HiroiImanishi
I'd like to take the element of the same index from multiple lists
Is there a way to take multiple lists and get all the elements at their n th index and pass them to the function?
For example, list1 (= [1,3,5]) and list2 (= [2,4,6]) are added,then list (=[3,7,11]) is created.
What I wrote below is wrong, but I imagine such usage.
defmodule TwoLists do
def sum(list1,list2) do
Enum.map(list1,list2,fn(a,b)->a+b end)
end
end
iex> list1 = [1,3,5]
iex> list2 = [2,4,6]
iex> TwoLists.sum(list1,list2)
#-->Enum.map/3 is undefined error
I’d like you to show me the way to implement.
Thanks
Marked As Solved
domvas
One way I see is the following one:
defmodule ListSum do
def sum(list1, list2, total \\ [])
def sum([], [], total) do
Enum.reverse(total)
end
def sum([h1 | t1], [], total) do
sum(t1, [], [h1 | total])
end
def sum([], [h2 | t2], total) do
sum([], t2, [h2 | total])
end
def sum([h1 | t1], [h2 | t2], total) do
sum(t1, t2, [h1 + h2 | total])
end
end
This implementation has the advantage to work with list of different lengths.
You can also code an implement using for and Enum.at but lists must have same length is this case
Also Liked
kokolegorille
You might use Enum.zip.
iex(1)> list1 = [1,3,5]
[1, 3, 5]
iex(2)> list2 = [2,4,6]
[2, 4, 6]
iex(3)> Enum.zip list1, list2
[{1, 2}, {3, 4}, {5, 6}]
iex(4)> list3 = [2,4,6,8]
[2, 4, 6, 8]
iex(5)> Enum.zip list1, list3
[{1, 2}, {3, 4}, {5, 6}]
but it will return a number of elements corresponding to the smallest list in case they are not the same length.
You can use more than two lists
iex(6)> Enum.zip [list1, list2, list3]
[{1, 2, 2}, {3, 4, 4}, {5, 6, 6}]
And for your use case…
iex(7)> list1 |> Enum.zip(list2) |> Enum.map(& (elem(&1, 0) + elem(&1, 1)))
[3, 7, 11]
hauleth
This is terrible solution, because Enum.at/2 has linear complexity. So in your case the complexity of this function is quadratic instead of linear.
joaoevangelista
the function without the do block is a header function, it is needed when we declare default arguments on matching functions, see that only that function has the total with a default value. Without it you would get a definitions with multiple clauses and default values require a header. error message from the compiler.
Roughly the compiler will create a sum/2 function calling the matching functions sum/3 with a default value on the third argument.
You can see more examples at Elixir School.
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