This overhead can be eliminated by simply -ing the expected size of the vector ahead of time. And you have obviously read only part of the thread. I did know all that. So, what we do essentially here is create a function which checks if a number is divisible by 2, then divisible by 3, and so on till it reaches the number before the number to be checked if it is prime of not. Here's a Euler's sieve implementation: Python 2. This means when you pass a large long long to sqrt , you will end up losing a few bits of precision, which can make your sqrt return slightly the wrong value.
What do you think: '''Euler's Sieve implementation. Maybe I have solution: Any number that can't be divisible with 2 and 3 is a prime number. That works, but when you want a list of many prime numbers, the is a much more efficient algorithm. A prime number is a natural number greater than one that has no positive divisors other than one and itself. A list of sieving primes related to a specific segment contains only those primes that have multiple occurrence s in that segment. On first blush I would say that this algorithm will be slow since it uses the function len so many times. Thus, 35 is not prime - but your test would say it is.
After a while, they ain't gonna fit. The fact that a number can't be divided by two or three does not mean it is prime. Natural number is a positive integer number. Here is the new working code, quite a bit faster too. Install primesieve The primesieve console application can be installed using your operating system's package manager.
However I couldn't find out how its supposed to work: apparently you generate a list of possible numbers s and mark the ones which are prime? To save on efficiency, you can eliminate all evens, except two, and any odd number that is evenly divible by 3,5,7,9,11. If the only number it is divisble by is in the same array location as the one you are currently opperating in, then that is a prime number. The remaining candidate primes are tested in random order with a standard probabilistic primality test such as the or the for. Use MathJax to format equations. When you come back to your old code six months later you will thank yourself. Remember two is the only even and the smallest prime number.
Prime numbers have many applications in computer science and mathematics. Finally, as mentioned in the comments, look at the Sieve of Eratosthenes. I've thinking about this and i finally made a program able to list all the combinations possible to for a number by multiplications. That's a lot of work to set up, why not just use a for loop that counts up to the number, and if it divides, then drop it, if it doesn't, put it into your array. Although, I've fixed the problem and am using an entirely different and more time efficient approach now, I'm still curious as to why that didn't work as it was supposed to. Prime numbers have many applications in computer science and mathematics.
Timings x Prime Count Intel Core i7-6700 4 x 3. Prime number is a natural number with exactly two distinct natural number divisors: 1 and itself. C program for prime number or not. Don't wait to see it here because i always forget addresses and might never enter this site again. If it can be divided by 2 then it is not prime. A double has only a 52 bit mantissa. If it can be divided by 2 then it is not prime.
Thus, 35 is not prime - but your test would say it is. This still equates to true. To read more please visit:. Additionally primesieve uses Tomás Oliveira e Silva's if needed. Thanks for pointing out the flaw.
Lecture Notes in Computer Science. Here's a un-onelined translation of what's going on, to help. If it has square root it's not prime. I will send you a copy of it. It also looks like it dumps the results to a text file which I think could be a substantial help to an isPrime function because then it takes away from all the repeated calculations. The biggest number a number can divided by is not the square root.
However it is easy enough to guess what is meant in C also. If there was no such number, stop. These numbers will be 2p, 3p, 4p, etc. Personally I always prefer the next line as else my eyes have trouble matching pairs of brackets but it is a valid style in some conventions. But it's also a working sieve, I think.
We know that there will always be the same number of factors less than the square root as greater than. This is an inefficient algorithm for finding primes - you certainly only need to check whether the number is divisible by a prime number up to the square root - not any number up to the square root. This capacity increase requires a reallocation of storage, and possibly a copying of the entire array contents to a new location. I like how you modularized checkprimeness, and the system calls are extremely useful for portability. Also all multiples of 5 are counted as primes. Thus, 35 is not prime - but your test would say it is. It's speed is mainly due to the segmentation of the sieve of Eratosthenes which prevents cache misses when crossing off multiples in the sieve array and the use of a bit array instead of a boolean sieve array.