      SUBROUTINE EXPPDF(X,PDF)
C
C     PURPOSE--THIS SUBROUTINE COMPUTES THE PROBABILITY DENSITY
C              FUNCTION VALUE FOR THE EXPONENTIAL DISTRIBUTION
C              WITH MEAN = 1 AND STANDARD DEVIATION = 1.
C              THIS DISTRIBUTION IS DEFINED FOR ALL NON-NEGATIVE X,
C              AND HAS THE PROBABILITY DENSITY FUNCTION
C              F(X) = EXP(-X).
C     INPUT  ARGUMENTS--X      = THE SINGLE PRECISION VALUE AT
C                                WHICH THE PROBABILITY DENSITY
C                                FUNCTION IS TO BE EVALUATED.
C     OUTPUT ARGUMENTS--PDF    = THE SINGLE PRECISION PROBABILITY
C                                DENSITY FUNCTION VALUE.
C     OUTPUT--THE SINGLE PRECISION PROBABILITY DENSITY
C             FUNCTION VALUE PDF.
C     PRINTING--NONE UNLESS AN INPUT ARGUMENT ERROR CONDITION EXISTS. 
C     RESTRICTIONS--X SHOULD BE NON-NEGATIVE.
C     OTHER DATAPAC   SUBROUTINES NEEDED--NONE.
C     FORTRAN LIBRARY SUBROUTINES NEEDED--EXP.
C     MODE OF INTERNAL OPERATIONS--SINGLE PRECISION.
C     LANGUAGE--ANSI FORTRAN. 
C     REFERENCES--JOHNSON AND KOTZ, CONTINUOUS UNIVARIATE
C                 DISTRIBUTIONS--1, 1970, PAGES 207-232.
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--JUNE      1972. 
C     UPDATED         --SEPTEMBER 1975. 
C     UPDATED         --NOVEMBER  1975. 
C
C---------------------------------------------------------------------
C
      IPR=6
C
C     CHECK THE INPUT ARGUMENTS FOR ERRORS
C
      IF(X.LT.0.0)GOTO50
      GOTO90
   50 WRITE(IPR,4)
      WRITE(IPR,46)X
      PDF=0.0
      RETURN
   90 CONTINUE
    4 FORMAT(1H , 96H***** NON-FATAL DIAGNOSTIC--THE FIRST  INPUT ARGUME
     1NT TO THE EXPPDF SUBROUTINE IS NEGATIVE *****)
   46 FORMAT(1H , 35H***** THE VALUE OF THE ARGUMENT IS ,E15.8,6H *****)
C
C-----START POINT-----------------------------------------------------
C
      PDF=EXP(-X)
C
      RETURN
      END 
