I wanted to Print to A pascal Triangle for a given Length.
main = do
l_str <- getLine
let l_int = read $ l_str :: Int
let why = print_row l_int 0
print why
return ()
print_row x y
| (x < y) = " "
| otherwise = (print_column y 0 ) ++ "\n" ++ print_row x (y+1)
print_column y r
| (y < r) = ""
| otherwise = (show $ fact y r ) ++ print_column y (r+1)
fact n r
| (n >= r) = truncate $ (fact' n)/((fact' (n-r))*(fact' r))
fact' n
| (n >= 0) = product [1..n]
I have checked all my functions "print_row" ,"print_column" everything works fine.
I am getting this error:
PascalTriangle.hs:4:17:
No instance for (RealFrac Int) arising from a use of ‘fact’
In the expression: fact l_int 0
In an equation for ‘why’: why = fact l_int 0
In the expression:
do { l_str <- getLine;
let l_int = ...;
let why = fact l_int 0;
print why;
.... }
I am not able Understand anything about this error.The pogram works fine when I use a constant instead of l_int in line 4.Like let why = print_row 4 0.
You need to use div instead of /.
div will take two Integral values and return another Integral value - e.g. div 5 2 == 2. Then you'll also need to get rid of the truncate call.
/ does "floating point" division.
fromIntegral will convert an Integral value to any other Num type.
Related
I'm trying to convert an integer from decimal integer to a string based on base 4, but my unfoldr doesn't work and I'm not sure why or how to replace it. I can't use imports either. Please help me fix this.
dec2Base4 :: Int -> String
dec2Base4 = map i2c . reverse . unfoldr decomp
where
decomp n = if n == 0 then Nothing else Just(n `mod` 4, n `div` 4)
i2c i = if i == 0 then '0' else if i == 1 then '1' else if i == 2 then '2' else '3'
Example: dec2Base4 10-> "22"
Your code is basically OK, but it would need you to import the unfoldr function from the Data.List package.
The fact that you are banned from using import clauses might just mean that the powers that be want you to use plain recursion.
A recursion-based solution:
Unfortunately, recursion will naturally produce the least significant digit (rightmost digit) first, because this rightmost digit is essentially mod n 4. You will have to use the reverse function to correct that, just like in your library-based code.
For example, without the help of any non-Prelude library functions, the dec2Base4 function can be written like this:
dec2Base4 :: Int -> String
dec2Base4 n
| (n < 0) = '-' : (reverse (auxStr (-n)))
| (n == 0) = "0"
| otherwise = reverse (auxStr n) -- when n > 0
where
i2c i = "0123" !! i
auxStr 0 = ""
auxStr n = let (q,r) = (divMod n 4) in (i2c r) : (auxStr q)
Testing code:
unitTest :: Int -> IO ()
unitTest n = do
let res = dec2Base4 n
putStrLn $ "Result for " ++ (show n) ++ " is: " ++ res
main = do
let testList = [0,11,2051,-2051]
mapM_ unitTest testList
Test program output:
Result for 0 is: 0
Result for 11 is: 23
Result for 2051 is: 200003
Result for -2051 is: -200003
I am working on a program to get the closest prime number by the exponent of 2, this is between an interval.
module Main where
import Data.Char
import System.IO
import Control.Monad (liftM)
data PGetal = G Bool | P Int
instance Show PGetal where
show (P n) = show n
show (G False) = "GEEN PRIEMGETAL GEVONDEN"
mPriem::(Int, Int) -> PGetal
mPriem (x,y) | (x > y) = G False
| (x > 1000000) = G False
| (y > 1000000) = G False
| (null (getAllPriem(x,y))) = G False
| otherwise = P (kleinsteVerschilF(getAllPriem(x,y),1000000,1))
kleinsteVerschilF:: ([Int], Int , Int) -> Int
kleinsteVerschilF ([],_, priemGetal) = priemGetal
kleinsteVerschilF (priem1:priemcss, kleinsteVerschil,priemGetal)=
if(kleinsteVerschil <= kleinsteVerschilMetLijst (priem1,(getMachtenVanTwee(0)),1000000))then kleinsteVerschilF(priemcss, kleinsteVerschil,priemGetal)
else kleinsteVerschilF (priemcss,kleinsteVerschilMetLijst(priem1,(getMachtenVanTwee(0)),1000000), priem1)
kleinsteVerschilMetLijst :: (Int,[Int],Int) -> Int
kleinsteVerschilMetLijst ( _,[],kleinsteVerschil) = kleinsteVerschil
kleinsteVerschilMetLijst (x,tweeMachten1:tweeMachtencss,kleinsteverschil)=
