L-99: Ninety-Nine Lisp Problems provides a good exercise for learning new language so here is my attempt at Erlang solutions of problems 1 through 25:

1 -module(lisp99). 2 -compile(export_all). 3 4 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 5 % My solutions to L-99: Ninety-Nine Lisp Problems from 6 % http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html 7 % Based on a Prolog problem list by werner.hett@hti.bfh.ch 8 % Note: I am using BIFs for a lot of solutions because learning language library APIs is just as 9 % important as learning language constructs. 10 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 11 12 %%%%%%%%%%%%%%%%%%%%%%%%% 13 %%% P01 (*) Find the last box of a list. 14 %%%%%%%%%%%%%%%%%%%%%%%%% 15 last(L) -> 16 lists:nthtail(length(L)-1, L). 17 18 %%%%%%%%%%%%%%%%%%%%%%%%% 19 %%% P02 (*) Find the last but one box of a list. 20 %%%%%%%%%%%%%%%%%%%%%%%%% 21 last_but(L) -> 22 lists:nthtail(length(L)-2, L). 23 24 %%%%%%%%%%%%%%%%%%%%%%%%% 25 %%% P03 (*) Find the K'th element of a list. 26 %%%%%%%%%%%%%%%%%%%%%%%%% 27 kth(L, K) -> 28 lists:nth(K, L). 29 30 %%%%%%%%%%%%%%%%%%%%%%%%% 31 %%% P04 (*) Find the number of elements of a list. 32 %%%%%%%%%%%%%%%%%%%%%%%%% 33 len(L) -> 34 length(L). 35 36 %%%%%%%%%%%%%%%%%%%%%%%%% 37 %%% P05 (*) Reverse a list. 38 %%%%%%%%%%%%%%%%%%%%%%%%% 39 rev(L) -> 40 lists:reverse(L). 41 42 %%%%%%%%%%%%%%%%%%%%%%%%% 43 %%% P06 (*) Find out whether a list is a palindrome. 44 %%%%%%%%%%%%%%%%%%%%%%%%% 45 palindrome(L) -> 46 L == lists:reverse(L). 47 48 49 %%%%%%%%%%%%%%%%%%%%%%%%% 50 %%% P07 (**) Flatten a nested list structure. 51 %%%%%%%%%%%%%%%%%%%%%%%%% 52 flatten(L) -> 53 lists:flatten(L). 54 55 %%%%%%%%%%%%%%%%%%%%%%%%% 56 %%% P08 (**) Eliminate consecutive duplicates of list elements. 57 %%%%%%%%%%%%%%%%%%%%%%%%% 58 compress(L) -> 59 lists:reverse(lists:foldl(fun compress/2, [], L)). 60 compress(H, [H|T]) -> 61 [H|T]; 62 compress(H, L) -> 63 [H|L]. 64 65 66 %%%%%%%%%%%%%%%%%%%%%%%%% 67 %%% P09 (**) Pack consecutive duplicates of list elements into sublists. 68 %%%%%%%%%%%%%%%%%%%%%%%%% 69 pack(L) -> 70 lists:reverse(lists:foldl(fun pack/2, [], L)). 71 pack(H, [[H|T]|TT]) -> 72 [[H,H|T]|TT]; 73 pack(H, L) -> 74 [[H]|L]. 75 76 %%%%%%%%%%%%%%%%%%%%%%%%% 77 %%% P10 (*) Run-length encoding of a list. 78 %%%%%%%%%%%%%%%%%%%%%%%%% 79 encode(L) -> 80 lists:reverse(lists:foldl(fun encode/2, [], L)). 81 encode(H, []) -> 82 [[1, H]]; 83 encode(H, [[N,H]|T]) -> 84 [[N+1,H]|T]; 85 encode(H, [[_N,_X]|_T]=L) -> 86 [[1,H]|L]. 87 88 %%%%%%%%%%%%%%%%%%%%%%%%% 89 %%% P11 (*) Modified run-length encoding. 90 %%%%%%%%%%%%%%%%%%%%%%%%% 91 encode_modified(L) -> 92 lists:reverse(lists:foldl(fun encode_modified/2, [], L)). 