Solving Problems in Fortran Code for Prime Numbers

[ƒ(x) → ∞]

Everytime I try to compile the followng code, it fails to give me the values when n is greater than one, I have tried and failed to correct that mistake, can you help?

	program sieve
	implicit none
	integer*1 s(1000000), offset (10), sequence
	integer i, j, n

c	INITIALISATIONS:
	n=0
	sequence=4

	do i=1, 10
	offset(i)=0
	enddo

	offset(1)=2
	offset(2)=4
	offset(3)=2

	do i=1, 1000000
	s(i) = 1
	enddo

	do i=2, 1000000
	  if (s(i).eq.1) then
	    do j=2, (1000000/i)
	       s(i*j)=0
	    enddo
	  endif
	enddo

	do i=2, 1000000

	if (s(i).eq.1) then
	  do j=1,sequence-1	  
	    if (s(i+offset(j)).ne.1)  goto 10
	  enddo
	  n=n+1
	  write(*,*) n, i,i+2,i+6,i+8
	endif
10	continue
	enddo


	end

It is looking for primes that differ by 2,4,2 like {5,7,11,13}.

Thank you.

 

[silverfrost]

Your code looks about right but if it doesn't work...

... have you tried debugging it? For example, just try printing the first 100 values in 's' and see if the 1s line up with the primes.

 

[minger]

Try using an EXIT rather than a GOTO in that bottom do loop. Without going line-by-line it's hard to tell.

Another tip, rather than using do loops do initialize things, you can implicitly define an entire array such as:

offset(:)=0