CODEWITHSUNDEEP
pythonsortingbubble-sort
atul gupta
atul gupta

Reputation: 19

Bubble sort in Python, what are these arguments for?

def bbsort(list1):
    for passnum in range(len(list1)-1,0,-1):
        for i in range (passnum):
            if list1[i]>list1[i+1]:
                temp =list1[i]
                list1[i] = list1[i+1]
                list1[i+1] = temp

This is code for a bubble sort, but what is the use of -1,0,-1 in the while condition?

Upvotes: 0

Views: 6114

Answers (2)

internet_user
internet_user

Reputation: 3279

The first -1 subtracts 1 from the length of the input list because the indexes start from 0, not 1. The 0 is the start number(only neccesary to get to the next parameter). The 2nd -1 is the step so that tells it to move backwards through the range.

Example: range(5, 2, -1) is [5, 4, 3]

equal to range(start, stop, step)

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1123410

You are looking at arguments to the range() function; the numbers set up the range to start at the list length, minus one, then stepping down to 1 (the end point is not included in the range).

So there are 3 arguments, the first is len(list1) - 1, the second is 0 and the third is -1.

Say the list is length 5, then range(5 - 1, 0, -1) will produce a list with 4, 3, 2, and 1:

>>> list(range(4, 0, -1))
[4, 3, 2, 1]

The for loop steps over these values:

>>> for i in range(4, 0, -1):
...     print(i)
... 
4
3
2
1

The for loop assigns these numbers to passnum, and the next nested loop uses that to create a new range(). The first time, that inner range will go from 0 to len(list1) - 1 (exclusive), the next time from 0 to len(list1) - 2, etc, until the last time, when it'll run from 0 to 1, always excluding the stop index. For a list of length 5, that means the inner loop first assigns 0, 1, 2, 3 to i, then 0, 1, 2, then 0, 1, then 0.

The code is missing an opportunity to use Python sequence assignment to swap to elements:

def bbsort(list1):
    for passnum in range(len(list1) - 1, 0, -1):
        for i in range (passnum):
            if list1[i] > list1[i + 1]:
                list1[i], list1[i + 1] = list1[i + 1], list[i]

Upvotes: 9

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