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iOS

Posted by swiftfun
swiftfun
4,039 Points

Dictionary Quiz - Is it wrong?

The quiz says the following statement is FALSE:

"The following code snippets are equivalent:"

 dict.updateValue("yetAnotherValue", forKey: 3)

and

dict[3] = "yetAnotherValue"

Why is it false? Given the context, both can be true. For instance in the following dictionary, both code snippets are TRUE.

var airportNumbers: [Int: String] = [
    1: "LGA",
    2: "HTR",
    3: "MCO"
]

airportNumbers.updateValue("yetAnotherValue", forKey: 3)

airportNumbers[3] = "yetAnotherValue"

Please let me know your thoughts.

Caleb Kleveter
Caleb Kleveter
Treehouse Moderator 37,862 Points

Can you post a link to the quiz?

3 Answers

Tommy Choe
Tommy Choe
38,156 Points
Tommy Choe
Tommy Choe
38,156 Points

So they're similar but NOT equivalent. The updateValue function actually RETURNS the value that was previously stored for the associated key whereas the subscript notation does not. The subscript notation simply replaces the value.

Hope that helps.

swiftfun
swiftfun
4,039 Points
swiftfun
swiftfun
4,039 Points

https://teamtreehouse.com/library/swift-20-collections-and-control-flow/dictionaries/recap-dictionaries

I assume the answer relies on the fact that the first snippet contains an "update" function. Nonetheless I still think, given the context, both expressions could be equivalents.

swiftfun
swiftfun
4,039 Points
swiftfun
swiftfun
4,039 Points

Still did not quite get what "returns" means.

What does returning means? Does it store ot somewhere? Please provide a practical example when developing an app

Tommy Choe
Tommy Choe
38,156 Points

Ok let me use the example that you provided.

Under the hood, when you invoke the updateValue method, the value stored in the previous key, "MCO" is first updated to the new value "yetAnotherValue". Then the method actually returns "MCO" so that you can store it a constant or variable like this:

let result = airportNumbers.updateValue("yetAnotherValue", forKey: 3)
print(result)  //"MCO"

Now you have the old value stored in result. At this point you can let the user know what value got replaced etc. etc.