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I am just learning to code so I decided to start with Swift. I am following the tour that mac has for it at here and I am at the section where it is calculating a sum of numbers and then it tells you to try to do a function that does an average of numbers. 

func averageOf(numbers: Int...) -> Int {
    var sum = 0
    var total = 0
    var average = 0
    for number in numbers {
        sum += number
        total++
    } **Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_1386_INVOP, subcode=0x0)**
    average = sum/total
    return average
}

What am I doing wrong(what do I need to learn to do it right)?

Jeremy Scott Peters
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2 Answers2

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I’m guessing you’ve called your function with no arguments, that is:

averageOf()

This is allowed with variadic arguments, and numbers will be an empty array. This will result in you attempting to divide an unchanged sum by an unchanged total (because you will go round the loop no times for no elements in numbers), so dividing 0 by 0, and you’re getting a divide-by-zero error.

To prevent this from being a possibility, you could require the user to supply at least one number:

func averageOf(first: Int, rest: Int...) -> Double {
    var sum = first
    var total = 1.0
    for number in rest {
        sum += number
        total++
    }
    return Double(sum)/total
}

This way, if you try to call it with no arguments, you’ll get a compiler error.

BTW I altered your version to return a Double rather than an Int, you might want to experiment with the two versions to see why.

(this technique is similar to how the standard lib max function is declared, which requires at least 2 arguments:

func max<T : Comparable>(x: T, y: T) -> T

but has an overloaded version for 3 or more:

func max<T : Comparable>(x: T, y: T, z: T, rest: T...) -> T

the reason for the first version instead of cutting straight to a variadic version that takes at least two being, you can then pass it into things like reduce to find the max in a collection e.g. reduce(a, 0, max))

Airspeed Velocity
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0

For me this error happened because an implicitly unwrapped property was not set. Setting it would fix the issue.

mfaani
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