Fwd: python script to give a list of prime no.

[inhahe]

On Sun, Apr 5, 2020 at 8:26 AM Sathvik Babu Veligatla <
sathvikveligatla at gmail.com> wrote:

> hi,
> I am new to python, and i am trying to output the prime numbers beginning
> from 3 and i cannot get the required output.
> It stops after giving the output "7" and that's it.
>
> CODE:
> a = 3
> l = []
> while True:
>     for i in range(2,a):
>         if a%i == 0:
>             l.append(1)
>         else:
>             l.append(0)
>
>     if l.count(1) == 0:
>         print(a)
>         a = a + 2
>     elif l.count(1) >> 0:
>         a = a + 2
>
>
>
> Any help is appreciated,
> Thank you.
> --
> https://mail.python.org/mailman/listinfo/python-list



"l = []" needs to go inside the "while True" loop. as it is, it keeps
adding 1's and 0's to the list with each new "a" value, and then counts the
1's and 0's, without ever resetting the list back to empty, so once it gets
to 9 the list has a 1 in it and l.count(1) will never be false again. 3, 5
and 7 are prime so a 1 doesn't get added yet with those values, which is
why 7 is the highest it gets to. so with "l = []" inside the while loop,
"l" would be reset to empty with each new "a" value.

btw, this is a very inefficient way to compute prime numbers. for example,
why add the 0's to the list? you never count the number of 0's so it's
unnecessary. another example.. why use a list at all? all that matters is
that it find *one* result where a%i==0, it doesn't matter how many results
it found after that, so all you need is a boolean. and you might as well
stop testing a%i for *all* i values up to i, because once it finds the
first result that's 0, you know it's prime so you can just go to the next
number. and you don't need i to go all the way to a, because a%i will
always be > 0 for any i over a/2. and since you're not testing evens, it's
actually any i over a/3. in actuality, though, computing any a%i where i >
the square root of a is redundant because for each factor of a above the
square root of a, you've already tested its cofactor, say a is 100. the
square root of 100 is 10. if you're testing 100%20, that's unnecessary
because 100/20 is 5 and you've already tested 100%5.