How can I rewrite following loop from JS to elixir?
let sum = 0
let results = []
for i in [1,2,3,4,5]{
{ num, res } = some_fun(sum)
results.push(res)
sum = sum + num
}
#Loop with accumulator in Elixir
mild inlet
burnt lance
naive sluice
you'll need to use Enum.reduce to accumulate both the sum and the results list
(or the reduce option of for)
mild inlet
I'm not sure how syntax would look like for both sum and res
I'm a newbie
I know how to do Enum.reduce over one variable
burnt lance
You can use a tuple that contains both variables
elfin hazel
In this case Enum.map_reduce/3 is what you want - https://hexdocs.pm/elixir/1.12/Enum.html#map_reduce/3
mild inlet
Thanks guys, I should have looked at documentation more carefully
Didn't know i can return tuple {result, accumulator }
Btw. is there a way to do the same thing with comprehensions?
sick salmon
Btw. is there a way to do the same thing with comprehensions?
Yes, there's a reduce option
Comprehensions are insanely powerful in elixir. Much more so than in, say, python
mild inlet
https://hexdocs.pm/elixir/Kernel.SpecialForms.html#for/1-the-reduce-option
The example in documentation is not showing how to set initial value for the accumulator
sick salmon
The example in documentation is not showing how to set initial value for the acc...
Yes it is, it's an empty map
mild inlet
so I should do acc -> {result, accumulator } ?
sick salmon
You need to give the tuple to reduce:
acc is the accumulator, so you'll need to pattern match out the tuple there and return a tuple from the body of the comprehension
So, {result, sum} -> something
Where something applies the computation and returns a new tuple
mild inlet
Like this?
initial = 10
for idx <- 1..10, reduce: {initial, []} do
{res, acc} -> some_fun(acc)
end
def some_fun(acc) do
{%{hello: "world"}, acc+1}
end
Well, acc in that instance is a list and you can't add to it with +
sick salmon
No, it's not. It's an empty list initially
Idx is a number
You don't seem to be doing anything with it
mild inlet
Correction
initial = 10
for idx <- 1..10, reduce: {[], initial} do
{res, acc} -> some_fun(acc, idx)
end
def some_fun(acc, idx) do
{%{hello: "world"}, acc+idx}
end
well it doesn't 😄
elfin hazel
It will not
sick salmon
Do you want to append a bunch of maps to the list?if you want to do that, you'll need to wither pass the list to the function and append in there, or take the return value of the function, pattern match out the parts and append the map to the list in the body of the for
mild inlet
Do you want to append a bunch of maps to the list?if you want to do that, you'll...
yes
elfin hazel
If you want to make it simple:
Enum.map_reduce(1..10, 0, &some_fun(&2, &1))
And it will do what you want
sick salmon
If you want to make it simple:
```elixir
Enum.map_reduce(1..10, 0, &some_fun(&2,...
They were interested in comprehensions
elfin hazel
They wanted to achieve what is in the question, it was latter sugested to use comprehesions, but first proposal was to use Enum.reduce/3 which was rejected only becasue @mild inlet didn't know how to do reduce over multiple values
elfin hazel
Ahhh, I missed it
sick salmon
And it can, but it's not as concise
elfin hazel
Indeed
mild inlet
Alright, I did it with Enum.map_reduce. Thanks for the help guys
sick salmon
I'm at a keyboard... Here's how you'd do it in a comp:
initial = 10
for idx <- 1..10, reduce: {[], initial} do
{res, acc} -> {[%{hello: "world"} | res], acc + idx}
end
You will need to reverse the result list afterwards
sick salmon
not in this example, since they're all the same 😉