Quite ofter it is needed to loop through each element in an array or dictionary to do certain operation on each element. Swift provides few helper methods to make the operation simple.
The array is used to demo these helper methods
1. sorted, sorted(by:)
Sort an array elements based on default or custom comparison logic. This is a swift standard library function.
Sample sort by default order
let values = [2, -24, 5, 7]
let sortedArray = values.sorted()
print(sortedArray)
//
[-24, 2, 5, 7]
Sample sort by custom comparison
let sortedArray2 = values.sorted (by: { (a: Int, b: Int) -> Bool in
return a*a < b*b
})
print(sortedArray2)
//[2, 5, 7, -24]
As the parameter is defined by sorted(by:) function, so the type information can be omitted as
let sortedArray3 = values.sorted (by: { a, b in
return a*a < b*b
})
print(sortedArray3)
As the block body only as a single expression, the return clause can also be omitted
let sortedArray4 = values.sorted (by: { a, b in
a*a < b*b
})
print(sortedArray4)
In addition, the parameter can also referred by their index as $0, $1, $2 from the parameter list, so the expression can be simplified as
let sortedArray5 = values.sorted (by:
{ $0*$0 < $1*$1
})
print(sortedArray5)
2. map
For each item in the array, do some operation on it and then return a new array with the processed items.
let values = [2, -24, 5, 7]
let square = values.map ({ (item:Int) -> Int in
return item * item
})
print(square)
//output
//[4, 576, 25, 49]
as the function only as a single unnamed block argument, the (), and parameter type information can be simplified as
let square2 = values.map{ $0 * $0}
print(square2)
3. filter
Loop through the collection and return a new collection with the satisfied elements, the returned element type is same as original type
To get the positive even number from the above array
let filtered = values.filter { (item: Int) -> Bool in
let i = item % 2
return i == 0 && item > 0
}
print((filtered))
Simplified express looks like below
let filtered2 = values.filter {
let i = $0 % 2
return i == 0 && $0 > 0
}
print((filtered2))
4. reduce
reduce can be used to go through each element and accumulate the result from previous element, for example, to calculate the sum
The first parameter is the initial value, which does not need to be the same type as collection element
let output = values.reduce("the array is: ", {(ret, item: Int) in
return ret + (String(item)) })
print(output)
//the array is: 2-2457
Simplified express is
let output2 = values.reduce("the array is: "){$0 + (String($1)) }
print(output2)
