PROGRAM Mean_Time_to_Failure
!-----------------------------------------------------------------------
! Program to read a list of failure times, count them, and find the
! mean time to failure.  Values are read until an end-of-data flag
! is read.  Identifiers used are:
!   FailureTime     : the current failure time read
!   EndDataFlag     : a constant -- the end-of-data flag
!   NumTimes        : the number of failure time readings
!   Sum             : sum of failure times
!   MeanFailureTime : the mean time to failure 
!
! Input:  A list of failure times
! Output: Number of failure times read and their mean or a message
!         indicating that no failure times were entered
!-----------------------------------------------------------------------

  IMPLICIT NONE
  INTEGER :: NumTimes
  REAL :: FailureTime, Sum, MeanFailureTime
  REAL, PARAMETER :: EndDataFlag = -1.0

  ! Initialize Sum and NumTimes and give instructions to user
  Sum = 0
  NumTimes = 0
  PRINT *, "Enter failure time of", EndDataFlag, "to stop."

  ! Repeat the following
  DO
     PRINT *, "Enter failure time:"
     READ *, FailureTime
 
     ! If end-of-data, terminate repetition
     IF (FailureTime == EndDataFlag) EXIT

     ! Otherwise, continue with the following:
     NumTimes = NumTimes + 1
     Sum = Sum + FailureTime
  END DO

  ! Calculate and display mean time to failure
  IF (NumTimes /= 0) THEN
     MeanFailureTime = Sum / NumTimes
     PRINT *
     PRINT *, "Number of failure time readings:", NumTimes
     PRINT *, "Mean time to failure:", MeanFailureTime
  ELSE
     PRINT *, "No failure times were entered."
  END IF

END PROGRAM Mean_Time_to_Failure