      SUBROUTINE GEOCDF(X,P,CDF)
C
C     PURPOSE--THIS SUBROUTINE COMPUTES THE CUMULATIVE DISTRIBUTION
C              FUNCTION VALUE AT THE SINGLE PRECISION VALUE X
C              FOR THE GEOMETRIC DISTRIBUTION
C              WITH SINGLE PRECISION
C              'BERNOULLI PROBABILITY' PARAMETER = P.
C              THE GEOMETRIC DISTRIBUTION USED HEREIN
C              HEREIN HAS MEAN = (1-P)/P
C              AND STANDARD DEVIATION = SQRT((1-P)/(P*P))). 
C              THIS DISTRIBUTION IS DEFINED FOR
C              ALL NON-NEGATIVE INTEGER X--X = 0, 1, 2, ... .
C              THIS DISTRIBUTION HAS THE PROBABILITY FUNCTION
C              F(X) = P * (1-P)**X.
C              THE GEOMETRIC DISTRIBUTION IS THE
C              DISTRIBUTION OF THE NUMBER OF FAILURES
C              BEFORE OBTAINING 1 SUCCESS IN AN
C              INDEFINITE SEQUENCE OF BERNOULLI (0,1)
C              TRIALS WHERE THE PROBABILITY OF SUCCESS
C              IN A SINGLE TRIAL = P.
C     INPUT  ARGUMENTS--X      = THE SINGLE PRECISION VALUE 
C                                AT WHICH THE CUMULATIVE DISTRIBUTION 
C                                FUNCTION IS TO BE EVALUATED.
C                                X SHOULD BE NON-NEGATIVE AND
C                                INTEGRAL-VALUED. 
C                     --P      = THE SINGLE PRECISION VALUE 
C                                OF THE 'BERNOULLI PROBABILITY'
C                                PARAMETER FOR THE GEOMETRIC
C                                DISTRIBUTION.
C                                P SHOULD BE BETWEEN
C                                0.0 (EXCLUSIVELY) AND
C                                1.0 (EXCLUSIVELY).
C     OUTPUT ARGUMENTS--CDF    = THE SINGLE PRECISION CUMULATIVE
C                                DISTRIBUTION FUNCTION VALUE.
C     OUTPUT--THE SINGLE PRECISION CUMULATIVE DISTRIBUTION
C             FUNCTION VALUE CDF
C             FOR THE GEOMETRIC DISTRIBUTION
C             WITH 'BERNOULLI PROBABILITY' PARAMETER = P.
C     PRINTING--NONE UNLESS AN INPUT ARGUMENT ERROR CONDITION EXISTS. 
C     RESTRICTIONS--X SHOULD BE NON-NEGATIVE AND INTEGRAL-VALUED.
C                 --P SHOULD BE BETWEEN 0.0 (EXCLUSIVELY)
C                   AND 1.0 (EXCLUSIVELY).
C     OTHER DATAPAC   SUBROUTINES NEEDED--NONE.
C     FORTRAN LIBRARY SUBROUTINES NEEDED--NONE.
C     MODE OF INTERNAL OPERATIONS--SINGLE PRECISION.
C     LANGUAGE--ANSI FORTRAN. 
C     COMMENT--NOTE THAT EVEN THOUGH THE INPUT
C              TO THIS CUMULATIVE
C              DISTRIBUTION FUNCTION SUBROUTINE
C              FOR THIS DISCRETE DISTRIBUTION
C              SHOULD (UNDER NORMAL CIRCUMSTANCES) BE A
C              DISCRETE INTEGER VALUE,
C              THE INPUT VARIABLE X IS SINGLE
C              PRECISION IN MODE.
C              X HAS BEEN SPECIFIED AS SINGLE
C              PRECISION SO AS TO CONFORM WITH THE DATAPAC
C              CONVENTION THAT ALL INPUT ****DATA****
C              (AS OPPOSED TO SAMPLE SIZE, FOR EXAMPLE)
C              VARIABLES TO ALL
C              DATAPAC SUBROUTINES ARE SINGLE PRECISION.
C              THIS CONVENTION IS BASED ON THE BELIEF THAT
C              1) A MIXTURE OF MODES (FLOATING POINT
C              VERSUS INTEGER) IS INCONSISTENT AND
C              AN UNNECESSARY COMPLICATION
C              IN A DATA ANALYSIS; AND
C              2) FLOATING POINT MACHINE ARITHMETIC
C              (AS OPPOSED TO INTEGER ARITHMETIC) 
C              IS THE MORE NATURAL MODE FOR DOING 
C              DATA ANALYSIS. 
C     REFERENCES--FELLER, AN INTRODUCTION TO PROBABILITY
C                 THEORY AND ITS APPLICATIONS, VOLUME 1,
C                 EDITION 2, 1957, PAGES 155-157, 210.
C               --NATIONAL BUREAU OF STANDARDS APPLIED MATHEMATICS
C                 SERIES 55, 1964, PAGE 929.
C     WRITTEN BY--JAMES J. FILLIBEN
C                 STATISTICAL ENGINEERING LABORATORY (205.03)
C                 NATIONAL BUREAU OF STANDARDS
C                 WASHINGTON, D. C. 20234
C                 PHONE:  301-921-2315
C     ORIGINAL VERSION--NOVEMBER  1975. 
C
C---------------------------------------------------------------------
C
      IPR=6
C
C     CHECK THE INPUT ARGUMENTS FOR ERRORS
C
      IF(P.LE.0.0.OR.P.GE.1.0)GOTO50
      IF(X.LT.0.0)GOTO55
      INTX=X+0.0001 
      FINTX=INTX
      DEL=X-FINTX
      IF(DEL.LT.0.0)DEL=-DEL
      IF(DEL.GT.0.001)GOTO60
      GOTO90
   50 WRITE(IPR,11) 
      WRITE(IPR,46)P
      CDF=0.0
      RETURN
   55 WRITE(IPR,4)
      WRITE(IPR,46)X
      CDF=0.0
      RETURN
   60 WRITE(IPR,5)
      WRITE(IPR,46)X
   90 CONTINUE
    4 FORMAT(1H , 96H***** NON-FATAL DIAGNOSTIC--THE FIRST  INPUT ARGUME
     1NT TO THE GEOCDF SUBROUTINE IS NEGATIVE *****)
    5 FORMAT(1H ,100H***** NON-FATAL DIAGNOSTIC--THE FIRST  INPUT ARGUME
     1NT TO THE GEOCDF SUBROUTINE IS NON-INTEGRAL *****)
   11 FORMAT(1H ,115H***** FATAL ERROR--THE SECOND INPUT ARGUMENT TO THE
     1 GEOCDF SUBROUTINE IS OUTSIDE THE ALLOWABLE (0,1) INTERVAL *****)
   46 FORMAT(1H , 35H***** THE VALUE OF THE ARGUMENT IS ,E15.8,6H *****)
C
C-----START POINT-----------------------------------------------------
C
      CDF=1.0-(1.0-P)**(X+1.0)
C
      RETURN
      END 
