Project Euler #8 - Largest product in a series
Let's start with a performance improvement: Your method repeatedly extracts substrings from the given digit string (which is a costly operation with Swift
strings), and then repeatedly converts the characters in a substring to an integer.
It is better to convert the entire string to an array of integers once with
a single string traversal:

func largestProduct(numDigits : Int) -&gt; Int {

    let str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
    let digits = Array(str).map { String($0).toInt()! }

    var maxNumber = 0
    let maxLength = digits.count - numDigits
    for var i = 0; i &lt; maxLength; ++i {

        var number = 1
        for var j = i; j &lt; i + numDigits; j++ {
            number *= digits[j]
        }

        if number &gt; maxNumber {
            maxNumber = number
        }
    }

    return maxNumber
}


On my computer (with the program compiled in Release mode), this reduced the
time to compute largestProduct(13) from 0.065 seconds to 0.002 seconds.

(The rest of this review is about the code organisation and might be opinion based.)

I would pass the given digits as an argument to the largestProduct() function.
One advantage is that it is easier to add test cases.

for var i = 0; i &lt; maxLength; ++i {  }


can be written more swifty using a range:

for i in 0 ..&lt; maxLength { }


and the multiplication of the digits to a number can be done with reduce().

Then your function would look like this:

func largestProduct(digits : [Int], numDigits : Int) -&gt; Int {

    var maxNumber = 0
    let maxLength = digits.count - numDigits
    for i in 0 ..&lt; maxLength {
        let number = reduce(i ..&lt; i + numDigits, 1) { $0 * digits[$1] }
        if number &gt; maxNumber {
            maxNumber = number
        }
    }

    return maxNumber
}


Your printTimeElapsedWhenRunningCode() function is not optimal for reuse.
It is restricted to functions taking an Integer argument and returning
an integer, and the argument (4 or 13 in this case) is part of the function.

I would change that to take a ()-&gt;Void closure:

func printTimeElapsedWhenRunningCode(operation:()-&gt;Void) {
    let startTime = CFAbsoluteTimeGetCurrent()
    operation()
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    println("Time elapsed : \(timeElapsed) s")
}


and define a function doing all the work:

func euler8()  {
    let str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"

    let digits = Array(str).map { String($0).toInt()! }

    let result = largestProduct(digits, 13)
    println(result)
}


Now you can measure the time with:

printTimeElapsedWhenRunningCode(euler8)


and the printTimeElapsedWhenRunningCode function is fully reusable.