Swift Learning Plan x day--Array (initialization problem)

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Array, mainly used to save data, in Swift, the creation of arrays is more powerful than OC, but at the time of initialization, found a few problems, perhaps because of self-study reasons, through the search, reluctantly persuade themselves (mainly initializing the problem)

As follows:

var arr = ["abc", 123, 234.323]

arr + = ["value"]

Print(" appended array \(arr)")

Results

[ABC, 123, 234.323, AAA]

But

var mutableA2 = []

MutableA2 + = ["123"]

Really error, obviously I have insufficient skills, but write here, but think can, check memory address to see. Then I looked at the official document because there was no type defined?!

But if I want to define a mutable array like OC, then what to do is to not specify a fixed type, that is, an initialization of a mutable array, after looking at the official document,

var anotherthreedoubles = Array (count:3, repeatedvalue:2.5) This looks like the initialization of a variable array of OC.

Swift Learning Plan x day--Array (initialization problem)