The “assignment” inside the for loop has no effect, it creates a new variable on each loop iteration. I think you might be looking for Enum.reduce:

Enum.reduce(0..9, %{sensor1: 0}, fn n, sensor_data ->
  Map.update!(sensor_data, :sensor1, &(&1 ||| (1 <<< n)))
end)

If you really want to use for you can. Since a few versions back it also supports reductions.

use Bitwise
sensor_data = %{sensor1: 0}
new_sensor_data = 
  for n <- 0..9, reduce: sensor_data do
    acc -> Map.update!(acc, :sensor1, &(&1 ||| (1 <<< n)))
  end

See also: https://elixir-lang.org/getting-started/case-cond-and-if.html#if-and-unless

This is also a good opportunity to talk about variable scoping in Elixir. If any variable is declared or changed inside if, case, and similar constructs, the declaration and change will only be visible inside the construct. For example:

iex> x = 1
1
iex> if true do
...>   x = x + 1
...> end
2
iex> x
1

In said cases, if you want to change a value, you must return the value from the if:

iex> x = 1
1
iex> x = if true do
...>   x + 1
...> else
...>   x
...> end
2

Not only, You also need the previous value.

That’s why You need the acc of a reducer…

Thanks @LostKobrakai and @kokolegorille !
I missed out this :reduce option in Comprehensions. I am able to recall now.
Documentation about :reduce for anyone to refer in future:
https://hexdocs.pm/elixir/Kernel.SpecialForms.html#for/1-the-reduce-option