root / src / blas / dnrm2.f @ 4
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DOUBLE PRECISION FUNCTION DNRM2(N,X,INCX) |
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* .. Scalar Arguments .. |
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INTEGER INCX,N |
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* .. |
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* .. Array Arguments .. |
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DOUBLE PRECISION X(*) |
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* .. |
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* |
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* Purpose |
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* ======= |
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* |
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* DNRM2 returns the euclidean norm of a vector via the function |
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* name, so that |
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* |
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* DNRM2 := sqrt( x'*x ) |
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* |
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* |
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* -- This version written on 25-October-1982. |
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* Modified on 14-October-1993 to inline the call to DLASSQ. |
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* Sven Hammarling, Nag Ltd. |
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* |
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* |
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* .. Parameters .. |
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DOUBLE PRECISION ONE,ZERO |
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PARAMETER (ONE=1.0D+0,ZERO=0.0D+0) |
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* .. |
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* .. Local Scalars .. |
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DOUBLE PRECISION ABSXI,NORM,SCALE,SSQ |
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INTEGER IX |
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* .. |
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* .. Intrinsic Functions .. |
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INTRINSIC ABS,SQRT |
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* .. |
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IF (N.LT.1 .OR. INCX.LT.1) THEN |
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NORM = ZERO |
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ELSE IF (N.EQ.1) THEN |
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NORM = ABS(X(1)) |
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ELSE |
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SCALE = ZERO |
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SSQ = ONE |
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* The following loop is equivalent to this call to the LAPACK |
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* auxiliary routine: |
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* CALL DLASSQ( N, X, INCX, SCALE, SSQ ) |
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* |
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DO 10 IX = 1,1 + (N-1)*INCX,INCX |
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IF (X(IX).NE.ZERO) THEN |
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ABSXI = ABS(X(IX)) |
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IF (SCALE.LT.ABSXI) THEN |
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SSQ = ONE + SSQ* (SCALE/ABSXI)**2 |
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SCALE = ABSXI |
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ELSE |
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SSQ = SSQ + (ABSXI/SCALE)**2 |
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END IF |
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END IF |
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10 CONTINUE |
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NORM = SCALE*SQRT(SSQ) |
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END IF |
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* |
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DNRM2 = NORM |
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RETURN |
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* |
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* End of DNRM2. |
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* |
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END |