Elixir: to assign variable in for generator(variable scope?) - for-loop

Question

I'm solving, find the largest prime factor of the number, Project Euler problem3.
Following Elixir code throw warnings, and do not evaluate in if block(assigning) I think:
num = 13195
range = num
|> :math.sqrt
|> Float.floor
|> round
for dv <- 2..range do
if rem(num, dv) == 0 and div(num, dv) != 1 do
num = div(num, dv)
end
end
num
|> IO.puts
Warnings are:
$ elixir 3.exs
warning: variable "num" is unused
3.exs:10
warning: the result of the expression is ignored (suppress the warning by assigning the expression to the _ variable)
3.exs:10
13195
$ elixir -v
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
Elixir 1.5.3
How can I update(assign) the num?
(following Python and Javascript codes are work for same the problem):
# 3.py
from math import ceil, sqrt
num = 600851475143
for div in range(2, ceil(sqrt(num)) + 1):
if num%div == 0 and num/div != 1:
num /= div
assert int(num) == 6857
// 3.js
var num = 600851475143;
var range = Array.from({length: Math.trunc(Math.sqrt(num))}, (x, i) => i + 2)
for (const div of range) {
if (num%div === 0 && num/div != 1) {
num /= div;
}
}
var assert = require('assert');
assert(num === 6857)

Answer 1

You are actually creating a new variable and shadowing the one from outer scope
You can rewrite it like this
num = 13195
range =
num
|> :math.sqrt()
|> Float.floor()
|> round
num =
2..range
|> Enum.reduce(num, fn elem, acc ->
if rem(acc, elem) == 0 and div(acc, elem) != 1 do
div(acc, elem)
else
acc
end
end)
IO.puts num
More on shadowing:
+------------------------------------------------------------+
| Top level |
| |
| +------------------------+ +------------------------+ |
| | Module | | Module | |
| | | | | |
| | +--------------------+ | | +--------------------+ | |
| | | Function clause | | | | Function clause | | |
| | | | | | | | | |
| | | +----------------+ | | | | +----------------+ | | |
| | | | Comprehension | | | | | | Comprehension | | | |
| | | +----------------+ | | | | +----------------+ | | |
| | | +----------------+ | | ... | | +----------------+ | | |
| | | | Anon. function | | | | | | Anon. function | | | |
| | | +----------------+ | | | | +----------------+ | | |
| | | +----------------+ | | | | +----------------+ | | |
| | | | Try block | | | | | | Try block | | | |
| | | +----------------+ | | | | +----------------+ | | |
| | +--------------------+ | | +--------------------+ | |
| +------------------------+ +------------------------+ |
| |
+------------------------------------------------------------+
Any variable in a nested scope whose name coincides with a variable from the surrounding scope will shadow that outer variable. In other words, the variable inside the nested scope temporarily hides the variable from the surrounding scope, but does not affect it in any way.
source