DEV Community

Shashwat Seth
Shashwat Seth

Posted on

Please help me with this haskell program.

Define a function dropOdds :: Int -> Int with the following behaviour. For any positive number m, dropOdds m is got by dropping all the odd digits in m. (If all the digits in the number are odd, the answer should be 0.)
Test cases:
dropOdds 0 = 0
dropOdds 8 = 8
dropOdds 1357 = 0

Discussion (8)

Collapse
totally_chase profile image
Phantz

Ok, I won't give you the answer directly since that'd be harmful to your learning. I'll ask 3 questions instead - try to answer them:-

  • How can you turn a number into a list of "digits".
  • How can you check if a "digit" is odd?
  • How do you "drop" certain elements of a list that match a criteria? (hint: filter)

You'll notice the quotes I put around the word "digits". I generally prefer using Char for "digits". But one may just as well use Int. Whichever you use, your answers will depend on it.

Collapse
benjioe profile image
Benjioe • Edited on

You can use recursion too :

  • How can you check if a "digit" is odd?
  • How can you apply a function only on first digit?
  • How can you apply that function on each other digits?
Collapse
shashwatseth profile image
Shashwat Seth Author

Thanks man

Collapse
shashwatseth profile image
Shashwat Seth Author

Thankyou so much, I got it right!!!!!

Collapse
benjioe profile image
Benjioe

...

Collapse
shashwatseth profile image
Shashwat Seth Author

I'm new to haskell. I want the solution for this so that I can understand the concepts

Collapse
gopinathvarad profile image
gopinathvarad

even I want this answer !!

Collapse
benjioe profile image
Benjioe

With Recursion :

dropOdds :: Int -> Int
dropOdds x
    | x >= 10 = 10 * dropOdds restDigits + filterDrops firstDigit
    | otherwise = filterDrops x
    where 
      firstDigit = x `mod` 10
      restDigits = x `div` 10


filterDrops x 
    | x `mod` 2 == 0 = x -- x is even => we keep it
    | otherwise = 0 -- x is drop => we 


main =  do
   print(dropOdds 0)
   print(dropOdds 8)
   print(dropOdds 10)
   print(dropOdds 1357)
Enter fullscreen mode Exit fullscreen mode