REAL FUNCTION LOG2 (A)
C
C This function computes the base two logarithm of A
C
      IMPLICIT NONE
      REAL A
      
      LOG2 = LOG10(A)/LOG10(2.0)

      RETURN
      END



      PROGRAM USRDEF
C
C This program motivates the utility of user-defined functions
C
      IMPLICIT NONE
      REAL LOG2
      REAL X, Y, Z
      
      PRINT *, 'Please enter a real number'
      READ *, X
      PRINT *, 'The base two logarithm is ', LOG2(X)

      PRINT *, 'Please enter another real number'
      READ *, Y
      PRINT *, 'The base two logarithm is ', LOG2(Y)

      PRINT *, 'Please enter a third real number'
      READ *, Z
      PRINT *, 'The base two logarithm is ', LOG2(Z)

      STOP
      END