Hi!
I've come across this piece of code of a (modified?) sieve and I'm trying to figure out what sieve[i]
means and how it works, specially the 2nd for.
Would you help me, please?
Thanks!
# | User | Rating |
---|---|---|
1 | ecnerwala | 3649 |
2 | Benq | 3581 |
3 | orzdevinwang | 3570 |
4 | Geothermal | 3569 |
4 | cnnfls_csy | 3569 |
6 | tourist | 3565 |
7 | maroonrk | 3531 |
8 | Radewoosh | 3521 |
9 | Um_nik | 3482 |
10 | jiangly | 3468 |
# | User | Contrib. |
---|---|---|
1 | maomao90 | 174 |
2 | awoo | 164 |
3 | adamant | 162 |
4 | TheScrasse | 159 |
5 | nor | 158 |
6 | maroonrk | 156 |
7 | -is-this-fft- | 151 |
8 | SecondThread | 147 |
9 | orz | 146 |
10 | pajenegod | 145 |
Name |
---|
sieve[i]
gives you the index of the biggest prime factor. The index is one-based.sieve[20] == 3
, because20 = 2*2*5
andprimes[3] == 5
(5
is the third prime number)Using this method, you generate the prime factorization pretty quick. You can find the biggest prime number using the
sieve
, divide by this number and repeat with the result.I think you meant
prime[5] = 3
, right?No,
primes
is a list of primes.primes = { 0, 2, 3, 5, 7, 11, ... }
Ah, sorry I totally misunderstood.
I thought you meant to give another example, which is
sieve[5] = 3
, and didn't notice the difference in arrays' names. Sorry about that :D