selection sort not understanding this code

Question

I found this code online for the selection sort:

 def selectionSort(alist):
    for fillslot in range(len(alist)-1,0,-1):
        positionOfMax=0
        for location in range(1,fillslot+1):
            if alist[location]>alist[positionOfMax]:
                positionOfMax = location

        temp = alist[fillslot]
        alist[fillslot] = alist[positionOfMax]
        alist[positionOfMax] = temp

 alist = [54,26,93,17,77,31,44,55,20]
 selectionSort(alist)
 print(alist)

list length = 9

i dont understand these lines of code:

 for fillslot in range(len(alist)-1,0,-1):

and

 for location in range(1,fillslot+1):

for fillslot if we go from those ranges it means we are looking at index: 8,7,6,5,4,3,2,1 why do we not look at index 0?

also for the location variable we would be looking at 1,2,3,4,...fillslot again why are we not looking at index 0?

positionOfMax starts at 0. No need to check itself.

Michael Bianconi
– Michael Bianconi

2019-08-06 19:21:43 +00:00
Commented Aug 6, 2019 at 19:21 — Michael Bianconi
– Michael Bianconi, Commented Aug 6, 2019 at 19:21

mahbubcseju · Accepted Answer · 2019-08-07 01:36:45Z

Using for fillslot in range(len(alist)-1,0,-1):, we are looking at index from right to left. When we are at a position (i), we will find the maximum number from index 0 to index i and swap the maximum number with element alist[i].

If we do so, then all the elements with index from 0 to i is smaller than all the elements with index i+1 to len(alist)-1.

After processing for index 1 , alist[0] will be smaller than any element from 1 to len(alist)-1, So we don,t need to consider index 0.

Now using for location in range(1,fillslot+1):, we are finding the maximum number from index 0 to index i. To find the maximum number firstly we let 0 index's number as maximum.For this, we declare a variable called positionOfMax=0 to hold the index of maximum number. Then we compare others numbers from position 1 to i, and are updating the positionOfMax variable.

For example,

alist=[2,5,3,9]

To find the maximum number of from index 0 to index 2:
let positionOfMax=0; which means let maximum number is at position 0.
Now check positions from 1 to 2.

When we are at position 1, alist[1] is greater than alist[positionOfMax], so positionOfMax will be updated by 1. Now positionOfMax is 1.

When we are at position 2, alist[1] is smaller than alist[positionOfMax], so positionOfMax will not be updated. So positionOfMax is still 1.

So maximum number from index 0 to index 2 is at position 1. I think now it is clear why the for loops are avoiding index 0.

Collectives™ on Stack Overflow

selection sort not understanding this code

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related