### SSPTRF implementation

Posted:

**Thu Jul 02, 2009 6:53 am**I was going through the SSPTR implementation at http://www.netlib.org/lapack/explore-html/ssptrf.f.html

I have a doubt in this piece of code.

Everything is clear until line 214. It is as per the comment at line 202.

The entire IMAX row is scanned (except the diagonal element) to find what is the maximum value.

Starting from line 216, the "IMAX"th COLUMN is scanned fora larger value.

At the end of 219, "JMAX" stands for a ROW# (if IMAX was > 1) and ROWMAX could EITHER be on the IMAXth row or IMAXth column.

My point of confusion is that "JMAX" is updated readily inside the IF statement without actually looking at the result of the "MAX" statement below.

Can some1 please clear my doubts?

Thank you,

Best Regards,

Sarnath

I have a doubt in this piece of code.

- Code: Select all
`201: *`

202: * JMAX is the column-index of the largest off-diagonal

203: * element in row IMAX, and ROWMAX is its absolute value

204: *

205: ROWMAX = ZERO

206: JMAX = IMAX

207: KX = IMAX*( IMAX+1 ) / 2 + IMAX

208: DO 20 J = IMAX + 1, K

209: IF( ABS( AP( KX ) ).GT.ROWMAX ) THEN

210: ROWMAX = ABS( AP( KX ) )

211: JMAX = J

212: END IF

213: KX = KX + J

214: 20 CONTINUE

215: KPC = ( IMAX-1 )*IMAX / 2 + 1

216: IF( IMAX.GT.1 ) THEN

217: JMAX = ISAMAX( IMAX-1, AP( KPC ), 1 )

218: ROWMAX = MAX( ROWMAX, ABS( AP( KPC+JMAX-1 ) ) )

219: END IF

Everything is clear until line 214. It is as per the comment at line 202.

The entire IMAX row is scanned (except the diagonal element) to find what is the maximum value.

Starting from line 216, the "IMAX"th COLUMN is scanned fora larger value.

At the end of 219, "JMAX" stands for a ROW# (if IMAX was > 1) and ROWMAX could EITHER be on the IMAXth row or IMAXth column.

My point of confusion is that "JMAX" is updated readily inside the IF statement without actually looking at the result of the "MAX" statement below.

Can some1 please clear my doubts?

Thank you,

Best Regards,

Sarnath