drotg.f.html

来自「famous linear algebra library (LAPACK) p」· HTML 代码 · 共 61 行

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<pre>
      SUBROUTINE <a name="DROTG.1"></a><a href="drotg.f.html#DROTG.1">DROTG</a>(DA,DB,C,S)
<span class="comment">*</span><span class="comment">     .. Scalar Arguments ..
</span>      DOUBLE PRECISION C,DA,DB,S
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  Purpose
</span><span class="comment">*</span><span class="comment">  =======
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     construct givens plane rotation.
</span><span class="comment">*</span><span class="comment">     jack dongarra, linpack, 3/11/78.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     .. Local Scalars ..
</span>      DOUBLE PRECISION R,ROE,SCALE,Z
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Intrinsic Functions ..
</span>      INTRINSIC DABS,DSIGN,DSQRT
<span class="comment">*</span><span class="comment">     ..
</span>      ROE = DB
      IF (DABS(DA).GT.DABS(DB)) ROE = DA
      SCALE = DABS(DA) + DABS(DB)
      IF (SCALE.NE.0.0d0) GO TO 10
      C = 1.0d0
      S = 0.0d0
      R = 0.0d0
      Z = 0.0d0
      GO TO 20
   10 R = SCALE*DSQRT((DA/SCALE)**2+ (DB/SCALE)**2)
      R = DSIGN(1.0d0,ROE)*R
      C = DA/R
      S = DB/R
      Z = 1.0d0
      IF (DABS(DA).GT.DABS(DB)) Z = S
      IF (DABS(DB).GE.DABS(DA) .AND. C.NE.0.0d0) Z = 1.0d0/C
   20 DA = R
      DB = Z
      RETURN
      END

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