A question.
Remco Gerlich
scarblac at pino.selwerd.nl
Tue Dec 12 07:00:30 EST 2000
DeHa <deha at poczta.onet.pl> wrote in comp.lang.python:
> I am new to Python, and just try to write my first program. But I have a
> small problem.
>
> My program should find first numbers in the set from 1 to 100000. I made
> such a code:
You probably meant prime numbers.
>
> first = range(100001)
> first = first[1:]
Instead, simply first = range(1,100001) would be easier. Actually,
the range should start at 2, since that is the first prime number
(and you don't want to delete numbers that are divided by 1).
> for i in first:
> if i != []:
> #until end of the list
"for i in first" means that i will become all the elements of first in turn.
So i never becomes [], since first only contains integers. And it already
loops until the end of the list.
> for j in first[first.index(i):]: #find all
> numbers divided by i
This also includes the number i itself, it shouldn't.
> if j % i == 0:
> first[first.index(j): first.index(j)] = [] #and delete it
> from the list
You are deleting from a list while you are walking through it with 'for',
that's a bit tricky.
> And it doesn't work, and I don't konw why? Could anyone help me? I try
> to find answer in the tutorial, but the information in it doesn't change
> anything.
It is easier to start with an empty list of primes, and add numbers that
can't be divided by the numbers in the list.
Something like this:
primes = [2] # We need one prime to start with
for candidate in range(3, 100001, 2):
for prime in primes:
if candidate % prime == 0:
break
else:
# The 'else' clause happens when the 'break' never happened
primes.append(candidate)
This is not the fastest solution, but it works.
--
Remco Gerlich
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