Swift while and repeat while Loop

In programming, loops are used to repeat a block of code. For example, if you want to show a message 100 times, then you can use a loop. It's just a simple example, you can achieve much more with loops.

In the previous tutorial, you learned about the Swift for-in Loop. Here, you are going to learn about while and repeat...while loops.

Swift while Loop

Swift while loop is used to run a specific code until a certain condition is met.

The syntax of while loop is:

while (condition){
  // body of loop
}

Here,

• A while loop evaluates condition inside the parenthesis ().
• If condition evaluates to true, the code inside the while loop is executed.
• condition is evaluated again.
• This process continues until the condition is false.
• When condition evaluates to false, the loop stops.

Flowchart of while Loop

Example 1: Swift while Loop

// program to display numbers from 1 to 5

// initialize the variable
var i = 1, n = 5

// while loop from i = 1 to 5
while (i <= n) {
  print(i)
   i = i + 1
}

Output

1
2
3
4
5

Here's how the program works.

Example 2: while Loop to Display Game Level

var currentLevel:Int = 0, finalLevel:Int = 5
let gameCompleted = true
while (currentLevel <= finalLevel) {

  if gameCompleted {
    print("You have passed level \(currentLevel)")
      currentLevel += 1
  }
}
print("Level Ends")

Output

You have passed level 0                                                                                                                 
You have passed level 1                                                                                                                 
You have passed level 2                                                                                                                 
You have passed level 3                                                                                                                 
You have passed level 4                                                                                                                 
You have passed level 5                                                                                                                 
Level Ends     

In the above example, we have used a while loop to check the current level and display it on the console.

repeat...while Loop

The repeat...while loop is similar to while loop with one key difference. The body of repeat...while loop is executed once before the test expression is checked.

The syntax of repeat..while loop is:

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

Here,

• The body of the loop is executed at first. Then condition is evaluated.
• If condition evaluates to true, the body of the loop inside the repeat statement is executed again.
• condition is evaluated once again.
• This process continues until condition evaluates to false. Then the loop stops.

Flowchart of repeat...while Loop

Example 3: repeat...while Loop

// program to display numbers

var i = 1, n = 5

// repeat...while loop from 1 to 5
repeat {
  
  print(i)

  i = i + 1

} while (i <= n)

In the above example, initially the value of i = 1 and n = 5. For the first time the statement inside the loop executes without checking the condition. The condition is checked after the first iteration is complete.

Here's how this program works,

Infinite while Loop

If the condition of a while loop is always true, the loop runs for infinite times (until the memory is full). This is called infinite while loop. For example,

while (true) {
    print("Endless Loop")
}

Output

Endless Loop
Endless Loop 
.
.
.

Here, the condition is always true. Hence, the while loop will run for infinite times.

for vs while loop

A for-in loop is usually used when the number of iterations is known. For example,

// this loop is iterated 5 times
for number in 1...5 {
   // body of loop
}

However, a while loop is usually used when the number of iterations are unknown. For example,

while (condition) {
    // body of loop
}

Note: The working of repeat...while is the same as the while. Hence, it is also used when the number of iterations is unknown.