I have the following code in erlang which gives solutions to the n-queens problem. It takes number of queens as argument. The program runs successfully on erl shell. But I need to run this program without shell on linux terminal so that I can use the 'time' command to measure execution time of the program.
-module(queen).
-export([queen/1]).
queen(N) ->
IJ = [{I, J} || I <- lists:seq(1, N), J <- lists:seq(1, N)],
lists:foreach(fun({I, J}) -> put_data(I, J, true) end, IJ),
solve(N, 1, [], 0).
solve(N, J, Board, Count) when N < J ->
print(N, Board),
Count + 1;
solve(N, J, Board, Count) ->
F = fun(I, Cnt) ->
case get_data(I, J) of
true ->
put_data(I, J, false),
Cnt2 = solve(N, J+1, [I|Board], Cnt),
put_data(I, J, true),
Cnt2;
false -> Cnt
end
end,
lists:foldl(F, Count, lists:seq(1, N)).
put_data(I, J, Bool) ->
put({row, I }, Bool),
put({add, I+J}, Bool),
put({sub, I-J}, Bool).
get_data(I, J) ->
get({row, I}) andalso get({add, I+J}) andalso get({sub, I-J}).
print(N, Board) ->
Frame = "+-" ++ string:copies("--", N) ++ "+",
io:format("~s~n", [Frame]),
lists:foreach(fun(I) -> print_line(N, I) end, Board),
io:format("~s~n", [Frame]).
print_line(N, I) ->
F = fun(X, S) when X == I -> "Q " ++ S;
(_, S) -> ". " ++ S
end,
Line = lists:foldl(F, "", lists:seq(1, N)),
io:format("| ~s|~n", [Line]).
You can use timer module to check speed of execution:
{Time, Value} = timer:tc(queen, queen, [N])
Or you can transform you code to escript:
#!/usr/bin/env escript
-mode(native). %% to fun faster
main([NStr]) ->
N = list_to_integer(NStr),
IJ = [{I, J} || I <- lists:seq(1, N), J <- lists:seq(1, N)],
lists:foreach(fun({I, J}) -> put_data(I, J, true) end, IJ),
solve(N, 1, [], 0).
solve(N, J, Board, Count) when N < J ->
print(N, Board),
Count + 1;
solve(N, J, Board, Count) ->
F = fun(I, Cnt) ->
case get_data(I, J) of
true ->
put_data(I, J, false),
Cnt2 = solve(N, J+1, [I|Board], Cnt),
put_data(I, J, true),
Cnt2;
false -> Cnt
end
end,
lists:foldl(F, Count, lists:seq(1, N)).
put_data(I, J, Bool) ->
put({row, I }, Bool),
put({add, I+J}, Bool),
put({sub, I-J}, Bool).
get_data(I, J) ->
get({row, I}) andalso get({add, I+J}) andalso get({sub, I-J}).
print(N, Board) ->
Frame = "+-" ++ string:copies("--", N) ++ "+",
io:format("~s~n", [Frame]),
lists:foreach(fun(I) -> print_line(N, I) end, Board),
io:format("~s~n", [Frame]).
print_line(N, I) ->
F = fun(X, S) when X == I -> "Q " ++ S;
(_, S) -> ". " ++ S
end,
Line = lists:foldl(F, "", lists:seq(1, N)),
io:format("| ~s|~n", [Line]).
and make it executable from linux shell:
chmod +x queue
And then run it from linux shell with command ./queue
or time ./queue