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How to transform a dictionary using mapValues()

Swift version: 5.0

Paul Hudson    @twostraws   

Although dictionaries have a general map() method, they also have a specialized form of map() called mapValues() – it transforms just the values of the dictionary, leaving the keys untouched.

This extra method is useful because dictionaries can’t have duplicate keys, but if you’re only transforming the values from a dictionary then this is not a problem.

Let’s try it out on a dictionary containing the height in centimeters of various people:

let peopleMetric = ["Taylor": 178.0, "Justin": 175.0, "Ed": 173.0]

If we want to convert those heights to inches without changing the keys, we can use mapValues() like this:

let peopleImperial = peopleMetric.mapValues { $0 / 2.54 }

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