SML Practice Problems

(* Return the last element *)
fun last_element([]) = 0
    | last_element(X::Y) = if Y=[] then X else last_element(Y);

last_element([3,4,5,1,2,99]);

(* Return the k'th element *)
fun kth_element([], k) = 0
    | kth_element(x::X, k) = if k=0 then x else kth_element(X, k-1);

kth_element([4,2,4,99,1,2], 3);

(* Reverse a list *)
fun reverse([]) = []
    | reverse(x::X) = reverse(X) @ [x];

reverse([1,2,3,4,5,6,7]);

(* Palindrome *)
fun palindrome(X) = if X=reverse(X) then true else false;

palindrome([1,2,3,2,1]);
palindrome([1,2,3,4,5]);

(* Flatten a nested list *)
fun flatten([]) = []
    | flatten(x::X) = x @ flatten(X);

flatten([[1], [2, 3, 4, 5]]);

(* Eliminate consecutive duplicates *)
fun elim_duplicates([]) = []
    | elim_duplicates(x::[]) = [x]
    | elim_duplicates(x::X) = if x=hd(X) then elim_duplicates(X)
        else x::elim_duplicates(X);

elim_duplicates([1,1,1,1,2,3,3,3,1,3,3,5,5,5,3]);

(* Pack consecutive duplicates *)
(* NOT WORKING *)
fun pack_duplicates([]) = []
    | pack_duplicates(x::[]) = [x]
    | pack_duplicates(x::X) = if x=hd(X) then [x] else X;

pack_duplicates([1,1,1,1,2,3,3,3,1,3,3,5,5,5,3]);

(* Run length encoding *)
(* Can use pack_duplicate(), then get the char of each array and the length *)

(* Run length decoding *)
fun create_run(n, char) = if n=0 then [] else char::create_run(n-1, char);

create_run(4, 9);

fun decode_run([]) = []
    | decode_run(x::X) = create_run(hd(x), hd(tl(x))) @ decode_run(X);

decode_run([[4,9],[1,8],[5,7]]);

(* Create a list of integers within a given range *)
fun range(a, b) = if a > b then [] else a::range(a+1, b);

range(4, 9);