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he conseguido este programa para el mastermind

MODULE CSEED
  INTEGER :: SEED
  INTEGER :: num
  CHARACTER(LEN=1),DIMENSION(6)::gameLetters !Define allowed letters
END MODULE CSEED

PROGRAM MasterMind
    USE CSEED
    IMPLICIT NONE
    REAL, EXTERNAL :: RANDN
    INTEGER :: i,j
    REAL :: x
    INTEGER, DIMENSION(50) :: first = (/( 0, i=1,50)/)
    CHARACTER(LEN=1), DIMENSION(4):: list
    CHARACTER(LEN=1), DIMENSION(4):: gameCombination,userGuess,gamePoints
    CHARACTER(LEN=4):: gamePointsChar='0000'
    list =(/'G','Y','B','R'/)
    gameLetters=(/'G','Y','B','R','V','S'/)
    PRINT *, "Welcome to the MasterMind", "Game in Fortran"
            !"*****************************************"
    PRINT *, "*****************************************"
    PRINT *, " Here are the rules: I choose a random "
    PRINT *, " combination of 4 colors out of 6. You have "
    PRINT *, " 10 tries to guess what I chose. "
    PRINT *, " The colors possible are: "
    PRINT *, " Green (G), Yellow (Y),Blue (, Red (R),   "
    PRINT *, " Violet (V), Silver (S) "

    call getGameCombination(gameCombination)
    !no real need to print the game combination and discover the user
    !what  it is
    PRINT *,  gameCombination, " FROM OUTSIDE"
    DO i=1,10
    call getGuess(userGuess)
    call compare(gameCombination, userGuess,gamePoints)
    !Reset Scores
    !gamePointsChar='0000'
    DO j=1,4
    gamePointsChar(j:j)=gamePoints(j)
    END DO
    IF (gamePointsChar=='BBBB') THEN
    print *, "That's right ! ", gameCombination, " Is the guess. &
    &You win after",  i, "rounds !"
    exit
    END IF
    END DO
    IF (i==10) THEN
    PRINT *, "You have reached the maximu allowed tries, you lose ...&
    & but you can always try again!"
    END IF
END PROGRAM MasterMind

SUBROUTINE compare(gameCombination, userGuess,roundPoints)
!SUBROUTINE compare(userGuess)
use CSEED
IMPLICIT NONE
INTENT(IN) :: gameCombination, userGuess
INTENT (OUT) :: roundPoints
CHARACTER(LEN=1),DIMENSION(4):: roundPoints, gameCombination, userGuess
INTEGER :: i,j
roundPoints = (/'0','0','0','0'/)
!Just for practicing character operations roundPoints is also converted
!to a string
DO i=1,4
    DO j=1,4
    IF (gameCombination(i) == userGuess(i)) THEN
        roundPoints(i)='B'
        exit
        !PRINT *, roundPoints
    END IF
    !else if (gameCombination(i) == userGuess(j)) THEN
    if (gameCombination(i) == userGuess(j)) THEN
        roundPoints(j)='W'
        !PRINT *, roundPoints
        !exit
    END IF

    !ELSE
    !IF (gameCombination(i) == userGuess(j)) THEN
    !    roundPoints(j)='W'
    !END IF
    END DO
END DO
PRINT *, "Your score " , roundPoints
!no need to give any output variable, just print scores to screen
END SUBROUTINE compare

SUBROUTINE getGameCombination(gameCombination)
! A subroutine to choose  the game combiantion from the game
! letters
  USE CSEED
  IMPLICIT NONE
  CHARACTER(LEN=1), DIMENSION(4), INTENT (OUT):: gameCombination
  REAL, EXTERNAL :: RANDN
  INTEGER :: I
  REAL :: Y
  INTEGER, DIMENSION (8) :: T
  CALL DATE_AND_TIME(VALUES = T)
  SEED = T(1)+70*(T(2)+12*(T(3)+31*(T(5)+23*(T(6)+59*T(7)))))
  IF (MOD(SEED,2).EQ.0) SEED = SEED-1
  DO I = 1, 4
    Y = RANDN()
  !get numbers from 1 to 6
  num = int(Y*6+1)
  gameCombination(I)=gameLetters(num)
  END DO
END SUBROUTINE getGameCombination

SUBROUTINE getGuess(userGuess)
    !get user guess
    IMPLICIT NONE
    INTENT(INOUT) :: userGuess
    !The subrutine has to know the shape and type of
    !userGuess
    CHARACTER(LEN=1), DIMENSION(4):: userGuess
    !subrutine to get players guess
    PRINT *, "Type your guess as 4 letters separated by spaces:"
    PRINT *, 'Valid input is:  "G B S V"'
    !PRINT *,'asd',userGuess
    READ *, userGuess
    PRINT *, "Here is your guess:   ", userGuess
END SUBROUTINE getGuess

REAL FUNCTION RANDN()
! Function to generate a uniform random number in [0,1]
! following x(i+1)=a*x(i) mod c with a=7** 5 and
! c=2** 31-1.  Here the seed is a global variable.
!
  USE CSEED
  IMPLICIT NONE
  INTEGER :: H, L, T, A, C, Q, R
  DATA A/16807/, C/2147483647/, Q/127773/, R/2836/
  !The DATA statement is used to specify initial values
  !for variables and array elements.
  H = SEED/Q
  L = MOD(SEED, Q)
  T = A*L - R*H
  IF (T .GT. 0) THEN
    SEED = T
  ELSE
    SEED = C + T
  END IF
  RANDN = SEED/FLOAT(C)
END FUNCTION RANDN



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pero no entiendo la funcion randn() y queria saber si existe otra forma de conseguir el numero aleatorio.He visto la subrutina RANDOM_NUMBER pero no se muy bien como utilizarla dentro de este programa