93 encode_modified(H, []) -> 94 [H]; 95 encode_modified(H, [H|T]) -> 96 [[2,H]|T]; 97 encode_modified(H, [[N,H]|T]) -> 98 [[N+1,H]|T]; 99 encode_modified(H, [[_N,_X]|_T]=L) -> 100 [H|L]; 101 encode_modified(H, [_X|_T]=L) -> 102 [H|L]. 103 104 %%%%%%%%%%%%%%%%%%%%%%%%% 105 %%% P12 (**) Decode a run-length encoded list. 106 %%%%%%%%%%%%%%%%%%%%%%%%% 107 decode(L) -> 108 lists:reverse(lists:foldl(fun decode/2, [], L)). 109 decode([N,H], L) -> 110 lists:duplicate(N, H) ++ L; 111 decode(H, L) -> 112 [H|L]. 113 114 %%%%%%%%%%%%%%%%%%%%%%%%% 115 %%% P13 (**) Run-length encoding of a list (direct solution). 116 %%%%%%%%%%%%%%%%%%%%%%%%% 117 direct_encode(L) -> 118 encode_modified(L). %% it already creates RLE by counting 119 120 %%%%%%%%%%%%%%%%%%%%%%%%% 121 %%% P14 (*) Duplicate the elements of a list. 122 %%%%%%%%%%%%%%%%%%%%%%%%% 123 dupli(L) -> 124 lists:reverse(lists:foldl(fun dupli/2, [], L)). 125 dupli(H, L) -> 126 [H,H|L]. 127 128 %%%%%%%%%%%%%%%%%%%%%%%%% 129 %%% P15 (**) Replicate the elements of a list a given number of times. 130 %%%%%%%%%%%%%%%%%%%%%%%%% 131 repli(L, N) -> 132 {N, L1} = lists:foldl(fun do_repli/2, {N,[]}, L), 133 lists:reverse(L1). 134 do_repli(H, {N,L}) -> 135 {N, lists:duplicate(N, H) ++ L}. 136 137 %%%%%%%%%%%%%%%%%%%%%%%%% 138 %%% P16 (**) Drop every N'th element from a list. 139 %%%%%%%%%%%%%%%%%%%%%%%%% 140 drop(L, N) -> 141 {_I, N, L1} = lists:foldl(fun do_drop/2, {1,N,[]}, L), 142 lists:reverse(L1). 143 do_drop(H, {I,N,L}) -> 144 Rem = I rem N, 145 if 146 Rem == 0 -> 147 {1,N,L}; 148 true -> 149 {I+1,N,[H|L]} 150 end. 151 152 %%%%%%%%%%%%%%%%%%%%%%%%% 153 %%% P17 (*) Split a list into two parts; the length of the first part is given. 154 %%%%%%%%%%%%%%%%%%%%%%%%% 155 split(L,N) -> 156 lists:split(N,L). 157 158 %%%%%%%%%%%%%%%%%%%%%%%%% 159 %%% P18 (**) Extract a slice from a list. 160 %%%%%%%%%%%%%%%%%%%%%%%%% 161 slice(L, Start, End) -> 162 lists:sublist(L, Start, End-Start+1). 163 164 165 %%%%%%%%%%%%%%%%%%%%%%%%% 166 %%% P19 (**) Rotate a list N places to the left. 167 %%%%%%%%%%%%%%%%%%%%%%%%% 168 rotate(L, N) -> 169 N1 = if 170 N > 0 -> 171 N; 172 true -> 173 length(L) + N 174 end, 175 Len = length(L), 176 lists:reverse(do_rotate(N1, Len, L, [])). 177 178 do_rotate(I, Len, In, Out) -> 179 I1 = (I rem Len)+1, % In Erlang offset of list starts from 1 180 Out1 = [lists:nth(I1, In)|Out], 181 if 182 length(Out1) == Len -> 183 Out1; 184 true -> 185 do_rotate(I1, Len, In, Out1) 186 end. 187 188 %%%%%%%%%%%%%%%%%%%%%%%%% 189 %%% P20 (*) Remove the K'th element from a list. 190 %%%%%%%%%%%%%%%%%%%%%%%%% 191 remove_at(L, N) -> 192 Elem = lists:nth(N,L), 193 lists:delete(Elem,L). 