Swift - for-in Loop

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Swift for-in Loop

The for-in loop iterates over collections of elements, such as ranges arrays, sets, dictionaries, etc. It iterates through each element of the specified collection without any explicit index manipulation. It is the most commonly used control flow statement because it expresses logic in a very clean and readable way.

It is also compatible with functional programming, which means you can easily use high-order functions like filter(), map(), reduce, etc with a for-in loop.

Syntax

Following is the syntax of the for-in loop −

for index in var{
   Statement(s)
}

Flow Diagram

The following flow diagram will show how the for-in loop works −

Example

Swift program to demonstrate the use of for-in loop.

import Foundation

var someInts:[Int] = [10, 20, 30]

for index in someInts {
   print( "Value of index is \(index)")
}

Output

It will produce the following output −

Value of index is 10
Value of index is 20
Value of index is 30

Swift for-in Loop with Underscore "_"

In a for-in loop, we can also ignore values from the given collection with the help of underscore "_". Here we use underscore "_" in place of variable Name in the for-in loop. It will ignore the current value while iterating. It is commonly used when we only want to iterate a given number of times and does not require the value from the collection.

Syntax

Following is the syntax of the for-in loop with underscore −

for _ in var{
   Statement(s)
}

Example

Swift program to demonstrate how to use for-in loop with underscore.

import Foundation

let numbers = [3, 5, 76, 12, 4]

// Using the underscore to ignore the values
for _ in numbers {

   // Following code will execute in each iteration
   print("Hello!! for-in loop!")
}

Output

It will produce the following output −

Hello!! for-in loop!
Hello!! for-in loop!
Hello!! for-in loop!
Hello!! for-in loop!
Hello!! for-in loop!

Swift for-in Loop with Ranges

We can also use range with a for-in loop. A range is the easiest way to represent a range of values in the for-in loop, it can be open, closed or half-open. The for-in loop iterates over each value in the given range and in each iteration, the loop variable takes the value of the current element in the range.

Syntax

Following is the syntax of the for-in loop with range −

// With a closed range
for variableName in start…end{
   Statement(s)
}

// With half-open range
for variableName in start..<end{
   Statement(s)
}

Example

Swift program to demonstrate how to use for-in loop with range.

import Foundation

// Using closed Range
print("Loop 1:")
for x in 1...6 {
   print(x)
}

// Using half-open Range
print("Loop 2:")
for y in 1..<6 {
   print(y)
}

Output

It will produce the following output −

Loop 1:
1
2
3
4
5
6
Loop 2:
1
2
3
4
5

Swift for-in Loop with stride() Function

We can use the stride() function with a for-in loop. The stride() function iterates over a range of values with a specific step between them. It is generally used to achieve more complex iteration patterns or can create a loop with a customized iteration pattern.

Syntax

Following is the syntax of the for-in loop with stride() function −

for variableName in stride(from: value, to: value, by: value){
   statement(s)
}

Parameters

The stride() function takes three parameters −

• from − It contains the starting value of the range.

• to − It contains the end value of the range. Its value does not include.

• by − It contains the number of steps or strides between the values. If its value is positive, then the stride iterates in an upward direction or if the value is negative, then the stride iterates in the downward direction.

Example

Swift program to demonstrate how to use for-in loop with stride() function.

import Foundation

// The stride() function moves in upward direction
print("Loop 1:")
for x in stride(from: 1, to: 7, by: 2) {
   print(x)
}

// The stride() function moves in downward direction
print("\nLoop 2:")
for y in stride(from: 7, to: 1, by: -2) {
   print(y)
}

Output

It will produce the following output −

Loop 1:
1
3
5

Loop 2:
7
5
3

Swift for-in Loop with where Clause

Swift also provides a where clause which we can use with a for-in loop. A where clause is used to filter out only those elements that satisfy the condition specified in the where clause from the given collection. It can work with all iterable collections such as arrays, sets, and dictionaries.

Syntax

Following is the syntax of the for-in loop with where clause −

for variableName in collectionName where condition{
   Statement(s)
}

Example

Swift program to demonstrate how to use for-in loop with where clause.

import Foundation
let nums = [43, 23, 66, 12, 2, 45, 33]

// Using the where clause to filter out only even elements from the array
for n in nums where n % 2 == 0 {
   print("Even number is \(n)")
}

Output

It will produce the following output −

Even number is 66
Even number is 12
Even number is 2