I am trying to use .join on T: PartialEq, which is giving errors

help
#1

#[derive(Debug, PartialEq)]
pub enum Comparison {
Equal,
Sublist,
Superlist,
Unequal,
}

pub fn sublist<T: PartialEq>(_first_list: &[T], _second_list: &[T]) -> Comparison {
let _first_list_string = _first_list.join(" ");
println!("{}", _first_list_string);
}

#[test]
#[ignore]
fn huge_sublist_not_in_huge_list_2() {
let v1: Vec = vec![0; 1_000_000];
let mut v2: Vec = vec![0; 500_000];
v2.push(1);
assert_eq!(Comparison::Unequal, sublist(&v1, &v2));
}

I want to convert the &[T] into a big string using .join method but I am getting these issues.

the method join exists for reference &[T], but its trait bounds were not satisfied

method cannot be called on &[T] due to unsatisfied trait bounds

note: the following trait bounds were not satisfied:
[T]: std::slice::Join<_>rustc(E0599)

main.rs(157, 42): method cannot be called on &[T] due to unsatisfied trait bounds

the method join exists for reference &[T], but its trait bounds were not satisfied
the following trait bounds were not satisfied:
[T]: Join<_>rustcE0599

I am new to rust and not able to figure out what is going wrong... Can anyone please help me to convert the slice into string?

#2

What are you expecting to get from _first_list.join(" ")? The only thing you know about T is that you can compare it with itself, but join will have to somehow build a string - how?

2 Likes
#3

For instance i need output of vec![0; 1_000_000]; as "0 1 2 .... 1_000_000" but i am not able to figure out how to do it.

#4

If you want to be able to print for any arbitrary T, it must have a trait bound which allows it to be convertible to a string. Two common such traits are Debug and Display. For numbers, you can use either - in general if you want to print something Display makes more sense, while Debug is as the name suggests more for debug output.
So, in your case, you need a trait bound of T: PartialEq + Display. Then you can do something like:

let first_list_str = first_list.into_iter().cloned().map(Display::to_string).collect::<Vec<_>>(); 
let first = first_list_str.join(" ");
println!("{}", first);
2 Likes
#5

This will not work - one must either collect to the intermediate vector first, or use something like Itertools::join.

#6

Sorry, my bad.

#7

Thanks for the help I guess I understood that I need format the type and use it

#[derive(Debug, PartialEq)]
pub enum Comparison {
Equal,
Sublist,
Superlist,
Unequal,
}

pub fn rem_first_and_last(value: &str) -> &str {
let mut chars = value.chars();
chars.next();
chars.next_back();
chars.as_str()
}
pub fn form_string<T: std::fmt::Debug> (_first_list: &[T], _second_list: &[T]) -> bool {
let mut _first_list_string = format!("{:?}",_first_list);
let mut _second_list_string = format!("{:?}", _second_list);

_first_list_string = rem_first_and_last(_first_list_string.as_str()).to_string();
_second_list_string = rem_first_and_last(_second_list_string.as_str()).to_string();

return _second_list_string.contains(_first_list_string.as_str())

}

pub fn check_equal<T: std::fmt::Debug> (_first_list: &[T], _second_list: &[T]) -> bool {
let mut _first_list_string = format!("{:?}",_first_list);
let mut _second_list_string = format!("{:?}", _second_list);

_first_list_string = rem_first_and_last(_first_list_string.as_str()).to_string();
_second_list_string = rem_first_and_last(_second_list_string.as_str()).to_string();

return _second_list_string == _first_list_string.as_str()

}

pub fn sublist<T: std::fmt::Debug>(_first_list: &[T], _second_list: &[T]) -> Comparison {
if check_equal(_first_list, _second_list) {
return Comparison::Equal
} else if _first_list.len() < _second_list.len() && form_string(_first_list, _second_list) {
Comparison::Sublist
} else if _first_list.len() > _second_list.len() && form_string(_second_list, _first_list) {
Comparison::Superlist
} else {
Comparison::Unequal
}
}

This solved everything I need and I passed all the test cases thanks for the help!

You can check my question and solution on this Exercism