break 如何在 for 循环中工作？

Question

I am new in Python and I got confused about the way that "break" works in a for loop. There is an example in Python documentation( break and continue Statements ) which calculates prime numbers in range (2, 10):

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')

and the output is:

2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

however when I outdent "break" in the code:

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')

the output will be:

2 is a prime number
4 equals 2 * 2
6 equals 2 * 3
8 equals 2 * 4

Can you please explain what happens in the code after I outdent "break"? Thank you

Answer 1

Sure - Simply put out-denting the "Break" means it's no longer subject to the "if" that precedes it.

The code reads the if statement, acts on it, and then regardless of whether that if statement is true or false, it executes the "break" and drops out of the for loop.

In the first example the code only drops out of the 'for' loop if the n%x==0 statement is true.

Answer 2

Try executing this code - it might make it more clear:

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

vs:

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

I'm sure the output would help you understand what is going on. #1 is breaking only if the if condition is satisfied, while #2 breaks always regardless of the if condition being satisfied or not.