23.4: Example

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In order to generate different random number for successive executions, the seed must be initialized with a different set of seed values each time.

The following example simulates the roll of two dice, which requires two random integer numbers, each between 1 and 6.

program diceRoll

implicit none
integer :: m, die1, die2, pair
real :: x
integer :: i, n, clock
integer, dimension(:), allocatable :: seed
character(10) :: nickname

    call random_seed(size = n)
    allocate(seed(n))
    call system_clock(count=clock)
    seed = clock + 37 * (/(i-1, i=1, n)/)
    call random_seed(put = seed)
    deallocate(seed)

    call random_number(x)
    die1 = int(x*6.0) + 1

    call random_number(x)
    die2 = int(x*6.0) + 1

    write (*,'(2(a,1x,i1/),a,1x,i2)') "Dice 1:", die1, &
                "Dice 2:", die2, "Dice Sum", (die1+die2)

end program diceRoll

Will generate different values each execution. For example, three executions of the example program are shown below:

C:\fortran> dice
Dice 1: 5
Dice 2: 4
Dice Sum  9

C:\fortran> dice
Dice 1: 2
Dice 2: 4
Dice Sum  6

C:\fortran> dice
Dice 1: 1
Dice 2: 6
Dice Sum  7

C:\fortran>

The dice values will be different for each execution.