JKT JKT - 6 months ago 12
Swift Question

Removing the smaller sets of values from an array of arrays in Swift

Given an array that consists of arrays containing integers.

[[2], [3], [2, 2], [5], [7], [2, 2, 2], [3, 3]]


What would be the preferred way in Swift to remove the arrays that contains a smaller number of elements with a certain value and keep only the larger arrays containing that value.

The result from the input above would be

[[5], [7], [2, 2, 2], [3, 3]]

Answer

Use a [Int: [Int]] dictionary to keep track of the largest array for the value specified by the key.

let arrays = [[2], [3], [2, 2], [5], [7], [2, 2, 2], [3, 3]]
var largest = [Int: [Int]]()

for arr in arrays {
    // Get the first value from the array
    if let first = arr.first {

        // current is the count for that key already in dictionary largest
        // If the key isn't found, the nil coalescing operator ?? will
        // return the default count of 0.
        let current = largest[first]?.count ?? 0

        // If our new array has a larger count, put it in the dictionary
        if arr.count > current {
            largest[first] = arr
        }
    }
}

// Convert the dictionary's values to an array for the final answer.   
let result = Array(largest.values)

print(result)  // [[5], [7], [2, 2, 2], [3, 3]]

Alternate Version

This version uses a [Int: Int] dictionary to just keep the counts of the largest array found for each key, and then reconstructs the arrays at the end using an array constructor.

let arrays = [[2], [3], [2, 2], [5], [7], [2, 2, 2], [3, 3]]
var counts = [Int: Int]()

for arr in arrays {
    if let first = arr.first {
        counts[first] = max(counts[first] ?? 0, arr.count)
    }
}

let result = counts.map { [Int](count: $1, repeatedValue: $0) }

print(result)  // [[5], [7], [2, 2, 2], [3, 3]]
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