On 25/12/2021 03.22, vani arul wrote:
> Hello,
> I am trying write a code.Can some help me find the error in my code.
> Thanks!
>
>
> def selectionsort(arr):
># le=len(arr)
> for b in range(0,len(arr)-1):
> pos=b
> for a in range(b+1,len(arr)-1):
> if arr[b]>arr[a+1]:
> arr[b],arr[a+1]=arr[a+1],arr[b]
> return arr
>
> arr=[3,5,9,8,2,6]
> print(selectionsort(arr))
This looks like a typical 'homework question'. Accordingly, we are happy
to help, but that means helping you to learn Python, NOT to help by
writing a homework-solution!
Evidently you are expected to learn some Python, but additionally an
algorithm (or method) for sorting. Also apparent, is that the current
attempt has been taken from another programming language - rather than
using a more 'pythonic' idiom. All fine, but with a "but...".
One assumes, the Python sort() built-in function may not be used. Even
so, you can ask the computer to test the success (or otherwise) of your
efforts, by adding to the end of the existing code, something like:
print( "Was the algorithm successful?",
selection_sort( arr ) == sorted( arr )
)
(I've recently 'enjoyed' an eye operation, so actively seek ways of
having the computer 'see' or check things that are difficult for me -
pending already-excellent recovery progress...)
The question has already been asked: "What are you expecting the code to
do?". This is vital. What is your target? (and when you ask us a
question, how do we know exactly what your target might be?).
Regardless, it is *always* the first question to be asked in attempting
to find a solution to any problem - thus, has much wider application
than computing then! How does one know if the aim is true, without first
defining "the target"?
The key to coding any algorithm is to (temporarily) forget Python, and
to first understand the method - how it works.
What I have often done in the past, is to ask trainees to bring a
pack/deck of Playing Cards - however a few scraps of paper with the
numbers: 3,5,9,8,2, and 6 written on them, will do just as well.
What we want is three sets of the same numbers. One set to move around
(the above), and the other two static - so could write them (in a row)
across a sheet of paper. The first 'static' set, should be the numbers
in the sequence given by the question, ie 'the start position'. Put this
at the 'top' of a desk or flat surface. The second 'static' set, is the
"target", ie the same digits, but in sorted-order. Artie them across a
second piece of paper, and place that at the 'bottom' of the desk. So,
now we can see a starting-position, and a final sequence.
The 'magic' is the bit that happens "in the middle" ("in-between")!
Now, using the space on your desk, between the 'start' and 'end',
arrange the 'numbers' in the given sequence, across a 'row'.
With your brain (and hands) as the 'computer', work the algorithm you've
already started using:
1 commence with the first item in the list:
ie take the "3" in your left hand
(or perhaps put a suitable finger on it)
2 then compare that with the "5" (right hand/finger)
3 given that three is less than five, what should happen?
4 next compare the "3" with the "9"
(left finger doesn't move, right hand/finger does!)
5 what should happen at this time?
6 keep going, until...
you compare the "3" with the "2"
what happens now?
That's the basic algorithm done!
Continuing the above, after a while there are no more items left to
compare with the one under your left hand/finger. What is the state of
the list?
Not much has changed, but what can we say about the situation
(placement) of the item which is in the first-position/at the left side
of the scraps of paper/playing cards? How does it relate to the
first-position in the target-row?
For the next stage, start with the second item in the list (move your
left hand/pointing-finger!):
1 run through, similar to the above
2 until you (your right hand) reach the end
(yes, it is starting to become a bit boring, but...)
When you have once-again 'run out' of items to compare, what can now be
said about the state of the list? About the values in the first two
positions (and their equivalents in the 'target')?
So, change of thinking: could it be considered that we have two
'sub-lists' (amongst our paper-scraps/playing-cards)? One with two items
in it, the other containing 'all the others'. What can we say about the
'left-sub-list' (and specifically its sequencing)?
So, at this time, please decide if you need to continue with the above
cycles in order to fully-understand how the method arrives at the
desired conclusion - or keep going to prove 'everything' to your
satisfaction, by running right-through. Has the algorithm managed to
arrange the slips of paper/playing cards into the correct sequence, as
portrayed by the bottom-row of numbers?
Once you have
