Cannot sort the array , index out of bound error

Question

I have 2 functions, one that swaps the values of 2 elements in an array and another that is capable of finding the index of the smallest number in an array slice, now when I use then both to find the sorted array , I am unable to do so as I am not able to understand as to how to use the swap function

The Swap function

func swapData(arr: [Int], firstIndex: Int, secondIndex: Int) -> [Int] {
    var temp = arr
    var a = firstIndex
    var b = secondIndex
    var x = temp[a]
    temp[a] = temp[b]
    temp[b] = x
    return temp
    
}

The function to find minIndex

func indexOfMinimum(arr:[Int], startIndex: Int) -> Int {
    var newArray = arr[startIndex...]
    var minValue = arr[newArray.startIndex]
    var minIndex = 0
    for i in newArray.startIndex + 1 ..< newArray.endIndex {
        if newArray[i] < minValue {
            minValue = newArray[i]
            minIndex = i - startIndex
        }
    }
   return minIndex
    
}

The function that attempts to sort array using above 2

func selectionSort(arr: [Int]) {
    
    var temp = arr
    var minIndex = 0
    for i in temp.startIndex  ..< temp.endIndex  {
        minIndex = indexOfMinimum(arr: Array(temp[i..<temp.count]), startIndex: temp.startIndex)
      
        
        swapData(arr: Array(temp[i..<temp.count]), firstIndex: temp.startIndex
                     , secondIndex: minIndex)
    }
      
    
   
}

var demoList = [18, 6, 1,66, 44,  78, 9, 22, 1,23]
selectionSort(arr: demoList)

I keep getting the original unsorted array

There are several problems with this. For instance. Your swapData function returns a new array. But you are not actually doing anything with that new array. So in your original array nothing is being swapped. — Fogmeister
– Fogmeister, Commented Jan 21, 2023 at 13:13

SimonL · Accepted Answer · 2023-01-21 12:55:07Z

1

Without getting into whether or not your sort function works, it has a bigger problem. It is neither mutating the input array or returning a new one. Consider the following code:

func f(a: [Int]) {
    // do stuff with `a`
}

var l = [1,2,3]

f(a: l)
// `l` is unchanged

Arrays in Swift are pass-by-value types. In order to change an array inside a function, you must return a new array or mark the array argument as inout and excplicitly pass a reference to it:

func f(a: inout [Int]) {
    // do stuff with `a`
}

var l = [1,2,3]

f(a: &l)
// `l` may be changed

answered Jan 21, 2023 at 12:55

SimonL

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