About a month ago, my brother and I started this site with the intention of going through Dr. Werner Hett's 99 Problems after being inspired by someone implementing them in Ruby. Since we were new to the language, we figured it would be a great way for us to gain a better understanding of the F# language; And it has been. Listed below are the first ten problems out of ninety-nine, so we still have a way to go. . .
These may not be the "best" implementations of each solution; I'm sure these aren't the "best" solutions. Actually, if you have a better solution; I implore you to post it in the comments so I can edit the post for future viewers to observe your acuity. ;)
(click the links for the problems to directed to more detailed posts. . . )
Problem #1: "Find the last element of a list" (detailed view)
1 #light
2
3 let myList =
4 [1; 2; 3; 4]
5 |> List.rev
6 |> List.hd
7 |> printf "Answer: %d"
Problem #2: "Find the last but one element of a list" (detailed view)
1 #light
2
3 let listerine = [1 .. 40]
4 let prob2 = List.nth listerine (List.length listerine - 2) |> printfn "Answer: %d"
Problem #3: "Find the K'th element of a list" (detailed view)
1 #light
2
3 let listerine k = [1 .. 40] |> fun x -> List.nth x k
4
5 printfn "Answer: %d" (listerine 27)
Problem #4: "Find the number of elements of a list" (detailed view)
1 #light
2
3 let listerine = [1 .. 40] |> List.length |> printfn "List length: %d"
1 #light
2
3 let listerine = [1 .. 40] |> List.rev |> List.iter (printfn "%d")
Problem #6: "Find out whether a list is a palindrome" (detailed view)
let printListsEqual (listy : List<char>) =
if listy.Equals(List.rev listy) then
printfn "It's a palindrome!"
else
printfn "Not a palindrome!"
Problem #7: "Flatten a nested list structure" (recursively) (detailed view)
1 #light
2
3 let someList = [[1;2;3];[4;5];[6;7];[8]]
4 let rec listFixer listy =
5 if List.nonempty listy then
6 let top = listy |> List.hd
7 let tail = listy |> List.tl
8 top @ (listFixer tail)
9 else
10 []
Problem #8: "Eliminate consecutive duplicates of list elements" (detailed view)
1 #light
2
3 let problem7 lister =
4 let rec checker head tail =
5 match tail with
6 | h::t -> if head = h then
7 checker h t
8 else
9 [head] @ checker h t
10 | [] -> [head]
11 match lister with
12 | h::t -> checker h t
13 | [] -> lister
14
15 let megaList = [1;1;1;1;1;2;2;2;2;3;3;3;3;3;3;4;4;4;4;5;5;5;6;6;7;7;7]
Problem #9: "Pack consecutive duplicates list elements into sublists" (detailed view)
1 #light
2
3 let oSnap = [1;1;1;2;2;2;2;2;2;3;3;3;3;3;3;4;]
4
5 let prob9 listegosaurus =
6 let rec packer listy tempy head tail =
7 match tail with
8 | h::t -> if head = h then
9 packer listy (tempy @ [head]) h t
10 else
11 packer (listy @ [tempy @ [head]] ) [] h t
12 | [] -> listy @ [[head]]
13
14 match listegosaurus with
15 | h::t -> packer [] [] h t
16 | [] -> [listegosaurus]
Problem #10: "Run-length encoding of a list" (detailed view)
1 #light
2
3 let oSnap = [1;1;1;2;2;2;2;2;2;3;3;3;3;3;3;4;]
4
5 let prob10 listupendous =
6 let rec packer listy tempy head tail =
7 match tail with
8 | h::t -> if head = h then
9 packer listy (tempy @ [head]) h t
10 else
11 packer (listy @ [(List.length (tempy @ [head]), head)]) [] h t
12 | [] -> listy @ [1 , head]
13
14 match listupendous with
15 | h::t -> packer [] [] h t
16 | [] -> [1, (listupendous |> List.hd)]
That's it so far. If you have any implementations of any of the problems, please toss them in the comments and I'll be sure to put them in the body.
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