Re: Machine epsilon

From: jkl "-at-" ccl.net
Subject: Re: Machine epsilon
Date: Tue, 25 Jun 91 11:32:27 -0400
I am forwarding the actual code for macheps kindly submitted by Eric:
 ---------- Begin Forwarded Message ----------
 >From stahlber "-at-" hydrogen.mps.ohio-state.edu Tue Jun 25 11:11:02 1991
 >Date: Tue, 25 Jun 91 11:15:22 EDT
 >From: Eric Stahlberg <stahlber "-at-" hydrogen.mps.ohio-state.edu>
 >To: jkl "-at-" ccl.net
 >Subject: Re: Machine epsilon
 The machine epsilon can easily be found with the following program:
       PROGRAM MACHPR
       REAL*4 R4,TOL4,ZERO4
       REAL*8 R8,TOL8,ZERO8
 *MDC*IF IBM CRAY
       REAL*16 R16,TOL16,ZERO16
 *MDC*ENDIF
 C
       ZERO4=0.0
       ZERO8=0.0
       R4=1.0
       TOL4=R4
   100 IF ((R4+TOL4).EQ.R4) GOTO 200
       PRINT *,TOL4
       TOL4=TOL4/2.0+D0
       GOTO 100
  200  PRINT *,'MACHINE PRECISION R4 IS GT :', TOL4
       R8=1.0
       TOL8=R8
   300 IF ((R8+TOL8).EQ.R8) GOTO 400
       PRINT *,TOL8
       TOL8=TOL8/2.0+D0
       GOTO 300
  400  PRINT *,'MACHINE PRECISION R8 IS GT :', TOL8
 *MDC*IF IBM CRAY
       R16=1.0
       TOL16=R16
   500 IF ((R16+TOL16).EQ.R16) GOTO 600
       PRINT *,TOL16
       TOL16=TOL16/2.0+D0
       GOTO 500
  600  PRINT *,'MACHINE PRECISION R16 IS GT :', TOL16
 *MDC*ENDIF
       STOP
       END
 I have used it to determine the machine precision for various machines
 and word sizes.
 Eric Stahlberg
 OSU Chemistry
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