loops.md

Published by Arunprasadh C on 04 May 2022 • Last Updated on 16 May 2022

Loops in Swift

As in any other language, Loops in Swift are used to execute a set of statements repeatedly until a given condition is true or until a termination condition or control statement is not reached. Swift supports three kind of loops :

for-in Loop

In Swift, the for-in loop is used to run a block of code for a certain number of times. It is used to iterate over any sequences such as an Array, Range, String, etc.

Syntax :

for val in sequence
{
  // statements
}

Here, val accesses each item of the sequence on each iteration. Loop continues until we reach the last item in the sequence.

Example 1:

let languages = ["Swift", "Java", "Kotlin", "C++"]
for language in languages
{
    print(language, terminator: ", ")
}

Output 1:

Swift, Java, Kotlin, C++,

The where clause can be used to add conditions to filter out the elements to be iterated.

Example 2:

let languages = ["Swift", "Java", "Kotlin", "C++"]

for language in languages where language != "C++"
{
    print(language, terminator: ", ")
}

Output 2:

Swift, Java, Kotlin,

We can also use Range in for-in Loop. But, if we want to increment by some fixed amount instead of 1, we can use the stride() function.

Example 3:

print("Even numbers from 2 to 20 : ", terminator: "")
for evenNum in stride(from: 2, to: 21, by: 2)
{
    print(evenNum, terminator: ", ")
}

Output 3:

Even numbers from 2 to 20 : 2, 4, 6, 8, 10, 12, 14, 16, 18, 20,

The for-in loop uses let constants by default for iterating elements. So, if we want mutable elements, we have to mention var in for-in loop.

Example 4:

var intArr = [1, 2, 3, 4, 5, 6]
print("Original Array: \(intArr)")
for var i in intArr
{
    print("Original i: \(i)")
    i *= 20
    print("Modified i: \(i)")
}
print("Modified Array: \(intArr)")

Output 4:

Original Array: [1, 2, 3, 4, 5, 6]
Original i: 1
Modified i: 20
Original i: 2
Modified i: 40
Original i: 3
Modified i: 60
Original i: 4
Modified i: 80
Original i: 5
Modified i: 100
Original i: 6
Modified i: 120
Modified Array: [1, 2, 3, 4, 5, 6]

However, the original array will remain unaffected. If you want to modify the original array inside the loop, you have to use indices.

Example 5:

var intArr = [1, 2, 3, 4, 5, 6]
print("Original Array: \(intArr)")
for i in intArr.indices
{
    intArr[i] = 20*intArr[i]
}
print("Modified Array: \(intArr)")

Output 5:

Original Array: [1, 2, 3, 4, 5, 6]
Modified Array: [20, 40, 60, 80, 100, 120]

while Loop

In Swift, the while loop is used to run a specific code as long as a certain condition is met.

Syntax :

while(condition)
{
//loop body
}

Example :

var it = 2
print("Even numbers from 2 to 20 : ", terminator: "")
while(it <= 20)
{
    print(it, terminator: ", ")
    it += 2
}

Output :

Even numbers from 2 to 20 : 2, 4, 6, 8, 10, 12, 14, 16, 18, 20,

repeat-while Loop

In Swift, the repeat-while loop is similar to while loop with one key difference. The body of repeat-while loop is executed at least once before the test expression is checked (It is similar to do-while loop of C/C++/Java).

Syntax :

repeat 
{
  // body of loop
} while (condition)

Example :

var it = 22
print("Even numbers from 2 to 20 : ", terminator: "")
repeat
{
    print(it, terminator: ", ")
    it += 2
} while (it <= 20)

Output :

Even numbers from 2 to 20 : 22, //22 is shown although 22 > 20 as it is a repeat-while loop

A while or repeat-while loop should be used when the exact number of iterations is not known. When the number of iterations is known beforehand or when a sequence needs to be iterated, the for-in loop should be used.

Infinite while loop

An infinite while loop will keep on executing inifinitely until the program is terminated explicitly. It can be constructed by giving an always true condition and by not providing any loop control statements inside the body. However, giving a termination condition or using a loop constrol statement is required in realtime situations.

Example :

var count = 0
while(true)
{
    print(count)
    count += 1
}

Output :

0
1
2
3
.
.
.
.

As we have seen about Loops in Swift, let's move on to see about Collections in Swift.

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