Ggtcf

I have a map and I am modifying each element on it, I am confused which approach is better(faster) to do it with Enum.map and then Enum.into(%) or to use for comprehension like

for key, value <- my_map, into: % do
 key, new_value
end

You can use Benchee to run this kind of comparisons.

A simple Benchee test will show that Enum is faster for cases like this one.

iex(1)> m = %a: 1, b: 2, c: 3, d: 4
%a: 1, b: 2, c: 3, d: 4
iex(2)> with_enum = fn -> Enum.map(m, fn k, v -> k, v * v end) end
#Function<20.127694169/0 in :erl_eval.expr/5>
iex(3)> with_for = fn -> for k, v <- m, into: %, do: k, v * v end
#Function<20.127694169/0 in :erl_eval.expr/5>
iex(4)> Benchee.run(%
...(4)> "with_enum" => fn -> with_enum.() end,
...(4)> "with_for" => fn -> with_for.() end
...(4)> )
Operating System: Linux
CPU Information: Intel(R) Core(TM) i7-6500U CPU @ 2.50GHz
Number of Available Cores: 4
Available memory: 7.71 GB
Elixir 1.7.4
Erlang 21.0

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 14 s


Benchmarking with_enum...
Benchmarking with_for...

Name ips average deviation median 99th %
with_enum 28.27 K 35.37 μs ±16.16% 34.37 μs 55.21 μs
with_for 19.55 K 51.14 μs ±9.16% 50.08 μs 59.94 μs

Comparison: 
with_enum 28.27 K
with_for 19.55 K - 1.45x slower

In general, for isn't the best option for these cases in Elixir, it's best suited for list comprehensions, which it can do pretty fast and with an easy to read syntax.

Enum's functions are optimized to handle these scenarios which are more iterative, like what you would do with a for construct in other programming languages.