comparison

[ndun78]

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;
}else{
status_bit = 0;
}

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

[ndun78]

On Oct 21, 3:10 pm, [e-mail] wrote:

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for

signed int i;
signed int j;

if(i < j){
{
status_bit = 1;

}else{
status_bit = 0;
}

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

[Martin Ambuhl]

nd****@gmail.com wrote:

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;
}else{
status_bit = 0;
}

Try the simple
status_bit = (i < j);

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

Is there a reason for avoiding the obvious
status_bit = (i < j);
?

[Charlie Gordon]

"Martin Ambuhl" <ma*****@earthlink.neta écrit dans le message de news:
5o************@mid.individual.net...

nd****@gmail.com wrote:

>I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;
}else{
status_bit = 0;
}

Try the simple
status_bit = (i < j);

>This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

Is there a reason for avoiding the obvious
status_bit = (i < j);

In other words: if there is a way to compute this without tests and jumps,
the compiler should know.

--
Chqrlie.

[ndun78]

Is there a reason for avoiding the obvious
status_bit = (i < j);

I think that it is not guaranteed to get only 1 or 0 as a result of i
< j, this may result in 0 or non zero value is it not?

[santosh]

nd****@gmail.com wrote:

>

Is there a reason for avoiding the obvious
status_bit = (i < j);

I think that it is not guaranteed to get only 1 or 0 as a result of i
< j, this may result in 0 or non zero value is it not?

No, the relational operators yield either 1 or 0.

[ndun78]

No, the relational operators yield either 1 or 0.

Is it guaranteed by ANSI C standard?

[pete]

nd****@gmail.com wrote:

>

Is there a reason for avoiding the obvious
status_bit = (i < j);

I think that it is not guaranteed to get only 1 or 0 as a result of i
< j, this may result in 0 or non zero value is it not?

No.
(i < j) is can only be equal to either one or zero.

--
pete

[santosh]

nd****@gmail.com wrote:

>

>No, the relational operators yield either 1 or 0.

Is it guaranteed by ANSI C standard?

Yes. From n1256.pdf:

6.5.8 Relational operators

....

6 Each of the operators < (less than), (greater than), <= (less
than or equal to), and >= (greater than or equal to) shall yield 1
if the speci?ed relation is true and 0 if it is false.92)
The result has type int.

[Army1987]

On Sun, 21 Oct 2007 10:10:51 +0000, ndun78 wrote:

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;
}else{
status_bit = 0;
}

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?

If *you* don't know that...

2. what could be the solution?

status_bit = (i < j);
Boolean operators evaluate to 0 if false and 1 if true.

--
Army1987 (Replace "NOSPAM" with "email")
A hamburger is better than nothing.
Nothing is better than eternal happiness.
Therefore, a hamburger is better than eternal happiness.

[abhy]

On Oct 21, 3:10 pm, [e-mail] wrote:

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;

}else{
status_bit = 0;
}

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

you as well write this as status_bit = i - j 1 ? 0 : 1

[Martin Ambuhl]

nd****@gmail.com wrote:

>>Is there a reason for avoiding the obvious
status_bit = (i < j);

I think that it is not guaranteed to get only 1 or 0 as a result of i
< j, this may result in 0 or non zero value is it not?

You are wrong. Check your elementary C textbook.

[Charlie Gordon]

"abhy" <ab************@gmail.coma écrit dans le message de news:
11*********************@z24g2000prh.googlegroups.c om...

On Oct 21, 3:10 pm, [e-mail] wrote:

>I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){
{
status_bit = 1;

}else{
status_bit = 0;
}

This actually works for most of the cases but fails for two values
i = 0x80000000
j = 0x7fffffff

I want a linear code for the above i.e. without jumps, so
1. what could be the issue?
2. what could be the solution?

you as well write this as status_bit = i - j 1 ? 0 : 1

No, it does not work at all: the expression i - j 1 is not equivalent to
!(i < j) and even the more accurate i - j < 0 does not produce the correct
result if i - j overflows (as the OP correctly pointed out).

If you *really* want to compute the status bit with plain arithmetics then
you can use this:

status_bit = ((unsigned long long)((long long)i - j) >(CHAR_BIT *
sizeof(int) + 1)) & 1;

It should work if sizeof(long long) sizeof(int).
It may even be acceptably efficient on 64 bit architectures.

