CODEWITHSUNDEEP
pythonlistreference
eagertoLearn
eagertoLearn

Reputation: 10132

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

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

Upvotes: 1

Views: 80

Answers (3)

brain storm
brain storm

Reputation: 31252

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

Upvotes: 1

CT Zhu
CT Zhu

Reputation: 54330

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]

Upvotes: 2

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798606

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

Upvotes: 2

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