YATES@A.CHEM.UPENN.EDU ("Yates, John H.") (07/10/87)
C makes me nervous! Below is an example of the dangers involved with
floating point square roots. Is something wrong here or does one just
have to be extremely careful in C with:
1) assigning the proper lnk$libraries for your specific case
2) doing the type conversions explicitly, extra care for read in
variables
3) being extremly careful with the /G switch, when needed vs. not
needed.
Is there a bug here or is it just that C must never, ever be treated
as a high level language for floating point operations?
$ ty gsqrt.c
/* This program gives an example of a G floating problem when a variable
is input with scanf . First, the lnk$library logicals must be set
in the proper order to get the G code, second, when this is done
and the CC/G compiles the program, input variables evidently are
NOT G type, you must use mth$cvt_d_g() to convert them!
$! The following logicals allow /G_Float qualifier to be used
$! with the CC command and Curses (a screen management package)
$!
$ define lnk$library sys$library:vaxccurse
$ define lnk$library_1 sys$library:vaxcrtlg
$ define lnk$library_2 sys$library:vaxcrtl
and use
$ cc/g gsqrt
$ link gsqrt
to compile and link the code.
*/
#include math
#include stdio
main( )
{
double x1 ,x2, y1, y2, x3, y3;
extern double mth$cvt_d_g();
double sqrt();
printf("Enter 9.0 for this test: ");
scanf("%f", &x1);
y1 = sqrt(x1);
x2 = 9.0;
y2 = sqrt(x2);
x3 = mth$cvt_d_g(&x1);
y3 = sqrt(x3);
printf("\n\nsqrt of input number %e = %e\n",x1, y1);
printf("sqrt of hardwired number %e = %e\n",x2, y2);
printf("sqrt of d->g input number %e = %e\n",x3, y3);
}
$ ty assign1.com
$! The following logicals allow /G_Float qualifier to be used
$! with the CC command and Curses (a screen management package)
$! according to the C manual, anyway
$!
$ define lnk$library sys$library:vaxccurse
$ define lnk$library_1 sys$library:vaxcrtlg
$ define lnk$library_2 sys$library:vaxcrtl
$ @assign1
$ sh log lnk$*
(LNM$PROCESS_TABLE)
"LNK$LIBRARY" = "SYS$LIBRARY:VAXCCURSE"
"LNK$LIBRARY_1" = "SYS$LIBRARY:VAXCRTLG"
"LNK$LIBRARY_2" = "SYS$LIBRARY:VAXCRTL"
(LNM$JOB_8028F360)
(LNM$GROUP_000011)
(LNM$SYSTEM_TABLE)
$ cc/lis gsqrt
$ link gsqrt
$ run gsqrt
Enter 9.0 for this test:
9.0
sqrt of input number 9.000000e+00 = 2.250000e+00
sqrt of hardwired number 9.000000e+00 = 2.250000e+00
sqrt of d->g input number 7.578125e-01 = 6.562500e-01
$ cc/g gsqrt
$ link gsqrt
$ run gsqrt
Enter 9.0 for this test:
9.0
sqrt of input number 4.294967e+09 = 6.553600e+04
sqrt of hardwired number 9.000000e+00 = 3.000000e+00
sqrt of d->g input number 9.000000e+00 = 3.000000e+00
$ ty gsqrt.lis (to show C version no.)
GSQRT 9-JUL-1987
23:59:39 VAX C V2.2-015 Page 1
V1.0 9-JUL-1987
23:52:57 SYS$SYSDEVICE:[JY.CBUG]GSQRT.C;1 (1)
Is it any wonder I keep putting away my K and R? Johntli@sargas.usc.edu (Tony Li) (07/12/87)
In article <8707100533.AA24900@super.upenn.edu> YATES@A.CHEM.UPENN.EDU ("Yates, John H.") writes:
C makes me nervous! Below is an example of the dangers involved with
floating point square roots.
double x1 ,x2, y1, y2, x3, y3;
...
printf("Enter 9.0 for this test: ");
scanf("%f", &x1);
...
printf("\n\nsqrt of input number %e = %e\n",x1, y1);
printf("sqrt of hardwired number %e = %e\n",x2, y2);
printf("sqrt of d->g input number %e = %e\n",x3, y3);
Is it any wonder I keep putting away my K and R? John
Please get out your K&R and your Vax C manual. Floats and doubles and
G floating are *NOT* interchangeable. When you called scanf, you
asked it to read a float. It did, and stored it in x1. Scanf has no
idea that x1 is a g-float, so it just read an f-float and stored the
binary representation into x1. Since the binary representation for
f-float and g-float is radically different, you would seem to get
garbage.
The trick here is to tell the runtimes to expect a double precision
value. This can easily be done by specifying "%lf" and "%le".
;-)
Tony Li - USC University Computing Services "Fene mele kiki bobo"
Uucp: oberon!tli -- Joe Isuzu
Bitnet: tli@uscvaxq, tli@ramoth
Internet: tli@sargas.usc.eduik.naggum-erik@siri.uio.no (07/13/87)
C makes me nervous! Below is an example of the dangers involved with
floating point square roots. Is something wrong here or does one just
have to be extremely careful in C with:
1) assigning the proper lnk$libraries for your specific case
2) doing the type conversions explicitly, extra care for read in
variables
3) being extremly careful with the /G switch, when needed vs. not
needed.
