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Technotools (Chestnut CD-ROM)(1993).ISO
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lagrint
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lagint.for
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1989-06-18
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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