HP MLIB User's Guide Vol. 1 7th Ed.

102 HP MLIB User’s Guide
SLSTxx/DLSTxx/ILSTxx/CLSTxx/ZLSTxx List selected vector elements
INTEGER*4 I,N
REAL*8 A,B,D,DLIM,R
REAL*8 F(20000),X(20000),Y(20000),Z(20000)
A = ...
B = ...
DLIM = ...
R = ...
N = 20000
DO 10 I = 1, N
D = SQRT( X(I)**2 + Y(I)**2 + Z(I)**2 ) - R
IF ( D .GT. DLIM ) THEN
F(I) = A * EXP( B * D )
ELSE
CALL FORCE (D,F(I))
END IF
10 CONTINUE
Change D to an array and introduce array INDX to hold the indices
corresponding to the ELSE clause. Split the body of the DO loop into two parts.
The first part corresponds to the body of the loop before the IF statement and
the THEN clause. It fully optimizes, so, even though it computes a few more
exponentials than the original code, it is still considerably faster. DLSTLE is
then called to determine the indices for which the ELSE clause must be
executed, and the second DO loop executes the ELSE clause for those indices.
The resulting program segment is:
INTEGER*4 I,J,N,NINDX,INDX(20000)
REAL*8 A,B,DLIM,R
REAL*8 D(20000),F(20000),X(20000),Y(20000),Z(20000)
A = ...
B = ...
DLIM = ...
R = ...
N = 20000
DO 10 I = 1, N
D(I) = SQRT( X(I)**2 + Y(I)**2 + Z(I)**2 ) - R
F(I) = A * EXP( B * D(I) )
10 CONTINUE
CALL DLSTLE (N,D,1,DLIM,NINDX,INDX)
DO 20 J = 1, NINDX
I = INDX(J)
CALL FORCE (D(I),F(I))
20 CONTINUE