erlang - How to use lists:duplicate? -


i know if possible use lists:duplicate in case:

decompress_1([])->       []; decompress_1(l)->       mynum = lists:map(fun(t)-> element(1,t) end,l),       res = lists:map(fun(t)-> element(2,t) end,l).

to :

decompress_1([{3,1},{3,2},{1,5},{1,4},{1,1},{1,0},{1,1}]) == [1,1,1,2,2,2,5,4,1,0,1]

i manage retrieve first , second elements of tuple. there solution list comprehension know without.

decompress([]) ->      []; decompress(l) ->      [y || {x, y} <- l, _ <- lists:seq(1, x)].

without using list comprehension, can use lists:duplicate/2 create result, have flatten desired final answer:

decompress([]) ->     []; decompress(l) ->     lists:flatten(lists:map(fun({x,y}) ->                                     lists:duplicate(x,y)                             end, l)).

without flatten we'd first result shown below, instead of second correct result:

1> decompress_no_flatten([{3,1},{3,2},{1,5},{1,4},{1,1},{1,0},{1,1}]). [[1,1,1],[2,2,2],[5],[4],[1],[0],[1]] 2> decompress([{3,1},{3,2},{1,5},{1,4},{1,1},{1,0},{1,1}]). [1,1,1,2,2,2,5,4,1,0,1]

by way, can use lists:duplicate/2 in original list comprehension approach well:

decompress([]) ->     []; decompress(l) ->     [y || {x,y} <- l, _ <- lists:duplicate(x,y)].

this works because here don't use values produced lists:seq/2 or lists:duplicate/2, rather use number of items produce.


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