How do I concatenate or Merge Arrays in Swift?

Swift provides us with two different ways to concatenate or merge arrays in the Swift language. 

You can use the + (plus) operator or the append() method. You will see other methods also of how you can merge multiple arrays in Swift. These methods are −

• Using plus (+) operator

• Using append(contentOf:) method

• Using flatMap() high-order function

• Using joined() method

• Using reduce() high-order function

Method 1: Merge Arrays Using The + Operator

Syntax

let outputArray = firstArray + secondArray

In order to use + operator to merge the arrays, simply it works as a binary operator that returns a newly merged array.

Algorithm

• Step 1 − Declare arrays

• Step 2 − Merge both arrays using the + operator

• Step 3 − Store the new merged array in another variable

Example

import Foundation
let firstArray: [String] = ["1", "2", "3", "4", "5"]
let secondArray: [String] = ["6", "7", "8", "9", "10"]
let outputArray = firstArray + secondArray
print("Array 1 = ",firstArray,"\nArray 2 = ",secondArray,"\nMerged Array =", outputArray)

Output

Array 1 =  ["1", "2", "3", "4", "5"] 
Array 2 =  ["6", "7", "8", "9", "10"] 
Merged Array ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]

Note that the + (plus) operator returns a newly created array that is the result of concatenation.

Example

Remember that the + (plus) operator takes care of type safety. It means you cannot perform this operator on two different types of arrays. If you do that, the compiler will give you an error like the one below −

import Foundation
let firstArray: [String] = ["1", "2", "3", "4", "5"]
let secondArray: [Int] = [6, 7, 8, 9, 10]
let outputArray = firstArray + secondArray
print(outputArray)

Output

Error

main.swift:4:30: error: binary operator '+' cannot be applied to operands of type '[String]' and '[Int]'
let outputArray = firstArray + secondArray

In the above example, you are trying to merge the strings' array and the integers' array which is not allowed.

Example

Merge more than two arrays −

In the following example we are going to merge 3 arrays in one array and print the output −

import Foundation
let firstArray: [Int] = [1, 2, 3]
let secondArray: [Int] = [4, 5, 6]
let thirdArray: [Int] = [7, 8, 9]
let mergedArray = firstArray + secondArray + thirdArray
print(mergedArray)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Method 2: Merge Arrays Using The Append(contentsOf:) Method

Syntax

firstArray.append(contentsOf: secondArray)

In order to append the content of an array, you have to call the append(contentsOf:) method with the target array.

Algorithm

Before moving to an example, you should note that this method does not return any new array. It appends the content of the target array to the caller array.

Example

import Foundation
var firstArray: [String] = ["1", "2", "3", "4", "5"]
let secondArray: [String] = ["6", "7", "8", "9", "10"]
firstArray.append(contentsOf: secondArray)
print(firstArray)

Output

["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]

You can see that firstArray is a variable (not a constant) to append the content of secondArray.

Example

You can also use the += operator to concatenate arrays, which is equivalent to calling the append(contentsOf:) method like below −

import Foundation
var firstArray: [String] = ["1", "2", "3", "4", "5"]
let secondArray: [String] = ["6", "7", "8", "9", "10"]
firstArray += secondArray

Example

Merge more than two arrays −

import Foundation
var firstArray: [Int] = [1, 2, 3]
let secondArray: [Int] = [4, 5, 6]
let thirdArray: [Int] = [7, 8, 9]
firstArray.append(contentsOf: secondArray)
firstArray.append(contentsOf: thirdArray)
print(firstArray)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Method 3: Merge Arrays Using FlatMap()

Syntax

In Swift, flatMap is a method on Sequence that transforms each element of the sequence by applying a transformation function and flattening the resulting sequence into a single sequence. The transformation function can return an optional value, and the method will automatically discard nil values.

let mergedArray = [firstArray, secondArray, thirdArray].flatMap({ $0 })

In order to use the flatMap() function, you need to pass an array of the combined array which results in the new merged array.

Algorithm

Here is an example of how flatMap can be used to merge multiple arrays −

• Step 1 − Initialize your arrays

• Step 2 − Combine all the arrays into one array 

• Step 3 − Call the flatMap() function on the combined array.

• Step 4 − Store the resulting array in a new variable.

Example

import Foundation
let firstArray: [Int] = [1, 2, 3]
let secondArray: [Int] = [4, 5, 6]
let thirdArray: [Int] = [7, 8, 9]
let mergedArray = [firstArray, secondArray, thirdArray].flatMap({ $0 })
print(mergedArray)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Method 4: Merge Arrays Using joined()

Syntax

let flattenCollection = [firstArray, secondArray, thirdArray].joined()
let mergedArray = Array(flattenCollection)

You can call the join function on a combined array which results in the elements being into a flat collection.

Now, you need to create a new array from a flat array to get all the elements into a single array.

Algorithm

• Step 1 − Initialize your arrays

• Step 2 − Join them using the joined() function

• Step 3 − Store the result from the joined function

• Step 4 − Convert the joined array using Array() into a single array

Example

This method returns the elements of this sequence of sequences, concatenated. For example,

import Foundation
let firstArray: [Int] = [1, 2, 3]
let secondArray: [Int] = [4, 5, 6]
let thirdArray: [Int] = [7, 8, 9]
let flattenCollection = [firstArray, secondArray, thirdArray].joined()
let mergedArray = Array(flattenCollection)
print(mergedArray)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Method 5: Merge Arrays Using reduce()

Syntax

This method returns the result of combining the elements of the sequence using the given closure.

let flattenArray = [firstArray, secondArray, thirdArray].reduce([]) { partialResult, element in
   partialResult + element
}

Basically, you have to pass combined arrays with reduce() function. In order to use the reduce() function, you need to assign an initial result which is the initialization of the result.

Algorithm

• Step 1 − Initialise your arrays

• Step 2 − Combine all the arrays into a single array

• Step 3 − Assign the initial result

• Step 4 − Merge each element into a partial result

Example

In this code, we are going to merge 3 arrays using reduce() function.

import Foundation
let firstArray: [Int] = [1, 2, 3]
let secondArray: [Int] = [4, 5, 6]
let thirdArray: [Int] = [7, 8, 9]
let flattenArray = [firstArray, secondArray, thirdArray].reduce([]) { partialResult, element in
   partialResult + element
}
print(flattenArray)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Conclusion

You learned about different methods to merge arrays in Swift. Each method can be used for a different purpose in different scenarios.