In this article i am proudly presenting solutions to 11-20 problems from classical 99 Haskell problem set.
Problem 11
Modify the result of problem 10 in such a way that if an element has no duplicates it is simply copied into the result list. Only elements with duplicates are transferred as (N E) lists.Example:
Haskell Example:
* (encode-modified '(a a a a b c c a a d e e e e)) ((4 A) B (2 C) (2 A) D (4 E))
Solution:
julia>function modifiedEncoding(x)
lest=[]
for c in unique(x)
if counter(x,c)>1
lest=[lest...,(counter(x,c),c)]
else
lest=[lest...,c]
end
end
lest
end
julia>function counter(l,el)
temp=0
for i=1:endof(l)
if el==l[i]
temp+=1
end
end
temp
end
Problem 12
(**) Decode a run-length encoded list.Given a run-length code list generated as specified in problem 11. Construct its uncompressed version.
Example in Haskell:
P12> decodeModified [Multiple 4 'a',Single 'b',Multiple 2 'c', Multiple 2 'a',Single 'd',Multiple 4 'e']
Solution:
julia>function de_encode(x)
lest=[]
for c in x
if typeof(c)==(Int32,Int32)
for i=1:c[1]
lest=[lest...,c[2]]
end
else
lest=[lest...,c]
end
end
lest
end
Problem 13
(**) Run-length encoding of a list (direct solution).Implement the so-called run-length encoding data compression method directly. I.e. don't explicitly create the sublists containing the duplicates, as in problem 9, but only count them. As in problem P11, simplify the result list by replacing the singleton lists (1 X) by X.
Example:
* (encode-direct '(a a a a b c c a a d e e e e)) ((4 A) B (2 C) (2 A) D (4 E))
Solution:
Same as problem 11.We used shortcut solution already in our early 11th problem.
Problem 14
(*) Duplicate the elements of a list.Example:
* (dupli '(a b c c d))
(A A B B C C C C D D)
Solution:
julia>function duplicate(x)
lest=[]
for c in x
lest=[lest...,c,c]
end
lest
end
Problem 15
(**) Replicate the elements of a list a given number of times.Example:
* (repli '(a b c) 3) (A A A B B B C C C)
Solution:
julia>function replicate(x,k)
lest=[]
for c in x
for i=1:k
lest=[lest...,c]
end
end
lest
end
Problem 16
(**) Drop every N'th element from a list.Example:
* (drop '(a b c d e f g h i k) 3) (A B D E G H K)
Solution:
julia>function dropN(x,n)
lest=[]
for i=1:endof(x)
if i%n!=0
lest=[lest...,x[i]]
end
end
lest
end
Problem 17
(*) Split a list into two parts; the length of the first part is given.Do not use any predefined predicates.
Example:
* (split '(a b c d e f g h i k) 3) ( (A B C) (D E F G H I K))
Solution:
julia>split(x,n)=x[1:n],x[n+1,end]
Problem 18
(**) Extract a slice from a list.Given two indices, i and k, the slice is the list containing the elements between the i'th and k'th element of the original list (both limits included). Start counting the elements with 1.
Example:
* (slice '(a b c d e f g h i k) 3 7)
(C D E F G)
Solution:
julia>slice(x,i,j)=x[i:j]
Problem 19
(**) Rotate a list N places to the left.Hint: Use the predefined functions length and (++).
Examples:
* (rotate '(a b c d e f g h) 3) (D E F G H A B C) * (rotate '(a b c d e f g h) -2) (G H A B C D E F)
Solution:
julia>rotLeft(x,n)=[x[n+1:end],x[1:n]]Problem 20
(*) Remove the K'th element from a list.Example in Prolog:
?- remove_at(X,[a,b,c,d],2,R). X = b R = [a,c,d]
Solution:
julia>removeAt(x,n)=delete!(x,n) This delete!() method will go to jail because it changes original list which
is anti philosophy of functional programming.Here is functional version.
julia>function removeAt(x,n)
lest=[]
for i=1:endof(x)
if i!=n
lest=[lest...,x[i]]
end
end
lest
end
Thanks for your patience.see you next time.
