Understanding python3 docs example code

Question

Why doesn't the very first iteration go to the first print statement. After all, isn't 2%2==0?

>>> for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print(n, 'equals', x, '*', n//x)
...             break
...     else:
...         # loop fell through without finding a factor
...         print(n, 'is a prime number')
...
2 is a prime number
3 is a prime number

cmd · Accepted Answer · 2013-06-13 15:58:34Z

1

The first time through the loop, n = 2 so x is in range(2, 2) which is an empty list. Iterating over an empty list doesn't go into the inner loop but does executes the else clause.

answered Jun 13, 2013 at 15:58

cmd

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1 Comment

GreenDog Over a year ago

Thanks! I thought that range would have a single element: [2].

recursive · Accepted Answer · 2013-06-13 16:00:42Z

0

2 is not in range(2, 2). The upper bound of range is exclusive, so it stops before reaching it.

answered Jun 13, 2013 at 16:00

recursive

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