PROGRAM BREAK
      REAL HI,HR,HLOW,HIGH,HALF,XL,XH,XM,D,TOL
      PARAMETER (TOL=10E-6)
C PROBLEM - FIND HI FROM EXPRESSION GIVEN IN FUNCTION F
      F(A,B,C)=A*(1.0-0.8*EXP(-0.6*C/A))-B
C HI IS INCIDENT WAVE HEIGHT (C)
C HR IS REFORMED WAVE HEIGHT (B)
C D IS WATER DEPTH AT TERRACE EDGE (A)
      PRINT*,' GIVE REFORMED WAVE HEIGHT, AND WATER DEPTH'
      READ*,HR,D
C
C FOR HLOW- LET HLOW=HR
C FOR HIGH- LET HIGH=HLOW*2.0
C
C CHECK THAT SIGNS OF FUNCTION RESULTS ARE DIFFERENT
C
      HLOW=HR
      HIGH=HLOW*2.0
      XL=F(HLOW,HR,D)
      XH=F(HIGH,HR,D)
C BEGINNING OF WHILE
1     IF((XL*XH).GE.0.0) THEN
        HIGH=HIGH*2.0
        XH=F(HIGH,HR,D)
        GOTO 1
      ENDIF
C BEGINNING OF REPEAT UNTIL
2     HALF=(HLOW+HIGH)*0.5
      XM=F(HALF,HR,D)
      IF((XL*XM).LT.0.0)THEN
        XH=XM
        HIGH=HALF
      ELSE
        XL=XM
        HLOW=HALF
      ENDIF
      IF(ABS(HIGH-HLOW).GT.TOL)GO TO 2
C END OF REPEAT UNTIL
      PRINT*,' INCIDENT WAVE HEIGHT LIES BETWEEN'
      PRINT*,HLOW,' AND ',HIGH,' METRES'
      END