Why Python range() in for loop running twice

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I am using python range() with for loop but situation is 2 loops, expect the outer loop run once, then inter loop runs completely, then outer loop runs its 2nd... question: why the outer loop run twice before the inter loop get a chance to run?

What I have tried:

Python

def sort(a_list):
    for i in range(1,len(a_list)):
        print("i=",i)
        for j in range(i-1,0,-1):
            print("j=",j)

Test: L=[9,6,1,3]

sort(L)

Result:

i= 1
i= 2   # here, the outer lopp ran twice then inter loop began.
j= 1
i= 3
j= 2
j= 1

Posted 14-Jul-18 8:50am

Member 13911588

Updated 15-Jul-18 8:41am

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3 solutions

Solution 2

Look at the values of your inner loop. The first time it runs the variable i holds the value 1. So the loop will not run since i - 1 is zero, and the loop is defined to run from values zero to zero.

Posted 14-Jul-18 19:30pm

Richard MacCutchan

Solution 3

Quote:
why the outer loop run twice before the inter loop get a chance to run?

For a good explanation, read the documentation:
4. Built-in Types — Python 3.7.0 documentation[^]

Posted 14-Jul-18 22:35pm

Patrice T

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Dave Kreskowiak · Accepted Answer · 2018-07-14T11:09:00

If you want to find out exactly how this code work, use the debugger. It's there to debug YOU and your understanding of the code.

It appears the i loop executes twice in a row because it doesn't. The inner j loop executes but range returns an array of values from i - 1 to 0, with the upper limit of 0 being non-inclusive.

On the first iteration of i, that would be range(1 - 1, 0, -1), which will return no values for the j loop.

This is the expected behavior.