Haskell flag pattern writing function?

0
votes

I really need help in writing this function in Haskell, I don't even know where to start. Here are the specs:

Define a function flagpattern that takes a positive Int value greater than or equal to five and returns a String that can be displayed as the following `flag' pattern of dimension n, e.g.

Main> putStr (flagpattern 7)

#######  
##   ##  
# # # #  
#  #  #  
# # # #  
##   ##  
#######
3
haskell
I don't understand. Is this supposed to look like an ASCii-art number 7? Could you specify the problem in words and not in a single example? - Thomas M. DuBuisson
@Thomas: It looks like a cross in a box of size 7x7 to me. You can use recursion to create the cross. - Niklas B.
How does it then look for an even n? Also, I don't see how this is haskell-specific, its mostly problem solving in general, no? - Tarrasch
Its hard o help you if you don't even know where to start. StackOverflow works better on m ore specific questions - hugomg

3 Answers

1
votes

Assuming you want a "X" enclosed in 4 lines, you need to write a function that given a coordinate (x,y) returns what character should be at that position:

coordinate n x y = if i == 0 then 'X' else ' '

(This version outputs only the leftmost X'es, modify it, remember indices start with 0)

Now you want them nicely arranged in a matrix, use a list comprehension, described in the linked text.

1
votes

You should start from your problem definition:

main :: IO ()
main = putStr . flagPattern $ 7

Then, you should ask yourself about how much dots flag has:

flagPattern :: Int -> String
flagPattern = magic $ [1..numberOfDots]

Then, (hard) part of magic function should decide for each dot whether it is   or #:

partOfMagic ...
  | ...       = "#" -- or maybe even "#\n" in some cases?
  | otherwise = " "

Then, you can concatenate parts into one string and get the answer.

1
votes

Start with the type signature.

flagpattern :: Int -> String

Now break the problem into subproblems. For example, suppose I told you to produce row 2 of a size 7 flag pattern. You would write:

XX   XX

Or row 3 of a size 7 flag pattern would be

X X X X

So suppose we had a function that could produce a given row. Then we'd have

flagpattern :: Int -> String
flagpattern size = unlines (??? flagrow ???)

flagrow :: Int -> Int -> String
flagrow row size = ???

unlines takes a list of Strings and turns it into a single String with newlines between each element of the list. See if you can define flagrow, and get it working correctly for any given row and size. Then see if you can use flagrow to define flagpattern.