if((abs(x-tweeMachten1)) < kleinsteverschil)
then kleinsteVerschilMetLijst(x,tweeMachtencss, (abs(x-tweeMachten1)))
else kleinsteVerschilMetLijst(x,tweeMachtencss, kleinsteverschil)
getAllPriem :: (Int, Int) ->[Int]
getAllPriem (x,y) = filter isPriem [x..y]
getMachtenVanTwee ::(Int) -> [Int]
getMachtenVanTwee (macht)
|(functieMachtTwee(macht)< 1000000) = (functieMachtTwee(macht)) : (getMachtenVanTwee ((macht+1)))
| otherwise = []
functieMachtTwee:: (Int) -> Int
functieMachtTwee (x) = 2^x
isPriem n = (aantalDelers n)==2
aantalDelers n = telAantalDelersVanaf n 1
telAantalDelersVanaf n kandidaatDeler
| n == kandidaatDeler = 1
| mod n kandidaatDeler == 0
= 1 + telAantalDelersVanaf n (kandidaatDeler+1)
| otherwise
= telAantalDelersVanaf n (kandidaatDeler+1)
aantalDelers2 getal = telDelers getal 1 0
where telDelers n kandidaat teller
| n == kandidaat = 1+teller
| mod n kandidaat == 0
= telDelers n (kandidaat+1) (teller+1)
| otherwise
= telDelers n (kandidaat+1) teller
transform :: [String] -> [PGetal]
transform [] = []
transform (cs:css) =
let (a : b: _ ) = words cs
in (mPriem ((read(a)),(read(b))): transform css)
main :: IO ()
main = do
n <- read `liftM` getLine :: IO Int
lss <- lines `liftM` getContents
let cases = take n lss
let vs = (transform (lss))
putStr $ unlines $ map show vs
When I use the mPriem function, it works fine.
But it needs to work with an input txt file, so I made a .exe file with the ghc command. I also added this .txt file in the folder.
10
1 1
1 3
1 100
200 250
14 16
5 10
20 31
16 50
100 120
5200 7341
When I use in command line this command, it does nothing. There is no output. I can't CTRL+C to stop the program, so I think it crashes. But I don't know what's wrong.
type invoer.txt | programma.exe
Your program works, but is not that efficient and personally I find it not that elegant (sorry :S) because you introduce a lot of "noise". As a result it takes a lot of time before output is written.
If I understand the problem statement correctly, each line (except the first), contains two integers, and you need to count the amount of prime numbers between these two numbers (bounds inclusive?)
First of all, you can do this more elegantly by defining a function: cPrime :: Int -> Int -> Int that takes as input the two numbers and returns the amount of prime numbers:
cPrime :: Int -> Int -> Int
cPrime a b = count $ filter isPrime [a .. b]
You can improve performance by improving your prime checking algorithm. First of all, you do not need to check whether 1 is a divisor, since 1 is always a divisor. Furthermore, you can prove mathematically that there is no divisor greater than sqrt(n) (except for n) that divides n; unless there is another divider that is smaller than sqrt(n). So that means that you can simply enumerate all numbers between 2 and sqrt n and from the moment one of these is a divisor, you can stop: you have proven the number is not prime:
isPrime :: Int -> Bool
isPrime 1 = False
isPrime 2 = True
isPrime n = all ((0 /=) . mod n) (2:[3,5..m])
where m = floor $ sqrt $ fromIntegral n
Now I'm not sure what you aim to do with kleinsteVerschilF.
This is to take a number, get its factorial and double it, however because of the base case if you input 0 it gives 2 as the answer so in order to bypass it i used an if statement, but get the error
parse error on input ‘if’. Really appreciate if you guys could help :)
fact :: Int -> Int
fact 0 = 1
fact n = n * fact(n-1)
doub :: Int -> Int
doub r = 2 * r
factorialDouble :: IO()
factorialDouble = do
putStr "Enter a Value: "
x <- getLine
let num = (read x) :: Int
if (num == 0) then error "factorial of zero is 0"
else let y = doub (fact num)
putStrLn ("the double of factorial of " ++ x ++ " is " ++ (show y))
I've spotted two issues
that should be addressed
You have a let that has no continuation:
(else let y = doub (fact num) ...).
Because you're not inside a do, you would probably want to change it into a let ... in statement.
Your if is indented too far in. It should be under the let.
