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The sieve of Sundaram: Difference between revisions
→{{header|Fortran}}
Line 278:
=={{header|Fortran}}==
<lang Fortran>
PROGRAM SUNDARAM
IMPLICIT NONE
!
! Local variables
!
INTEGER(8) :: curr_index
INTEGER(8) :: i
INTEGER(8) :: j
INTEGER :: lim
INTEGER(8) :: mid
INTEGER :: primcount
LOGICAL*1 , ALLOCATABLE , DIMENSION(:) :: primes !Array of booleans representing integers
lim = 10000000 ! Not the number of primes but the storage where the prime marker is held for the millionth prime
ALLOCATE(primes(lim))
primes(1:lim) = .TRUE.
!Set all to .True., we will later block out the known non-primes
mid = lim/2
!Generate primes
DO j = 1 , mid
DO i = 1 , j
curr_index = i + j + (2*i*j)
IF( curr_index>lim )EXIT ! Too big already, leave the loop.
primes(curr_index) = .FALSE. !This candidate will not produce a prime
END DO
END DO
!Find first number still prime below lim
i = 0
j = 0
WRITE(6 , *)'The first 100 primes:'
DO WHILE ( i < 100 )
j = j + 1
IF( primes(j) )THEN
WRITE(6 , 34 , ADVANCE = 'no')j*2 + 1 !Take the candidate, multiply by 2, add 1, and you have a prime
34 FORMAT(I0 , 1x)
i = i + 1 ! Counter used for printing
IF( MOD(i,10)==0 )WRITE(6 , *)' '
END IF
END DO
! Now print the millionth prime
primcount = 0
DO i = 1 , lim
IF( primes(i) )THEN
primcount = primcount + 1
IF( primcount==1000000 )THEN
WRITE(6 , 35)'1 millionth Prime Found: ' , (i*2) + 1
35 FORMAT(/ , a , i0)
EXIT
END IF
END IF
END DO
DEALLOCATE(primes)
STOP
END PROGRAM SUNDARAM
</lang>
{{out}}
<pre>
The first 100 primes:
3 5 7 11 13 17 19 23 29 31
37 41 43 47 53 59 61 67 71 73
79 83 89 97 101 103 107 109 113 127
131 137 139 149 151 157 163 167 173 179
181 191 193 197 199 211 223 227 229 233
239 241 251 257 263 269 271 277 281 283
293 307 311 313 317 331 337 347 349 353
359 367 373 379 383 389 397 401 409 419
421 431 433 439 443 449 457 461 463 467
479 487 491 499 503 509 521 523 541 547
1 millionth Prime Found: 15485867
</pre>
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