scala for-loop issue assignment issue

Question

simple selectionSort is not working

• doSort - not working
• doSortWorking method is working properly

Here doSort method is not working just because of manipulating "minimum" inside the for loop, can someone tell me what is the root cause, i couldnt understand the root cause

object SelectionSort extends App {
    val unSortedArray = ArrayBuffer(5, -1, 8, 4, 6, 21, 4, 5, -0, 0)

    def doSort (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
        var minimum = 0
        for {
            index <- startIndex to endIndex
            _ = minimum = ary(index)
            loopIndex <- index until endIndex
        } {
            if (minimum > ary(loopIndex + 1)) {
                minimum = ary(loopIndex + 1)
                val swap = ary(index)
                ary(index) = ary(loopIndex + 1)
                ary(loopIndex + 1) = swap
            }
        }
    }

    def doSortWorking (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
        var minimum = 0
        for {
            index <- startIndex to endIndex
            loopIndex <- index until endIndex
        } {
            minimum = ary(index)
            if (minimum > ary(loopIndex + 1)) {
                minimum = ary(loopIndex + 1)
                val swap = ary(index)
                ary(index) = ary(loopIndex + 1)
                ary(loopIndex + 1) = swap
            }
        }
    }

    doSort(unSortedArray, 0, unSortedArray.size - 1)
    println(unSortedArray)
    doSortWorking(unSortedArray, 0, unSortedArray.size - 1)
    println(unSortedArray)
}

Answer 1

Just swap 2nd and third lines in for comprehension inside doSort():

def doSort (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
    var minimum = 0
    for {
        index <- startIndex to endIndex
        loopIndex <- index until endIndex
        _ = minimum = ary(index)
    } {
        if (minimum > ary(loopIndex + 1)) {
            minimum = ary(loopIndex + 1)
            val swap = ary(index)
            ary(index) = ary(loopIndex + 1)
            ary(loopIndex + 1) = swap
        }
    }
}

You should reassign new value to minimum after finding loopIndex, right inside doSortWorking function.

EDIT: Why doesn't your version of doSort work?

Let's try simplified version of it:

scala> val startIndex = 0
firstIndex: Int = 0

scala> val endIndex = 3
lastIndex: Int = 3

scala> var minimum = 0
minimum: Int = 0

scala> for {
     |   index <- startIndex to endIndex
     |   _ = minimum = index
     | } {
     |   println(minimum)
     | }
3
3
3
3

Why doesn't it print 0 1 2 3? Because for comprehension doesn't work as you thought here. Check this answer: https://stackoverflow.com/a/3754568/5053865 Here are some rules about for-comprehension evaluating. Let's apply them:

scala> for ((p, _) <-
     |   for (index <- startIndex to endIndex)
     |   yield { val x0 = minimum = index; (index, x0) }
     | ) {
     |   println(minimum)
     | }
3
3
3
3

scala> startIndex to endIndex map {
     |   index => val x0 = minimum = index; (index, x0)
     | } foreach {
     |   _ => println(minimum)
     | }
3
3
3
3

Now you should see what's really is going on here. Firstly, you assign values from startIndex to endIndex to variable minimum and only then execute body with maximum = endIndex every time!