But it would be crazy to use such a convoluted expression when ``status_bit
= (i < j)'' is vastly more readable and quite unlikely to be a performance
killer.

--
Chqrlie

[Ben Pfaff]

nd****@gmail.com writes:

I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){

You should buy a copy of the book _Hacker's Delight_ by Henry
S. Warren. It is a compendium of tricks like this. This
particular problem, and related inequalities, takes up a few
pages in chapter 2, for example.
--
Ben Pfaff
http://benpfaff.org

[Charlie Gordon]

"Ben Pfaff" <bl*@cs.stanford.edua écrit dans le message de news:
87************@blp.benpfaff.org...

nd****@gmail.com writes:

>I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){

You should buy a copy of the book _Hacker's Delight_ by Henry
S. Warren. It is a compendium of tricks like this. This
particular problem, and related inequalities, takes up a few
pages in chapter 2, for example.

You will find this "Bit Twiddling Hacks" entertaining as well:

http://graphics.stanford.edu/~seander/bithacks.html

Good night!

--
Chqrlie.

[Charlie Gordon]

"Ben Pfaff" <bl*@cs.stanford.edua écrit dans le message de news:
87************@blp.benpfaff.org...

nd****@gmail.com writes:

>I want to write something like
status_bit = (unsigned)(i - j) >31

as an equivalent code for
if(i < j){

You should buy a copy of the book _Hacker's Delight_ by Henry
S. Warren. It is a compendium of tricks like this. This
particular problem, and related inequalities, takes up a few
pages in chapter 2, for example.

You will find this "Bit Twiddling Hacks" entertaining as well:

http://graphics.stanford.edu/~seander/bithacks.html

Good night!

--
Chqrlie.

[CBFalconer]

nd****@gmail.com wrote:

>

>>Is there a reason for avoiding the obvious
status_bit = (i < j);

I think that it is not guaranteed to get only 1 or 0 as a result
of i < j, this may result in 0 or non zero value is it not?

Yes, the result of a logical statement is guaranteed to be 0 or 1.

Please do not strip attributions (the "joe wrote:" headers) for
material you quote.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[santosh]

Keith Thompson wrote:

CBFalconer <cb********@yahoo.comwrites:

>nd****@gmail.com wrote:

[...]

>>I think that it is not guaranteed to get only 1 or 0 as a result
of i < j, this may result in 0 or non zero value is it not?

Yes, the result of a logical statement is guaranteed to be 0 or 1.

By "logical statement", of course, you mean a logical *operator*.

Off the top of my head, the following operators yield logical results
that are guaranteed to be either 0 or 1:

= != < <= = ! && ||

Here don't you mean `==` instead of `=`?

<snip>

[Keith Thompson]

santosh <sa*********@gmail.comwrites:

Keith Thompson wrote:

[...]

>Off the top of my head, the following operators yield logical results
that are guaranteed to be either 0 or 1:

= != < <= = ! && ||

Here don't you mean `==` instead of `=`?

Yes, of course, thanks.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Mark McIntyre]

On Tue, 23 Oct 2007 13:03:25 -0700, in comp.lang.c , Keith Thompson
<ks***@mib.orgwrote:

>
<OT>
A personal note: Don't expect a lot of postings from me in the next
few days. We've had to evacuate our home due to the huge fires here
in San Diego. We're ok (and so is our house so far), but my Internet
access will be sporadic until we can go back home.
</OT>

Hope everything is ok.
--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

[Keith Thompson]

Mark McIntyre <ma**********@spamcop.netwrites:

On Tue, 23 Oct 2007 13:03:25 -0700, in comp.lang.c , Keith Thompson
<ks***@mib.orgwrote:

>>
<OT>
A personal note: Don't expect a lot of postings from me in the next
few days. We've had to evacuate our home due to the huge fires here
in San Diego. We're ok (and so is our house so far), but my Internet
access will be sporadic until we can go back home.
</OT>

Hope everything is ok.

Yes, thanks. We're back home now after staying at my mother's house
for two days. The air still stinks, but the worst is past, at least
for us (though not for people in other parts of the county where the
fires are still burning).

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"