<Option<Vec<u64>>> from char.to_digit

help
1

I've got a start on a Luhn algorithm that takes a credit card number as string, as an impl of Digits trait

impl Digits for String {
    fn digits(self) -> Result<Vec<u32>, String> {
        self.chars()
            .map(|c| c.to_digit(10))
            .collect::<Option<Vec<u32>>>()
            .ok_or("Invalid Digits".to_string())
    }
}

pub trait Digits {
    fn digits(self) -> Result<Vec<u32>, String>;
}

It works, but I'm now trying to create trait implementation that allows me to pass an integer instead of a string. The problem I'm running into is that since

let ex2 = 4539148803436467; is literal out of range for u32

I need to let ex2: u64 = 4539148803436467;, which requires refactoring the Digits trait to return a result with Vec<u64>.

Obviously there are ways I can refactor that don't require me refactor the trait to

pub trait Digits {
    fn digits(self) -> Result<Vec<u64>, String>;
}

But now I'm curious to how I would do it. I've tried various ways of converting u32 to u64 for

        .map(|c| c.to_digit(10))
        .collect::<Option<Vec<u32>>>()

but I can't seem to figure it out.

2

.map(From::from) will convert iterated elements (it's same as .map(|x| x.into())).

1 Like
3 
c.to_digit(10).map(u64::from)

will convert the Option<u32> from to_digit into a Option<u64>

1 Like
4

thank you. i knew there was a simple way

5

thank you. nice and simple.

6

Can somebody please explain why that line works? char::to_digit returns an Option, but I thought you only can collect a Vec<Option<u32>>. How does it behave if one digit cannot be converted? Does it then early out? Is there any documentation for this?

7

An iterator of Options can be collected into an Option of a collection. If one of the options is None, the method stops early and returns None. The documentation for that is here.

Similarly, you can .collect() an Iterator<Item=Result<T, E>> into a Result<Vec<T>, E> and the method would stop at the first error. This is made possible by just the same trait.

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