Fortran - random

Author

random_seed

Kurda Yon

2004-07-28, 9:06 pm

Hi,

in the posting from 2003-09-23 13:07:12 PST
http://groups.google.com/groups?hl=...ws.t-online.com
I have found the following example of the usage "random seed":

program random3
implicit none
real, dimension(10) :: randArray
integer, allocatable, dimension(:) :: seed
integer :: size

call random_seed(size=size) ! get size of seed
allocate(seed(size)) ! allocate its space
call random_seed()
call random_number(randArray)
write(*,*) randArray
call random_seed(get=seed) ! get last value of seed
call random_seed(put=seed) ! restore seed to last value
call random_number(randArray)
write(*,*) randArray
end program random3

What I cannot understand is what functions "random_seed" performs.
In the above program I have deleted all lines related with
random_seed:

program random3
implicit none
real, dimension(10) :: randArray
integer :: size
call random_number(randArray)
write(*,*) randArray
call random_number(randArray)
write(*,*) randArray
end program random3

and obtained in such way program gives the same output as the previous
one.
Can anybody explain me what for author wanted to use "random_seed" and
why it does not work?

Jan Vorbrüggen

2004-07-28, 9:06 pm

> Can anybody explain me what for author wanted to use "random_seed" and
> why it does not work?

The seed is the internal state of a pseudo-random number generator that
determines the next number being generated. Getting the seed now and setting
it later allows you to (re-)start the sequence of numbers. As written, your
sample program could indeed dispense with all calls involving the seed,
because it calls the getting and setting back-to-back. You need to allocate
the seed to the size specified by the first call because the standard
allows each implementation to have internal state of different size.

Jan

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