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- SUBROUTINE LAGINT (X,Y,N,XINT,YOUT)
- C
- C FROM APPLIED NUMERICAL ANALYSIS, THIRD EDITION (GERALD / WHEATLEY)
- C CHAPTER 3, PROGRAM 1
- C
- C --------------------------------------------------------------
- C
- C THIS SUBROUTINE PERFORMS LAGRANGIAN INTERPOLATION WITHIN A SET
- C OF (X,Y) PAIRS TO GIVE THE Y VALUE CORRESPONDING TO XINT. THE
- C DEGREE OF THE INTERPOLATING POLYNOMIAL IS ONE LESS THAN THE
- C NUMBER OF POINTS SUPPLIED.
- C
- C --------------------------------------------------------------
- C
- C PARAMETERS ARE:
- C X ARRAY OF VALUES OF THE INDEPENDENT VARIABLE
- C Y ARRAY OF FUNCTION VALUES CORRESPONDING TO X
- C N NUMBER OF POINTS
- C XINT THE X-VALUE FOR WHICH ESTIMATE OF Y IS DESIRED
- C YOUT THE Y VALUE RETURNED TO CALLER
- C
- C --------------------------------------------------------------
- C
- DIMENSION X(N),Y(N)
- C
- C --------------------------------------------------------------
- C
- YOUT=0.0
- DO 20 I=1,N
- TERM = Y(I)
- DO 10 J= 1,N
- IF (I.NE.J) THEN
- TERM = TERM * (XINT - X(J))/(X(I)-X(J))
- END IF
- 10 CONTINUE
- YOUT = YOUT + TERM
- 20 CONTINUE
- RETURN
- END