Repeat-While Loop

The repeat-while loop in Swift is a post-test control flow statement that executes a block of code at least once before evaluating its termination condition. It is Swift’s equivalent to the do-while loop found in C-based languages.

Syntax

repeat {
    // Statements to execute
} while condition

Execution Flow

1. Unconditional Execution: The program enters the repeat block and executes the enclosed statements sequentially. This first pass happens regardless of the state of the condition.
2. Condition Evaluation: Upon reaching the while clause at the end of the block, the boolean condition is evaluated.
3. Branching:
   • If the condition evaluates to true, control flow jumps back to the beginning of the repeat block for another iteration.
   • If the condition evaluates to false, the loop terminates, and control flow proceeds to the next statement immediately following the loop structure.

Code Example

var iterationCount = 0

repeat {
    iterationCount += 1
    print("Execution number: \(iterationCount)")
} while iterationCount < 3

Technical Characteristics

• Post-Condition Evaluation: Because the condition is evaluated at the end of the loop rather than the beginning (pre-test), the loop body is structurally guaranteed a minimum of one execution.
• Strict Boolean Requirement: The expression provided to the while clause must evaluate explicitly to a Bool type (true or false). Swift’s type safety prevents implicit type coercion; passing an integer (like 1 or 0) or an optional will result in a compile-time error.
• Variable Scope: Variables declared inside the repeat block are scoped locally to that block and are not accessible within the while condition clause. To evaluate a variable in the while condition, it must be declared in the outer scope prior to the repeat keyword.

Scope Example

// Correct Scope
var index = 0
repeat {
    index += 1
} while index < 5

// Compilation Error (Unresolved Identifier)
repeat {
    var localIndex = 0
    localIndex += 1
} while localIndex < 5 

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