why the list is not changed when I modify it within the function in python

Question

I am a beginner in python and writing a program to calculate the primes numbers till given N. here is my code. what I see is that my list called numbers is changed after each iteration through for-loop. however, I see the list from which I am iterating over still contains reference to old values while this list is changed with for loop. kindly look at the output below.

def getprimes(n):
         numbers = [ i for i in range (2,n+1)]
         primes=[]
         print "numbers in the beginning",numbers

         for i in numbers:
                print i
                primes.append(i)
                print "primes",primes
                print "numbers",numbers
                numbers=[k for k in numbers if (k%i!=0)]

         print "numbers at end",numbers
         return primes


print getprimes(10)

my output is below:

numbers in the beginning [2, 3, 4, 5, 6, 7, 8, 9, 10]
2
primes [2]
numbers [2, 3, 4, 5, 6, 7, 8, 9, 10]
3
primes [2, 3]
numbers [3, 5, 7, 9]
4
primes [2, 3, 4]
numbers [5, 7]
5
primes [2, 3, 4, 5]
numbers [5, 7]
6
primes [2, 3, 4, 5, 6]
numbers [7]
7
primes [2, 3, 4, 5, 6, 7]
numbers [7]
8
primes [2, 3, 4, 5, 6, 7, 8]
numbers []
9
primes [2, 3, 4, 5, 6, 7, 8, 9]
numbers []
10
primes [2, 3, 4, 5, 6, 7, 8, 9, 10]
numbers []
numbers at end []
[2, 3, 4, 5, 6, 7, 8, 9, 10]

Any suggestions and comments are most welcome!. Thanks

Ignacio Vazquez-Abrams · Accepted Answer · 2013-08-27 01:36:21Z

2

Rebinding the name doesn't change the iterable being iterated over. Consider rewriting the code to use a while loop instead.

answered Aug 27, 2013 at 1:36

Ignacio Vazquez-Abrams

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Comments

CT Zhu · Accepted Answer · 2013-08-27 02:05:06Z

2

Try to write your code simple and straight forward. Overcomplexity always leads to confusions. Your algorithm can be translate into a two line function.

>>> def getprimes(n):
         numbers = [ i for i in range (2,n+1)]
         primes=[k for k in numbers if all((k%i!=0) for i in numbers if i!=k)]
         return primes

>>> print getprimes(10)
[2, 3, 5, 7]

>>> print getprimes(50)
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]

answered Aug 27, 2013 at 2:05

CT Zhu

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2 Comments

brain storm Over a year ago

+1. can you explain how the keyword all is used? It is nice you could write this in just two lines of code.

CT Zhu Over a year ago

@user1988876, all(some_iterable) basically returns True if all elements of the iterable (e.g. a list) are True. So, that line means, if (k%i!=0) is True for all the numbers in the list numbers, returns True. Of course, we want to exclude k for the list numbers, that's what the final if i!=k is here for.

brain storm · Accepted Answer · 2013-08-27 03:53:21Z

1

Try this, as suggested by Ignacio, try the while loop instead as

def getprimes(n):
     numbers = [ i for i in range (2,n+1)]
     primes=[]

     while (numbers):
        print numbers[0]
        primes.append(numbers[0])
        print "primes",primes
        numbers=[k for k in numbers if k%numbers[0]!=0 ]
        print numbers

     print numbers
     return primes

answered Aug 27, 2013 at 3:53

brain storm

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