Swift Tutorial Swift References

Swift - For-In Loop



The for-in loop in Swift is used to iterate over a given sequence and executes a set of statements for each element in the sequence. A sequence can be anything like array, string, range of numbers, tuple, set, and dictionary etc.

Flow Diagram:

Swift For-In Loop

Iterating over a range of numbers

In the example below, the program uses for-in to iterate over range of numbers.

//iterating over a closed range to
//print all elements in the range
print("iterating over closed range")
for i in (1...4) {
  print("i = \(i)")
}

//iterating over a half-open range to
//print all elements in the range
print("\niterating over half-open range")
for j in (1..<4) {
  print("j = \(j)")
}

The output of the above code will be:

iterating over closed range
i = 1
i = 2
i = 3
i = 4

iterating over half-open range
j = 1
j = 2
j = 3

Iterating using stride() function

The stride(from: to: by:) or stride(from: through: by:) function can be used to skip the unwanted marks.

//stride(from: to: by:) creates half-open range
print("for-in loop using stride(from: to: by:)")
for i in stride(from:1, to:9, by: 2) {
  print("i = \(i)")
}

//stride(from: through: by:) creates closed range
print("\nfor-in loop using stride(from: through: by:)")
for j in stride(from:1, through:9, by: 2) {
  print("j = \(j)")
}

The output of the above code will be:

for-in loop using stride(from: to: by:)
i = 1
i = 3
i = 5
i = 7

for-in loop using stride(from: through: by:)
j = 1
j = 3
j = 5
j = 7
j = 9

Iterating over an Array

In the example below, the for-in is used to iterate over an array.

var MyArray:[Int] = [10, 20, 30, 40]

//iterating over an array to print
//all its elements
print("MyArray contains:")
for i in MyArray {
  print(i)
}

The output of the above code will be:

MyArray contains:
10
20
30
40

Iterating over a Dictionary

In the example below, the for-in is used to iterate over a dictionary to access all key/value pairs.

var MyDict:[Int:String] = [1:"MON",
                           2:"TUE",
                           3:"WED",
                           4:"THU",
                           5:"FRI"]

//iterating over a dictionary to print
//all its elements
print("MyDict contains:")
for (key, value) in MyDict {
  print("\(key) = \(value)")
}

The output of the above code will be:

MyDict contains:
5 = FRI
2 = TUE
3 = WED
4 = THU
1 = MON

When no iterating variable is required

Use _, if no iterating variable is required. Consider the example below:

var x = 2
var result = 1
var power = 10

//use _ when no iterating variable is required
for _ in 1...power {
  result *= x
}

print("\(x) raised to the power of \(power) = \(result)")

The output of the above code will be:

2 raised to the power of 10 = 1024

5