194 195 196 %%%%%%%%%%%%%%%%%%%%%%%%% 197 %%% P21 (*) Insert an element at a given position into a list. 198 %%%%%%%%%%%%%%%%%%%%%%%%% 199 insert_at(Elem, L, N) -> 200 {L1,L2} = lists:split(N-1,L), % there got to be better way 201 L1 ++ [Elem] ++ L2. 202 203 %%%%%%%%%%%%%%%%%%%%%%%%% 204 %%% P22 (*) Create a list containing all integers within a given range. 205 %%%%%%%%%%%%%%%%%%%%%%%%% 206 range(Start, End) -> 207 lists:seq(Start, End). 208 209 %%%%%%%%%%%%%%%%%%%%%%%%% 210 %%% P23 (**) Extract a given number of randomly selected elements from a list. 211 %%%%%%%%%%%%%%%%%%%%%%%%% 212 rnd_select(L, N) -> 213 rnd_select(N, L, []). 214 rnd_select(N, In, Out) -> 215 I = random:uniform(length(In)), 216 Out1 = [lists:nth(I, In) | Out], 217 In1 = remove_at(In, I), 218 if 219 length(Out1) == N -> 220 Out1; 221 true -> 222 rnd_select(N, In1, Out1) 223 end. 224 225 226 %%%%%%%%%%%%%%%%%%%%%%%%% 227 %%% P24 (*) Lotto: Draw N different random numbers from the set 1..M. 228 %%%%%%%%%%%%%%%%%%%%%%%%% 229 lotto_select(Count, Max) -> 230 L = range(1, Max), 231 rnd_select(L, Count). 232 233 %%%%%%%%%%%%%%%%%%%%%%%%% 234 %%% P25 (*) Generate a random permutation of the elements of a list. 235 %%%%%%%%%%%%%%%%%%%%%%%%% 236 rnd_permu(L) -> 237 rnd_select(L, length(L)). 238 239 240 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 241 % Tests 242 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 243 test_last() -> 244 last("abcd"). 245 246 test_last_but() -> 247 last_but("abcd"). 248 249 test_kth() -> 250 [kth("abcd", 3)]. 251 252 test_len() -> 253 len("abcd"). 254 255 test_rev() -> 256 rev("abcd"). 257 258 test_flatten() -> 259 flatten([$a, [$b, [c, d], e]]). 260 261 test_compress() -> 262 compress("aaaabccaadeeee"). 263 264 test_pack() -> 265 pack("aaaabccaadeeee"). 266 267 test_encode() -> 268 encode("aaaabccaadeeee"). 269 270 test_encode_modified() -> 271 encode_modified([1,1,1,1,2,3,3,1,1,4,5,5,5,5]). 272 273 test_decode() -> 274 decode(encode_modified([1,1,1,1,2,3,3,1,1,4,5,5,5,5])). 275 276 test_dupli() -> 277 dupli("abccd"). 278 279 test_repli() -> 280 repli("abccd",3). 281 282 test_drop() -> 283 drop("abcdefghik",3). 284 285 test_split() -> 286 split("abcdefghik",3). 287 288 test_slice() -> 289 slice("abcdefghik",3,7). 290 291 test_rotate1() -> 292 rotate("abcdefgh",3). 293 294 test_rotate2() -> 295 rotate("abcdefgh",-2). 296 297 test_remove_at() -> 298 remove_at("abcd", 2). 299 300 test_insert_at() -> 301 insert_at("alfa", "abcd", 2). 302 303 test_range() -> 304 range(4,9). 305 306 test_rnd_select() -> 307 rnd_select("abcdefgh", 3). 308 309 test_lotto_select() -> 310 lotto_select(6,49). 311 312 test_rnd_permu() -> 313 rnd_permu("abcdef"). 314 315