Swiftのループチットシート (日本語)
This is a cheat sheet of the loops available in Swift 5.0+
For
Since Swift 3.0, C-style for loops have been deprecated! But I include them for people who are migrating old code bases.
Loop over Ranges with For-In
Looping n times
Deprecated C-style version:
for var i = 0; i < 10; i++ {
print(i)
}
for-in over a closed range with the ... (closed range) operator :
for i in 0...10 {
print(i)
}
for-in over a half-open range with the ..< (half-open range) operator :
for i in 0..<10 {
print(i)
}
Looping n times in reverse
Deprecated C-style version:
for var i = 10; i > 0; i-- {
print(i)
}
for-in over a closed range that has been reversed using reversed():
for i in (1...10).reversed() {
print(i)
}
Looping with Stride
Deprecated C-style version:
for var i = 0; i < 10; i += 2 {
print(i)
}
for-in over a half-open range, stepping by the specified amount using stride(from:to:by:):
for i in 0.stride(to: 10, by: 2) {
print(i)
}
Loop over through Arrays
Looping through Array Values
Deprecated C-style version:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for var i = 0; i < someNumbers.count; i++ {
print(someNumbers[i])
}
for-in over all the values of an Array:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for number in someNumbers {
print(number)
}
forEach over all the values of an Array:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
someNumbers.forEach { number in
print(number)
}
Reverse Looping through Array Values
Deprecated C-style version:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for var i = someNumbers.count - 1; i >= 0; i-- {
print(someNumbers[i])
}
for-in over all the values of an Array that has been reversed using reversed():
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for number in someNumbers.reversed() {
print(number)
}
forEach over all the values of an Array that has been reversed using reversed():
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
someNumbers.reversed().forEach { number in
print(number)
}
Looping Through an Array with Index
Deprecated C-style version:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for var i = 0; i < someNumbers.count; i++ {
print("\(i + 1): \(someNumbers[i])")
}
for-in over all the indexes and values of an Array enumerated using enumerated():
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for (index, number) in someNumbers.enumerated() {
print("\(index + 1): \(number)")
}
forEach over all the indexes and values of an Array enumerated using enumerated():
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
someNumbers.enumerated().forEach { (index, number) in
print("\(index + 1): \(number)")
}
Looping Through Array Indices
Deprecated C-style version:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for var i = 0; i < someNumbers.count; i++ {
print(i)
}
for-in over all the indexes of an Array enumerated using indices:
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
for index in someNumbers.indices {
print(index)
}
Looping Through a Dictionary
for-in over all the indexes and values of a Dictionary:
let scores = ["Bob": 42, "Alice": 99, "Jane": 13]
for (name, score) in scores
{
print("\(name)'s score is \(score)")
}
While
Repeats code until the condition evaluated at the start of the loop becomes false.
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
var i = 0
while(i < someNumbers.count) {
print(someNumbers[i])
i += 1
}
Repeat-While
Repeats code until the condition evaluated at the end of the loop becomes false.
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
var i = 0
repeat {
print(someNumbers[i])
i += 1
} while(i < someNumbers.count)
Break
Terminates the current iteration of the loop and the flow control of the program exits the loop.
unlabeled break
Terminates the current iteration and exits the loop:
for i in 1...4{
if i == 3 {
break
}
print("i = \(i)")
}
}
Outputs:
i = 1
i = 2
labeled break
When using nested loops with a labeled break statement, terminates the current iteration of the labeled loop and exits the labeled loop:
outerloop: for i in 1...3{
innerloop: for j in 1...3 {
if j == 2 {
break outerloop
}
print("i = \(i), j = \(j)")
}
}
Outputs:
i = 1, j = 1
Continue
Skips the current iteration of the loop and the flow control of the program goes to the next iteration.
unlabeled continue
Skips the current iteration of the loop and goes to the next:
for i in 1...4{
if i == 3 {
continue
}
print("i = \(i)")
}
}
Outputs:
i = 1
i = 2
i = 4
labeled continue
When using nested loops with a labeled continue statement, skips the current iteration of the labeled loop and goes to the next:
outerloop: for i in 1...3{
innerloop: for j in 1...3 {
if j == 2 {
continue outerloop
}
print("i = \(i), j = \(j)")
}
}
Outputs:
i = 1, j = 1
i = 2, j = 1
i = 3, j = 1
Higher-order functions
If you want to manipulate arrays, for instance to apply a function to each of the array elements, or to reduce the array to one result, then you should use the higher-order functions.
Array.filter(_:)
Iterates over all items in array, and return an array containing all the elements filtered by a condition.
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
let numbersUnder10 = someNumbers.filter { $0 < 10 }
print(numbersUnder10)
Array.map(_:)
Iterates over all items in array, transforming them and return an array containing the transformed elements.
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
let doubledNumbers = someNumbers.map { $0 * 2}
print(doubledNumbers)
Array.reduce(_:_:)
Iterates over all items in array, combining them together until you end up with a single value.
let someNumbers = [2, 3, 45, 6, 8, 83, 100]
let sum = someNumbers.reduce(0) { $0 + $1}
print(sum)