check if all elements of an array have the same value in Swift

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With Swift 5, you can use one of the four following ways in order to tests if all elements of an array are equal.

#1. Using Array‘s allSatisfy(_:) method

allSatisfy(_:) returns a Boolean value indicating whether every element of a sequence satisfies a given predicate. You can set the predicate to test if all elements of the array are equal:

let array = [1, 1, 1]

let hasAllItemsEqual = array.dropFirst().allSatisfy({ $0 == array.first })
print(hasAllItemsEqual) // prints: true

let array = [1, 1, 3]

let hasAllItemsEqual = array.dropFirst().allSatisfy({ $0 == array.first })
print(hasAllItemsEqual) // prints: false

let array = [Int]()

let hasAllItemsEqual = array.dropFirst().allSatisfy({ $0 == array.first })
print(hasAllItemsEqual) // prints: true

#2. Using Array‘s reduce(_:_:) method

As an alternative to allSatisfy(_:), you can use reduce(_:_:):

let array = [1, 1, 1]

let hasAllItemsEqual = array.dropFirst().reduce(true) { (partialResult, element) in
    return partialResult && element == array.first
}
print(hasAllItemsEqual) // prints: true

let array = [1, 1, 3]

let hasAllItemsEqual = array.dropFirst().reduce(true) { (partialResult, element) in
    return partialResult && element == array.first
}
print(hasAllItemsEqual) // prints: false

let array = [Int]()

let hasAllItemsEqual = array.dropFirst().reduce(true) { (partialResult, element) in
    return partialResult && element == array.first
}
print(hasAllItemsEqual) // prints: true

#3. Using elementsEqual(_:) method

elementsEqual(_:) returns a Boolean value indicating whether two sequences contain the same elements in the same order. Therefore you can create a new collection by repeating the first element of the initial array and compare the former with the latter:

let array = [1, 1, 1]

precondition(!array.isEmpty)
let repeated = repeatElement(array[0], count: array.count)

let hasAllItemsEqual = array.elementsEqual(repeated)
print(hasAllItemsEqual) // prints: true

let array = [1, 1, 3]

precondition(!array.isEmpty)
let repeated = repeatElement(array[0], count: array.count)

let hasAllItemsEqual = array.elementsEqual(repeated)
print(hasAllItemsEqual) // prints: false

#4. Using Set‘s init(_:) initalizer

If all elements of an array are equal, creating a set from this array should result in the set having only one element:

let array = [1, 1, 1]

let set = Set(array)
let hasAllItemsEqual = set.count <= 1
print(hasAllItemsEqual) // prints: true

let array = [1, 1, 3]

let set = Set(array)
let hasAllItemsEqual = set.count <= 1
print(hasAllItemsEqual) // prints: false

let array = [Int]()

let set = Set(array)
let hasAllItemsEqual = set.count <= 1
print(hasAllItemsEqual) // prints: true

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