Inserting element in [String] array changes its type

Question

struct Numbers {
    var list: [String] = [
        "one",
        "two",
        "three"
    ]
    
    func returnList() -> [String] {
        var sortedList: [String] = self.list.sorted()
        var finalList: [String] = sortedList.insert("four", at: 0)
        return finalList
    }
}

finalList is inferred to be a () instead of [String]. If I specify the type [String] I get the message:

Cannot convert value of type '()' to specified type '[String]'

Why on earth?

Because insert returns Void aka () and modifies the array inplace? Just remove the finalList variable and return sortedList. — Clashsoft
– Clashsoft, Commented Apr 16, 2021 at 7:03
See this very similar question about append rather than insert, or this question about subtract. — Sweeper
– Sweeper, Commented Apr 16, 2021 at 7:04

matt · Accepted Answer · 2021-04-16 07:04:49Z

0

You’ve misunderstood the error message.

The problem is that sortedList.insert does not return a value. (We express this by saying it returns ().) Hence it cannot be assigned to finalList: [String], for the simple reason that it is not a [String].

So just change

var sortedList: [String] = self.list.sorted()
var finalList: [String] = sortedList.insert("four", at: 0)
return finalList

To

var sortedList: [String] = self.list.sorted()
sortedList.insert("four", at: 0)
return sortedList

answered Apr 16, 2021 at 7:04

matt

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1 Comment

matt Over a year ago

I grant that it can be a little confusing knowing which Swift methods return a value and which ones operate directly on a mutable target. Your code exemplifies both.

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