Why duplicates when adding dictionary to array

Question

I'm new to Swift and experimenting. Trying to create a simple array of dictionary values. This is crude, but I simply replicated a short piece of code four times for the dictionary objects, changed values in each section and then added each of the four to the array.

When I print the array, I am finding it contain four versions of the last dictionary object. How can that be? I would have expected the array for have four dictionary objects i.e one of each of the objects added.

func countDown()  {
    let dict: NSMutableDictionary = [:]
    let enduroArrayFile = NSMutableArray()

    dict.setObject(1, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict.setObject(1.2, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict.setObject(2, forKey: "SectionDistance")
    dict.setObject("R", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict.setObject(2.1, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    print (enduroArrayFile)
}

The output is

(
    {
    ArrivalTime = 0;
    AverageSpeed = 0;
    Direction = S;
    SectionDistance = "2.1";
},
    {
    ArrivalTime = 0;
    AverageSpeed = 0;
    Direction = S;
    SectionDistance = "2.1";
},
    {
    ArrivalTime = 0;
    AverageSpeed = 0;
    Direction = S;
    SectionDistance = "2.1";
},
    {
    ArrivalTime = 0;
    AverageSpeed = 0;
    Direction = S;
    SectionDistance = "2.1";
    }
)

Because NSDictionary is a reference type. You create an array with 4 reference to the same one-and-only dictionary. — Martin R
– Martin R, Commented Sep 16, 2016 at 21:27
Consider to use Swift value types instead: Dictionary, Array, ... — Martin R
– Martin R, Commented Sep 16, 2016 at 21:33

Leo Dabus · Accepted Answer · 2016-09-16 21:59:19Z

4

You are mistakenly reusing dict over and over. You need to create a new instance each time.

func countDown()  {
    let enduroArrayFile = NSMutableArray()

    var dict: NSMutableDictionary = [:]
    dict.setObject(1, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict = [:]
    dict.setObject(1.2, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict = [:]
    dict.setObject(2, forKey: "SectionDistance")
    dict.setObject("R", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    dict = [:]
    dict.setObject(2.1, forKey: "SectionDistance")
    dict.setObject("S", forKey: "Direction")
    dict.setObject(0, forKey: "ArrivalTime")
    dict.setObject(0, forKey: "AverageSpeed")
    //saving dictionary to array
    enduroArrayFile.addObject(dict)

    print (enduroArrayFile)
}

I don't know Swift well so I may have gotten the syntax wrong.

edited Sep 16, 2016 at 21:59

Leo Dabus

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answered Sep 16, 2016 at 21:29

rmaddy

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2 Comments

Jeremy Over a year ago

Thanks. You nailed it. I was thinking along similar lines but I would have expected to see the first dictionary values repeated four times, not the last set of values. Seeing the last set four times has me totally confused. Any idea why?

rmaddy Over a year ago

Sure. The dictionary's values are being replaced over and over. The last one wins.

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