[Tutor] Tutor Digest, Vol 213, Issue 17

[481 Vinay]

Code for this one

On Sun, Nov 14, 2021, 10:31 PM <tutor-request at python.org> wrote:

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> Today's Topics:
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>    1. prime numbers (coskun arslan)
>    2. Re: prime numbers (Alan Gauld)
>
>
>
> ---------- Forwarded message ----------
> From: coskun arslan <coskunarslan at yahoo.com>
> To: "tutor at python.org" <tutor at python.org>
> Cc:
> Bcc:
> Date: Sat, 13 Nov 2021 14:43:59 +0000 (UTC)
> Subject: [Tutor] prime numbers
> Want to list primes to 20.Where is mistake?
> my_list=[i for i in range(2,21)]for i in my_list:       for k in
> range(2,i):              if i%k==0:
> my_list.remove(i)print(my_list)
> It gives eror List.remove(i) i is not in list.What is wronge?
>
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> ---------- Forwarded message ----------
> From: Alan Gauld <alan.gauld at yahoo.co.uk>
> To: tutor at python.org
> Cc:
> Bcc:
> Date: Sun, 14 Nov 2021 00:16:44 +0000
> Subject: Re: [Tutor] prime numbers
> On 13/11/2021 14:43, coskun arslan via Tutor wrote:
> Please always post code in plain text, the HTML text gets messed
> up, and formatting is very important in Python.
>
> In this case I'll try to reformat it as you probably had it...
>
>
> my_list=[i for i in range(2,21)]
> for i in my_list:
>        for k in range(2,i):
>               if i%k==0:
>                   my_list.remove(i)
> print(my_list)
>
> > It gives eror List.remove(i) i is not in list.What is wronge?
>
> Please always post the full error text not a summary,
> there is a lot of useful information contained in the
> error text.
>
> But in this case you are iterating over my list while at the
> same time removing elements of the list. That's a bit like
> cutting down the branch of the tree that you are sitting on.
>
> Python goes looking for the item at position 6, say,  but
> in the meantime you have deleted items 3 and 4 so item 6
> is no longer at position 6. Python gets confused!
> To fix that you can iterate over a copy of the list
> (mylist[:] is an easy way to create a copy) and remove
> from the original list.
>
> By the way, there are much more efficient algorithms for
> finding prime numbers. For example:
> Think about how high you need to go to find the factors.
> Take 24 as an example:
> (1,24),(2,12),(3,8),(4,6),(6,4), ...
> but wait, 6,4 is the same pair as 4,6.
> We are repeating ourselves. So we only need to test
> up to the square root of the target, int(24**0.5) => 4.
>
> That saves a bunch of tests.
>
> But there are even more powerful algorithms you can use.
> Try a google search...
>
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
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>
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