I've corrected what I mentioned and the code works for me...
fact :: Int -> Int
fact 0 = 1
fact n = n * fact(n-1)
doub :: Int -> Int
doub r = 2 * r
factorialDouble :: IO ()
factorialDouble = do
putStr "Enter a Value: "
x <- getLine
let num = (read x) :: Int
if num == 0 then (error "factorial of zero is 0")
else let y = doub (fact num)
in putStrLn ("the double of factorial of " ++ x ++ " is " ++ (show y))
I hope this works by just pasting and running it with "runghc euler4.hs 1000". Since I am having a hard time learning Haskell, can someone perhaps tell me how I could improve here? Especially all those "fromIntegral" are a mess.
module Main where
import System.Environment
main :: IO ()
main = do
args <- getArgs
let
hBound = read (args !! 0)::Int
squarePal = pal hBound
lBound = floor $ fromIntegral squarePal /
(fromIntegral hBound / fromIntegral squarePal)
euler = maximum $ takeWhile (>squarePal) [ x | y <- [lBound..hBound],
z <- [y..hBound],
let x = y * z,
let s = show x,
s == reverse s ]
putStrLn $ show euler
pal :: Int -> Int
pal n
| show pow == reverse (show pow) = n
| otherwise = pal (n-1)
where
pow = n^2
If what you want is integer division, you should use div instead of converting back and forth to Integral in order to use ordinary /.
module Main where
import System.Environment
main :: IO ()
main = do
(arg:_) <- getArgs
let
hBound = read arg :: Int
squarePal = pal hBound
lBound = squarePal * squarePal `div` hBound
euler = maximum $ takeWhile (>squarePal) [ x | y <- [lBound..hBound],
z <- [y..hBound],
let x = y * z,
let s = show x,
s == reverse s ]
print euler
pal :: Int -> Int
pal n
| show pow == reverse (show pow) = n
| otherwise = pal (n - 1)
where
pow = n * n
(I've re-written the lbound expression, that used two /, and fixed some styling issues highlighted by hlint.)
Okay, couple of things:
First, it might be better to pass in a lower bound and an upper bound for this question, it makes it a little bit more expandable.
If you're only going to use the first two (one in your previous case) arguments from the CL, we can handle this with pattern matching easily and avoid yucky statements like (args !! 0):
(arg0:arg1:_) <- getArgs
Let's convert these to Ints:
let [a, b] = map (\x -> read x :: Int) [arg0,arg1]
Now we can reference a and b, our upper and lower bounds.
Next, let's make a function that runs through all of the numbers between an upper and lower bound and gets a list of their products:
products a b = [x*y | x <- [a..b], y <- [x..b]]
We do not have to run over each number twice, so we start x at our current y to get all of the different products.
from here, we'll want to make a method that filters out non-palindromes in some data set:
palindromes xs = filter palindrome xs
where palindrome x = show x == reverse $ show x
finally, in our main function:
print . maximum . palindromes $ products a b
Here's the full code if you would like to review it:
import System.Environment
main = do
(arg0:arg1:_) <- getArgs
let [a, b] = map (\x -> read x :: Int) [arg0,arg1]
print . maximum . palindromes $ products a b
products a b = [x*y | x <- [a..b], y <- [x..b]]
palindromes = filter palindrome
where palindrome x = (show x) == (reverse $ show x)
I'm learning Haskell, and I'm trying to add preconditions to a (trivial, as an exercise) element_at function (code below). I've created a "helper" elem_at_r because otherwise, len x fails at some point (when x is a 'literal' rather than a list? - I still have trouble parsing ghci's error messages). elem_at now has all the error checking, and elem_at_r does the work. In elem_at, I'd like to add a check that x is indeed a list (and not a 'literal'). How can I do that?
len x = sum [ 1 | a <- x]
elem_at_r x n | n == 0 = head x
| 0 < n = elem_at_r (tail x) (n-1)
elem_at x n | x == [] = error "Need non-empty list"
| len x <= n = error "n too large " ++ show (len x)
| n < 0 = error "Need positive n"
| otherwise = elem_at_r x n
Thanks!
Frank
Due to Haskell's type system, elem_at can only take a list as its first argument (x); if you try to pass a non-list, GHC will detect this and give an error at compile time (or interpretation time in GHCi). I don't know why len would "fail"; could you post the error message that GHCi gives you?
It looks like you were getting errors because of the "x == []" line. The code below pattern matches for that condition and adds a few signatures. Otherwise it is the same. Hope it helps.
len x = sum [ 1 | a <- x]
elem_at_r :: [a] -> Int -> a
elem_at_r x n | n == 0 = head x
| 0 < n = elem_at_r (tail x) (n-1)
elem_at :: [a] -> Int -> a
elem_at [] _ = error "Need non-empty list"
elem_at x n | len x <= n = error ("n too large " ++ show (len x))
| n < 0 = error "Need positive n"
| otherwise = elem_at_r x n
You could also make your helper functions part of this function using a where clause:
elem_at :: [a] -> Int -> a
elem_at [] _ = error "Need non-empty list"
elem_at x n | len x <= n = error ("n too large " ++ show (len x))
| n < 0 = error "Need positive n"
| otherwise = elem_at_r x n
where
len :: [a] -> Int
len x = sum [ 1 | a <- x]
elem_at_r :: [a] -> Int -> a
elem_at_r x n | n == 0 = head x
| 0 < n = elem_at_r (tail x) (n-1)