Elixir - Convert a string number or empty string to a float or nil - ecto

Question

I am trying to convert the price field, which is a string (eg "2.22" or "") to a float or nil, and then add it to the database.
def insert_product_shop(conn, product_id, shop_id, price) do
priceFloat = nil
if price not in [""] do
price = elem(Float.parse(price), 0)
priceFloat = price / 1
IO.inspect(priceFloat)
else
priceFloat = nil
end
IO.inspect(priceFloat)
changeset = Api.ProductShop.changeset(%Api.ProductShop{
p_id: product_id,
s_id: shop_id,
price: priceFloat,
not_in_shop_count: 0,
is_in_shop_count: 0
})
errors = changeset.errors
valid = changeset.valid?
IO.inspect(changeset)
case insert(changeset) do
{:ok, product_shop} ->
{:ok, product_shop}
{:error, changeset} ->
{:error, :failure}
end
end
the output is:
2.22
nil
#Ecto.Changeset<action: nil, changes: %{}, errors: [], data: #Api.ProductShop<>,
valid?: true>
13:25:41.745 [debug] QUERY OK db=2.0ms
INSERT INTO "product_shops" ("is_in_shop_count","not_in_shop_count","p_id","s_id") VALUES ($1,$2,$3,$4) RETURNING "id" [0, 0, 40, 1]
As the output shows, priceFloat becomes nil, I assume because when I set it to 2.22 it was out of scope. Maybe my code is too imperative. How can I rewrite this to convert "2.22" to 2.22 without making it nil, and allow "" to be converted to nil?

Answer 1

As the output shows, priceFloat becomes nil, I assume because when I set it to 2.22 it was out of scope.
Almost right. Rather that the variable you are trying to set being out of scope, the problem is that the variable you assign to inside the if statement goes out of scope. It just happens to have the same name as the variable outside the if statement.
The solution is to assign the result of the if/else statement to the variable. Here is your code with minimal changes:
price = "2.22"
priceFloat =
if price not in [""] do
elem(Float.parse(price), 0)
else
nil
end
IO.inspect(priceFloat)
However, it's still not very idiomatic. You can take advantage of the fact that Float.parse/1 returns :error when the input is the empty string to write it like with a case expression:
priceFloat =
case Float.parse(price) do
{float, ""} -> float
:error -> nil
end

Answer 2

You can use case to evaluate the returned value by Float.parse and assign nil when it returns :error, assuming that the purpose of your if is to avoid the parsing error
def insert_product_shop(conn, product_id, shop_id, price) do
priceFloat = case Float.parse(price) do
{value, _remainder} -> value
:error -> nil
end
...
end

Answer 3

You can use a combination of pattern matching and method overloading to solve the problem:
defmodule Example do
def parsePrice(""), do: nil
def parsePrice(price) when is_float(price), do: price
def parsePrice(price) when is_binary(price) do
{float, _} = Float.parse(price)
float
end
end
Example.parsePrice(2.22) |> IO.inspect
Example.parsePrice("2.22") |> IO.inspect
(The equivalent is achievable using a case statement)
If you pass anything that is not a binary (a string) or a float to this function it will cause a pattern unmatched error. This may be good in case you have some error reporting in place, so you can detect unexpected usage of your code.
For a better debugging experience, I encourage you to use the built-in debugger via IEx.pry/0.

Answer 4

For the sake of diversity, I’d post another approach that uses with/1 special form.
with {f, ""} <- Float.parse("3.14"),
do: f,
else: (_ -> nil)
Here we explicitly match the float only. Any trailing garbage would be discarded. If the match succeeds, we return the float, otherwise, we return nil.
Beware of Float.parse/1 might be confused by garbage that looks like scientific notation.
(with {f, ""} <- Float.parse("3e14"), do: f) == 300_000_000_000_000
#⇒ true
Important sidenote: assigning priceFloat inside if does not change the value of the priceFloat variable outside of the scope. Scoping in elixir is pretty important and one cannot propagate local variables to the outermost scope, unlike most of the languages.
foo = 42
if true, do: foo = 3.14
IO.puts(foo)
#⇒ 42
Well, to some extent it’s possible to affect outermost scope variables from macros with var!/2, and if is indeed a macro, but this whole stuff is definitely far beyond the scope of this question.