Int]
Even though GHC is very good at inferring types, always adding
type signatures to your code is a good way of catching these kinds
of errors early.
I have a function save that take standard input, which is used individually like this:
./try < input.txt (* save function is in try file *)
input.txt
2
3
10 29 23
22 14 9
and now i put the function into another file called path.ml which is a part of my interpreter. Now I have a problem in defining the type of Save function and this is because save function has type in_channel, but when i write
type term = Save of in_channel
ocamlc complain about the parameter in the command function.
How can i fix this error? This is the reason why in my last question posted on stackoverflow, I asked for the way to express a variable that accept any type. I understand the answers but actually it doesn't help much in make the code running.
This is my code:
(* Data types *)
open Printf
type term = Print_line_in_file of int*string
| Print of string
| Save of in_channel (* error here *)
;;
let input_line_opt ic =
try Some (input_line ic)
with End_of_file -> None
let nth_line n filename =
let ic = open_in filename in
let rec aux i =
match input_line_opt ic with
| Some line ->
if i = n then begin
close_in ic;
(line)
end else aux (succ i)
| None ->
close_in ic;
failwith "end of file reached"
in
aux 1
(* get all lines *)
let k = ref 1
let first = ref ""
let second = ref ""
let sequence = ref []
let append_item lst a = lst # [a]
let save () =
try
while true do
let line = input_line stdin in
if k = ref 1
then
begin
first := line;
incr k;
end else
if k = ref 2
then
begin
second := line;
incr k;
end else
begin
sequence := append_item !sequence line;
incr k;
end
done;
None
with
End_of_file -> None;;
let rec command term = match term with
| Print (n) -> print_endline n
| Print_line_in_file (n, f) -> print_endline (nth_line n f)
| Save () -> save ()
;;
EDIT
Error in code:
Save of in_channel:
Error: This pattern matches values of type unit
but a pattern was expected which matches values of type in_channel
Save of unit:
Error: This expression has type 'a option
but an expression was expected of type unit
There are many errors in this code, so it's hard to know where to start.
One problem is this: your save function has type unit -> 'a option. So it's not the same type as the other branches of your final match. The fix is straightforward: save should return (), not None. In OCaml these are completely different things.
The immediate problem seems to be that you have Save () in your match, but have declared Save as taking an input channel. Your current code doesn't have any way to pass the input channel to the save function, but if it did, you would want something more like this in your match:
| Save ch -> save ch
Errors like this suggest (to me) that you're not so familiar with OCaml's type system. It would probably save you a lot of trouble if you went through a tutorial of some kind before writing much more code. You can find tutorials at http://ocaml.org.
getChar :: Int -> IO Char
getChar n = do
c <- getLine
return (c !! n)
The program must needs a number and a line and it will return char, but how do I catch exception, if the number is too big?
I tried like this but it doesnt seem to work
getChar n
= do
c <-getLine
| n>=0 && n < b
= return c !! n
| otherwise
= error "Too big number"
where
b = length c
This is not a homework, im trying to involve myself. Google didint give me useful answers
Couldn't implement catch in there. Examples?
You probably want to restructure things a bit as you've got IO mixed up in something it doesn't have to be. What about changing the signature to something like this?
getChar :: Int -> String -> Maybe Char
getChar n x | n < length x = Just (x !! n)
| otherwise = Nothing
Data.Maybe allows you to indicate that you are either going to return something (e.g. the length is within range) or Nothing (the length isn't within range). The function that calls getChar can then decide what to do with things. Data.Either provides a way of returning an error message with an error instead. From what I've seen (and I'm by no means an expert) exceptions are rarely used in Haskell, and choice types such as Either or Maybe are much more commonly used.
Now in the code that calls this, you can use pattern matching to see what happened e.g.
main :: IO ()
main = do
x <- getLine
let z = getChar' 5 x
case z of
(Just z) -> print $ "The 5th character is " ++ show z
Nothing -> print $ "The 5th character is out of range"
You can use the drop function to drop the first n characters of the line (drop will just give an empty result if there are fewer than n chars), and the listToMaybe function to turn a list to a Maybe (either Just c where c is the first element of the list, or Nothing if the list is empty):
import Data.Maybe (listToMaybe)
getchar :: Int -> IO (Maybe Char)
getchar n = do
line <- getLine
return . listToMaybe . drop n $ line
getChar' :: Int -> IO Char
getChar' n =
do
c <- getLine
if (n < length c)
then
return (c !! n)
else
getChar' n
You can do something like above. This is just an example though. But, since you are a beginner, it is strongly recommended not to play with IO and Monads. You can come to it after you get familiarized with pure functional concepts.