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Chandan Thour
Posts: **20**

in C and C++

What is the most optimum method of finding a prime number?

Till we use a method of incrimenting a number and then dividing it with all the preceding numbers.

Kindly tell the most efficient method of finidng a prime number.

Post the code or link to it.]

Regards

chandan

Till we use a method of incrimenting a number and then dividing it with all the preceding numbers.

Kindly tell the most efficient method of finidng a prime number.

Post the code or link to it.]

Regards

chandan

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## Comments

117: Till we use a method of incrimenting a number and then dividing it with all the preceding numbers.

: Kindly tell the most efficient method of finidng a prime number.

: Post the code or link to it.]

:

: Regards

: chandan

:

You can at least skip every second number since it is even and not prime. If you manage to deduce a formula of which the result is always a prime number, a math institute apparently will pay you one million dollars. I cant remember their name now.

1,784: : Till we use a method of incrimenting a number and then dividing it with all the preceding numbers.

: : Kindly tell the most efficient method of finidng a prime number.

: : Post the code or link to it.]

: :

: : Regards

: : chandan

: :

:

:

: You can at least skip every second number since it is even and not prime. If you manage to deduce a formula of which the result is always a prime number, a math institute apparently will pay you one million dollars. I cant remember their name now.

:

This is one of the unsolved problems of mathematics.

One way to speed up the algorithm is to do like this:

[code]

do forever

a++

b=1

do

inc b

until b = a OR (a mod b) > 0

if b = a, it's a prime

else it's not

[/code]

Or even faster in assembler:

[code]

start:

inc ax

xor bx,bx

inc bx

loopstart:

inc bx

cmp ax,bx

je prime

mov cx,ax

mod cx,bx

jnz notprime

jmp loopstart

prime:

print "It's prime" ;OK, I don't got the time to write all the memmory.

jmp start

notprime:

print "It's not prime"

jmp start

[/code]

I wrote it in psuedo asm, I don't got time right now to write it in real asm.

[b]Niklas Ulvinge[/b] [white]aka [b]IDK[/b][/white]

2,444To speed it up a bit more, the following two hints:

1. To check if a number is prime you only need to divide it by 2 to the square root of that number, including the square root.

2. Actually, but this is only faster if you're creating a row of primes, you only need to try and divide the number by every prime number that is between 2 and it's square root. But like I said, for a single number this is rarely faster.

And another remark:

For checking a row of number for primality, you could also use "The Sieve of Erasthotenes". Might be worth checking out if you're interested ;-)

Greets...

Richard