Swift array with mixed elements,why return different result

Question

var arr1: Array = [34, 8.7, "hello"]
var arr2: Array<Any> = [34, 8.7, "hello"]

println(arr1)
println(arr2)

[34, 8.699999999999999, hello] [34, 8.7, hello]

different result,anyone can explain it?

Floating point precision. The computer can represent numbers only to a certain precision. 8.7 and 8.6999999999999 are essentially the same, though I can only speculate why they are displayed differently. — Atomix
– Atomix, Commented Sep 17, 2014 at 13:48
As @vacawama has noted, you can simplify this question to "why do println(NSNumber(double: 8.7)) and println(Double(8.7)) give different results?" — Matt Gibson
– Matt Gibson, Commented Sep 17, 2014 at 15:50

vacawama · Accepted Answer · 2014-09-17 16:31:57Z

4

In the case of arr1, since you didn't specify the type and you have Foundation imported, Swift made the array type [NSObject]. Then, the types of 34, 8.7, and "hello" are NSNumber, NSNumber, and NSString respectively.

For whatever reason, an NSNumber with a value of 8.7 prints as 8.699999999999999. Try this is a Playground:

var a: NSNumber = 8.7
println(a)            // prints 8.699999999999999

In arr2, the values have the types Int, Double, and String, so the 8.7 prints as you would expect:

var b: Double = 8.7
println(b)           // prints 8.7

As Matt Gibson states in the comments:

the output difference may be because NSNumber's description method formats the number as "%0.16g"; if you do NSString(format: "%0.16g", 8.7) gives you "8.699999999999999". I'm sure they're both just the same double "underneath".

To test the theory that they are the same number underneath, I did:

if (arr1[1] as NSNumber).doubleValue == (arr2[1] as Double) {
    println("same")
}

and this does indeed print "same".

edited Sep 17, 2014 at 16:31

answered Sep 17, 2014 at 14:08

vacawama

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1 Comment

Matt Gibson Over a year ago

I think the output difference may be because NSNumber's description method formats the number as "%0.16g"; if you do NSString(format: "%0.16g", 8.7) gives you "8.699999999999999". I'm sure they're both just the same double "underneath".

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