Swift Sort an array with multiple sort criteria [d

问题:

This question already has an answer here:

• How do I sort an array of structs by multiple values? 3 answers

How is an array sorted by multiple criteria in Swift? For example, an array of dictionaries as shown:

items = [
    [
        "item":"itemA"
        "status":"0"
        "category":"B"
    ],[
        "item":"itemB"
        "status":"1"
        "category":"C"
    ],[
        "item":"itemC"
        "status":"0"
        "category":"A"
    ],[
        "item":"itemD"
        "status":"2"
        "category":"A"
    ]
]

This needs to be sorted as follows:

1. category ASC
2. status DESC

I have successfully sorted this array based on either condition 1 OR 2, but not both. Below is the code for that:

itemArray.sort({
        $0["category"] < $1["category"])
    })

How can this be expanded to include multiple sort criteria in a given order?

回答1:

You want a lexicographic comparison i.e. if the first fields are equal, compare the second fields, otherwise compare the first fields.

The dictionary complicates it a bit since you don’t want to fetch the data out twice and keys may be missing, but that’s actually fine since == and < can handle optional comparisons.

let result = items.sorted {
    switch ($0["category"],$1["category"]) {
    // if neither “category" is nil and contents are equal,
    case let (lhs,rhs) where lhs == rhs:
        // compare “status” (> because DESC order)
        return $0["status"] > $1["status"]
    // else just compare “category” using <
    case let (lhs, rhs):
        return lhs < rhs
    }
}

There’s actually a lexicographicCompare that you could use under some circumstances to do the comparison – but it won’t work in this case because a) optionals, while they can be compared with <, don’t conform to Comparable so you’d have to check for nils manually, and b) you want to sort the second entry in descending order.