Is there a bug here or is it just that C must never, ever be treated
as a high level language for floating point operations?
$ ty gsqrt.c
/* This program gives an example of a G floating problem when a variable
is input with scanf . First, the lnk$library logicals must be set
in the proper order to get the G code, second, when this is done
and the CC/G compiles the program, input variables evidently are
NOT G type, you must use mth$cvt_d_g() to convert them!
$! The following logicals allow /G_Float qualifier to be used
$! with the CC command and Curses (a screen management package)
$!
$ define lnk$library sys$library:vaxccurse
$ define lnk$library_1 sys$library:vaxcrtlg
$ define lnk$library_2 sys$library:vaxcrtl
and use
$ cc/g gsqrt
$ link gsqrt
to compile and link the code.
*/
#include math
#include stdio
main( )
{
double x1 ,x2, y1, y2, x3, y3;
extern double mth$cvt_d_g();
double sqrt();
printf("Enter 9.0 for this test: ");
scanf("%f", &x1);
y1 = sqrt(x1);
x2 = 9.0;
y2 = sqrt(x2);
x3 = mth$cvt_d_g(&x1);
y3 = sqrt(x3);
printf("\n\nsqrt of input number %e = %e\n",x1, y1);
printf("sqrt of hardwired number %e = %e\n",x2, y2);
printf("sqrt of d->g input number %e = %e\n",x3, y3);
}
$ ty assign1.com
$! The following logicals allow /G_Float qualifier to be used
$! with the CC command and Curses (a screen management package)
$! according to the C manual, anyway
$!
$ define lnk$library sys$library:vaxccurse
$ define lnk$library_1 sys$library:vaxcrtlg
$ define lnk$library_2 sys$library:vaxcrtl
$ @assign1
$ sh log lnk$*
(LNM$PROCESS_TABLE)
"LNK$LIBRARY" = "SYS$LIBRARY:VAXCCURSE"
"LNK$LIBRARY_1" = "SYS$LIBRARY:VAXCRTLG"
"LNK$LIBRARY_2" = "SYS$LIBRARY:VAXCRTL"
(LNM$JOB_8028F360)
(LNM$GROUP_000011)
(LNM$SYSTEM_TABLE)
$ cc/lis gsqrt
$ link gsqrt
$ run gsqrt
Enter 9.0 for this test:
9.0
sqrt of input number 9.000000e+00 = 2.250000e+00
sqrt of hardwired number 9.000000e+00 = 2.250000e+00
sqrt of d->g input number 7.578125e-01 = 6.562500e-01
$ cc/g gsqrt
$ link gsqrt
$ run gsqrt
Enter 9.0 for this test:
9.0
sqrt of input number 4.294967e+09 = 6.553600e+04
sqrt of hardwired number 9.000000e+00 = 3.000000e+00
sqrt of d->g input number 9.000000e+00 = 3.000000e+00
$ ty gsqrt.lis (to show C version no.)
GSQRT 9-JUL-1987
23:59:39 VAX C V2.2-015 Page 1
V1.0 9-JUL-1987
23:52:57 SYS$SYSDEVICE:[JY.CBUG]GSQRT.C;1 (1)
Is it any wonder I keep putting away my K and R? Johnik.naggum-erik@siri.uio.no (07/15/87)
"The best way to predict the future is to invent it." (EVB S/W Eng, Inc.) Message-ID: <12317866627.139.IK.NAGGUM-ERIK@SIRI> Yates@a.chem.upenn.edu writes: > C makes me nervous! Below is an example of the dangers involved with > Floating point square roots. Is something wrong here or does one just > have to be extremely careful in C with: ...... > Is there a bug here or is it just that C must never, ever be treated > as a high level language for floating point operations? ...... > $! The following logicals allow /G_Float qualifier to be used > $! with the CC command and Curses (a screen management package) > $! according to the C manual, anyway ...... > Is it any wonder I keep putting away my K and R? John John, I regard myself as an expert in C and Unix. I have read the Ansi Standard Draft of 10/10/86, and Kernighan and Richie: The C Programming Language. With 3 years of experience in C programming, and 200K source lines on my conscience, I have some comments to your article, blaming C for various flaws in floating point operations on a VAX (model X?), using VAX C version 2.2 under VMS. I took the time to go through a lot of documentation tonight, and visited those for (AT&T) Unix Version 7, System III and System V, for SVID, for BSD release 4.3 and Ultrix 2.0, for XENIX 3 and XENIX 5, and finally those of SUN OS. (Insert list of trademark owners here.) Nowhere can I find a reference to G-floating or, in fact, any other specific implementation of floating point numbers, as part of the C language. Rather, I have found that each machine and each implementor decides which formats to use, and that they are more an attribute of the hardware than of the language definition. You ask, > Is it any wonder I keep putting away my K and R? Why not put away your VAX VMS or your VAX C, instead? Erik Naggum ARPA: enag@siri.uio.no or enag%siri@ifi.uio.no Manager SNAIL: POB 1560 Vika, N-0118 OSLO 1, NORWAY Naggum Software PHONE: (intl)+47-2-549-163 (0600-1200 GMT) -------