CODEWITHSUNDEEP
fortranprimes
Ruthvik
Ruthvik

Reputation: 840

Getting the prime numbers till 10000 in fortran?

Im trying to print prime numbers till 10000. (display the first five for testing) This is my program

program primes
    implicit none
    
    integer :: array(1229)
    integer :: i, ind
    logical :: is_prime
    ind = 1
    do i = 2, 10000, 1
        if (is_prime(i) .eqv. .true.) then
            array(ind) = i
            ind = ind + 1
        end if
    end do
    
    print *, array(1)
    print *, array(2)
    print *, array(3)
    print *, array(4)
    print *, array(5)
end program primes

function is_prime(n) result(ispr)
    implicit none
    
    integer :: c, i
    integer, intent(in) :: n
    logical :: ispr

    c = 0
    do i = 2, n
        if (mod(i,2) == 0) then
            c = c + 1
        end if
    end do
    
    ispr = (c == 0)
    
end function is_prime

I don't know why but this is the output

     9175178
     6417360
     5374044
     6750309
     7536745

Why does this happen and how to correct?

Upvotes: 2

Views: 494

Answers (2)

Old Dog Programmer
Old Dog Programmer

Reputation: 1251

While I am not familiar with modern Fortran, it looks to me as if function is_prime(n) result(ispr) is not working.

In the do loop in that function, you want a loop that tests thus:

  • is n divisible by 2?
  • is n divisible by 3?
  • is n divisible by 4?
  • is n divisible by 5?

and so on.

But, what it is actually doing is asking these:

  • is 2 divisible by 2?
  • is 3 divisible by 2?
  • is 4 divisible by 2?
  • is 5 divisible by 2?

and so on.

As a result, your counter will always have a non-zero value, and your function will always return false.

But, that's not the only problem. From your results, it looks like your Fortran implementation does not automatically initialize variables. Suppose I have statements like the following:

integer :: b
print *,b

What will be the result?

Remember, the names of variables represent locations in the computer's memory. If a variable is not initialized, it's value will be what was in the memory location before your program started to run. This value will not be related to your program.

I have 2 suggestions to fix the 2nd problem:

  1. Prior to do i = 2, 10000, 1, have another loop that sets each value in array.

  2. Set a values of each array (i) inside your do i = 2, 10000, 1 loop. One way to do this is to set one value when (is_prime(i) .eqv. .true.) is true and a different value when it is false.

Upvotes: 2

Turo
Turo

Reputation: 4924

is_prime should be(n is the only divider of n besides 1 <=> c == 1)

function is_prime(n) result(ispr)
    implicit none
    
    integer :: c, i
    integer, intent(in) :: n
    logical :: ispr

    c = 0
    do i = 2, n
        if (mod(n,i) == 0) then
            c = c + 1
        end if
    end do
    
    ispr = (c == 1)
    
end function is_prime

Could be optimezed by leaving the loop when c == 1 and i < n(after adding 1 to c)...

See on online fortran compiler

version with exit loop

Upvotes: 4

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