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)
}

Have you debugged yet? – Carcigenicate Mar 03 '17 at 13:00 — Carcigenicate, Mar 03 '17 at 13:00
Yes, but I couldn't understand it :( – sharath chandra Mar 03 '17 at 14:06 — sharath chandra, Mar 03 '17 at 14:06

score 1 · Accepted Answer · edited May 23 '17 at 12:17

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!

Why reassigning is not happening in my case ? "Minimalvalue" should be reassigned every time for each outer loop ,even in my case , but not !!! What would be the reason ?? — sharath chandra, Mar 03 '17 at 14:08
@sharathchandra I explained the reason, check edit of my answer. — Evgeny Veretennikov, Mar 03 '17 at 19:43

scala for-loop issue assignment issue

1 Answers1