CODEWITHSUNDEEP
python
Subhojit Samonta
Subhojit Samonta

Reputation: 1

What am I doing wrong with the While Loop here?

I am getting an error that says "IndexError: List index out of range" with the While Loop that I am using. Pls help.

from array import *

limit = 20
limitn = limit+1
p = 2
nums = range(p,limitn)
primes = []

while p < limitn+1: 
    for i in nums:
        if i == p:
           primes.append(i)
        if i%p == 0:
           nums.remove(i)
    p = nums[0]


print primes

Upvotes: 0

Views: 27

Answers (2)

Wang Liang
Wang Liang

Reputation: 4444

With nums.remove(i), nums will be [].
So, IndexError: List index out of range is no problem

from array import *

limit = 20
limitn = limit+1
p = 2
nums = range(p,limitn)

primes = []

while p < limitn+1: 
    for i in nums:
        if i == p:
           primes.append(i)
        if i%p == 0:
           print(i)
           nums.remove(i)

    # Check nums array size
    if len(nums) == 0:
        break
    p = nums[0]


print primes

Upvotes: 0

Rahul
Rahul

Reputation: 577

Your list goes out of index because at the last iteration the length of nums is 0 i.e the list is empty and you are referencing the first element.
Because you are in a for loop nums.remove(index) removes the index but does not update the list.
So nums still refers to the previous nums and it's length has been changed.
So instead of .remove() use slice:

nums = nums[: nums.index(i)] + nums[nums.index(i) + 1 :]

slice operator creates a new nums removing the element i.
Your program for finding primes is not a good approach

for i in range(p, limitn):
     for j in range(2, i):
          if i % j == 0:
                break
     else:
          primes.append(i)

The code above will give you primes in your range. If there is anything that can be improved please comment

Upvotes: 1

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