Problem using break command in Python tutorial

Question

I'm following the python tutorial at their site and I'm currently at the break continue section. I just tried this sample 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'
... 
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

And instead of spitting out what it says above I get

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

It seems to me that it continues to run the inside for loop, but why would the tutorial not take this into account? Is it outdated for the latest interpreter build (I'm running xubuntu jaunty)?

I was able to fix it by adding the line

     else:
...                     if n != y:
...                             print n, 'is a prime number'
...                             y = n

but I am concerned this may be bad coding practice. Thank you for your help.

Answer 1

The output which you show contains ten times the string "x is a prime number". However, this string is printed in the else clause of the inner loop, and is as such executed at most once for each execution of the inner loop.

Since the outer loop performs eight iterations, "x is a prime number" cannot be printed more than eight times. Thus the output which you display cannot be brought about by the code shown.

Conclusion: something is fishy. Can you show the code as you executed it?

---

Edit: solved!

You incorrectly indented the else clause, such that Python interpreted it as belonging to the if statement. Python treats a tab as 8 spaces. Perhaps your editor displays tabs as 4 spaces. That way you may have missed this bug. As per PEP 8, please don't mix tabs and spaces, and preferably use four spaces to indent your 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'
... 
3 is a prime number
4 equals 2 * 2
5 is a prime number
5 is a prime number
5 is a prime number
6 equals 2 * 3
7 is a prime number
7 is a prime number
7 is a prime number
7 is a prime number
7 is a prime number
8 equals 2 * 4
9 is a prime number
9 equals 3 * 3

Answer 2

My best guess is that your 'else:' statement isn't properly indented, and then your result is logical, check that you else's indentation is on level with 'for x'.

I.e. you use:

for n in range(2,10):
    for x in range(2,n):
        if n%x == 0:
            print(n, '=', x, '*', n/x)
            break
        else:
            print(n, 'is a prime')

instead of:

for n in range(2,10):
    for x in range(2,n):
        if n%x == 0:
            print(n, '=', x, '*', n/x)
            break
    else:
        print(n, 'is a prime')

Answer 3

You may need to update your Python interpreter.

This code runs correctly for me (notice Python version number):

Python 2.6.1 (r261:67515, Dec  6 2008, 16:42:21) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 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
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

Answer 4

I think you have got indents wrong. If I take your code, and indent the else so that it is under the if statement, I get exactly the output that you are getting.

The code below reproduces your output,

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'

while

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'

Does what you want it to do.

Note the different in the indenting of the else.

Answer 5

I thought that else should always be aligned with if. That is what I have read. But in this prime number generator code, the only way to get the primes written once, is aligning else with for x. So I have no explanation for such identation. Although I am just starting to learn Python.