How can I sort by a method that returns Boolean in Kotlin? I was using `Comparator` which returns an...

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[Joan Colmenero]

How can I sort by a method that returns Boolean in Kotlin? I was using Comparator which returns an Int but now I need more complex stuff, so I have a method that returns True or False depending of a check, is there any way I'd use this method to sort it? Is getting Foo1 Foo2 as a parameter and then do the comparation.

[diesieben07]

You can use the compareBy helper probably, which creates a comparator.
Example:
val comparator = compareBy<String> { it[0].isUpperCase() }
This is a Comparator, which sorts Strings by whether they start with an upper case letter or not.
compareBy can also take multiple lambdas and will check them in order (if first one compares them as equal it will check the 2nd one and so on)

[Joan Colmenero]

doing it[0] and it[1] I'm comparing 2 Strings then?

[diesieben07]

No, with compareBy you are not explaining how to compare, you explaining what to compare. In this case I am saying "compare by whether or not the first character (it[0]) is upper case".

[Joan Colmenero]

Can you check my second message?

[Joan Colmenero]

That's what I have and I'd like to sort depending of it

[diesieben07]

Well, a boolean is not enough to determine whether an element is greater or smaller than another

[diesieben07]

Given two elements, you need to determine which of the two is larger

[diesieben07]

You can't do that with just a boolean

[diesieben07]

3 cases:
both equal
first larger
second larger

[Joan Colmenero]

But if I'm checking 2 different values how I'm supposed to retutnr equal first or second? Like this :

else {
            (foo1.first()
                    || (foo1.second() && !foo2.third())
                    || (foo1.fourth() && foo2.fifth()))
        }

[diesieben07]

I don't know, you need to decide on which order you want. If that is not clear, you can't start implementing a comparator.

[diesieben07]

And an order always means you can clearly decide which element of two is bigger

[Joan Colmenero]

I know the order

[Joan Colmenero]

Is that one I sent to you

[diesieben07]

I don't see an order...?

[diesieben07]

An order needs to return three possible values:
• a is greater
• b is greater
• a == b

[diesieben07]

You cannot do that by returning a boolean.

[Joan Colmenero]

But why I can not use Boolean.compare(something==somethingelse)

[Joan Colmenero]

as a return?

[diesieben07]

You can. But it returns an Int, not a Boolean.

[Joan Colmenero]

Yes, but comparator needs an int

[Joan Colmenero]

So perhaps I can do this :S

[diesieben07]

Yes, that's what I have been saying all along...

[Joan Colmenero]

Why I can not use Boolean.compareTo()?

[Joan Colmenero]

is in red

[diesieben07]

firstBoolean.compareTo(otherBoolean)

[Joan Colmenero]

No, I mean can I return this?

foo1.endDate <= foo2.endDate

[Joan Colmenero]

Converting to an int?

[diesieben07]

No, that's a Boolean again, not an Int.

[diesieben07]

It doesn't make sense to convert that to an int.
The int needs to be one of -1, 0 and 1.
A boolean is just true and false...

[diesieben07]

If you want to sort by end date you need to do foo1.endDate.compareTo(foo2.endDate)

[diesieben07]

OR you can use the compareBy helper:
compareBy { it.endDate }

[diesieben07]

Which is equivalent.

[Joan Colmenero]

I have a List<Foo>

[Joan Colmenero]

And I want to use the compareBy there

[diesieben07]

Either:

val comparator = compareBy<Foo> { it.endDate }
list.sortedWith(comparator)

Or:

list.sortedBy { it.endDate }

Same thing

[Joan Colmenero]

And then can I do more operations inside of sortedBy

[diesieben07]

You can do however much you want in there, yes. In the end it needs to produce something comparable.
The list is then sorted by whatever you produce there.
If you just do { it.endDate } it just sorts by end date. But you can do complex calculations as well, it will then sort by those.

[Joan Colmenero]

But that's the thing I can only sort checking one it, I want to compare it with other one like I'm doing on my comparator check

object EndCompi : Comparator<Foo> {
        override fun compare(foo1: Foo, foo2: Foo): Int {
            return foo1.endDate.compareTo(foo2.endDate)
        }
    }

Then calling it.sortedWith(EndCompi)

[diesieben07]

Okay, then do that...? I am not sure what your question is...

[Joan Colmenero]

Thing is, if I have to do something like this :

(foo1.first()
                    || (foo1.second() && !foo2.third())
                    || (foo1.fourth() && foo2.fifth()))
        }

[Joan Colmenero]

I'm lost because sortedBy can only check the current it

[Joan Colmenero]

Can not check for it1.first() or it1.second && !it2.third

[diesieben07]

Then use a comparator... And not the sortedBy helper.

[diesieben07]

Then you get two values, but you need to be explicit about the result (-1 => foo1 is smaller, 0 => they are equal, 1 => foo2 is smaller)

[Joan Colmenero]

Ok... let me check