hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
21  16824 
"Abhishek Jha" <ab***********@yahoo.co.in> wrote in message
news:60**************************@posting.google.c om... hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
[ a=a+b;b=a-b;a=a-b; ] is a bad way to swap two numbers due to possible
overflow. A better method without using a temporary variable is to use the
XOR trick[*] (which works for non-integral types as well). But, the best
way is probably just to use a standard temporary variable and let the
compiler do the optimization.
[*] a^=b; b^=a; a^=b;
Pedro Graca wrote: Abhishek Jha wrote: this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also
swap without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was
also done in three statements. Can anyon suggest this swapping in
anything less then that.
a = a-(b=(a=a+b)-b);
This is even worse than the original silliness as it always invokes
undefined behavior.
Rob Gamble ab***********@yahoo.co.in (Abhishek Jha) writes: hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
C FAQ <http://www.eskimo.com/~scs/C-faq/q10.3.html>, question 10.3.
--
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.
On Sat, 16 Oct 2004 16:45:22 +0000, Pedro Graca wrote: Abhishek Jha wrote: this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
a = a-(b=(a=a+b)-b);
int a = INT_MAX;
int b = a;
... (a = a+b) whoops.
Abhishek Jha wrote: hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
What if a+b overflows ?
And is there any point to do "smart" tricks like this ? If one
needs to swap variables, do it the right way. If there is
anything to gain, trust your compiler to optimize it.
> [ a=a+b;b=a-b;a=a-b; ] is a bad way to swap two numbers due to possible overflow. A better method without using a temporary variable is to use the
XOR trick[*] (which works for non-integral types as well). But, the best
^^^^^^^^^^^^^^^^^^^^
What do you mean by this? The trick won't work for floats, doubles or
pointers
for instance.
"Nils O. Selåsdal" What if a+b overflows ?
And is there any point to do "smart" tricks like this ? If one
needs to swap variables, do it the right way. If there is
anything to gain, trust your compiler to optimize it.
You're basically right, but not about trusting the compiler. Let's say that
the XOR hack saves a cycle over use of a temporary, due to the architecture
of the instruction set. Seeing that assignment to a temporary is only for
the purposes of swapping is the sort of thing compilers tend not to be good
at, because they don't see the source with a human eye. However is saving a
cycle really worth it? If you are so time critical, wouldn't it be better to
resort to assembly?
In article <ck**********@newsg4.svr.pol.co.uk> "Malcolm" <ma*****@55bank.freeserve.co.uk> writes: "Nils O. Selåsdal" What if a+b overflows ?
And is there any point to do "smart" tricks like this ? If one
needs to swap variables, do it the right way. If there is
anything to gain, trust your compiler to optimize it.
You're basically right, but not about trusting the compiler. Let's say that
the XOR hack saves a cycle over use of a temporary, due to the architecture
of the instruction set. Seeing that assignment to a temporary is only for
the purposes of swapping is the sort of thing compilers tend not to be good
at,
Compilers tend to be very good at just that. Using the tmp variable
will much more likely get the optimal sequence:
load a to r1
load b to r2
store r2 to a
store r1 to b
on most machines. Using the xor trick (or whatever) may very well lead
to:
load a to r1
load b to r2
set r1 to r1 ^ r2
set r2 to r2 ^ r1
set r1 to r1 ^ r2
store r1 to a
store r2 to b
on the same machines.
The point is, tell the compiler what you want, do not obfuscate it.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/
"Abhishek Jha" <ab***********@yahoo.co.in> wrote in message
news:60**************************@posting.google.c om... hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
Trite academic exercises aside, who needs a 'clever' way to swap two
numbers?
>"Nils O. Selåsdal" What if a+b overflows ?
And is there any point to do "smart" tricks like this ? If one
needs to swap variables, do it the right way. If there is
anything to gain, trust your compiler to optimize it.
In article <news:ck**********@newsg4.svr.pol.co.uk>
Malcolm <ma*****@55bank.freeserve.co.uk> wrote:You're basically right, but not about trusting the compiler. Let's say that
the XOR hack saves a cycle over use of a temporary, due to the architecture
of the instruction set. Seeing that assignment to a temporary is only for
the purposes of swapping is the sort of thing compilers tend not to be good
at, because they don't see the source with a human eye.
No; they see it with one that makes no mistakes, instead. :-)
This is actually a very simple optimization in any compiler that
does live-range analysis of variables. The "live range" of a
variable begins from when it is first assigned and ends at the
last reference to it before it is assigned another value (or the
last reference, if never re-assigned). This means that, for
instance, in code like:
tmp = a; /* line 17 */
a = b;
b = tmp; /* line 19 */
tmp = c; /* line 20 */
c = d;
d = tmp; /* line 22 */
/* code that does not refer to "tmp" again */
the "tmp" variable has two separate live ranges: one covers lines
17 through 19, and the other lines 20 through 22.
Given this information, and assuming that a, b, c, and d are in
processor registers and the CPU has a "swap" instruction for swapping
a pair of registers, it is easy to generate code of the form:
swap rA, rB
swap rC, rD
for these six lines of code. The GNU C compiler does it, for
instance. (Of course, the person who writes the ".md" file for
the machine has to define the swap instruction.)
However is saving a cycle really worth it? If you are so time
critical, wouldn't it be better to resort to assembly?
This is a valid point -- but it turns out that many C programmers
can no longer write "fast assembly code" for modern (pipelined)
processors -- or at least, not as well as compilers with good
pipeline schedulers (gcc3's is far better than gcc2's, for instance).
To beat the compiler, you may need one of those special assembly
programmers stored in the glass cases (with the signs that read
"in case of emergency, break glass"). :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
On Sat, 16 Oct 2004 23:42:11 +0100, Malcolm wrote: What if a+b overflows ?
And is there any point to do "smart" tricks like this ? If one
needs to swap variables, do it the right way. If there is
anything to gain, trust your compiler to optimize it.
You're basically right, but not about trusting the compiler. Let's say that
the XOR hack saves a cycle over use of a temporary, due to the architecture
of the instruction set. Seeing that assignment to a temporary is only for
Then I would call it a non-optimal compiler. Take a look at
optimized compiler compiler generated assembly, there isn't always that
much resemblance with the corresponding C code ;) the purposes of swapping is the sort of thing compilers tend not to be
good at, because they don't see the source with a human eye. However is
saving a cycle really worth it? If you are so time critical, wouldn't it
be better to resort to assembly?
Lets just not hope the C compiler does funny things then;
"The order of evaluation of subexpressions within an expression is
undefined."
On Sun, 17 Oct 2004 00:24:10 +0000, Dik T. Winter wrote: In article <ck**********@newsg4.svr.pol.co.uk> "Malcolm" <ma*****@55bank.freeserve.co.uk> writes: > "Nils O. Selåsdal" > > What if a+b overflows ?
> > And is there any point to do "smart" tricks like this ? If one
> > needs to swap variables, do it the right way. If there is
> > anything to gain, trust your compiler to optimize it.
> >
> You're basically right, but not about trusting the compiler. Let's say that
> the XOR hack saves a cycle over use of a temporary, due to the architecture
> of the instruction set. Seeing that assignment to a temporary is only for
> the purposes of swapping is the sort of thing compilers tend not to be good
> at,
Compilers tend to be very good at just that. Using the tmp variable
will much more likely get the optimal sequence:
load a to r1
load b to r2
store r2 to a
store r1 to b
looking at compiler output using a
inline void swap(int *a,int *b);
For 4 real world examples I've tested now, the compiler
optimizes away the entire swap altogether, it just seems to
reorder things so the swap isn't even needed :)
In article <pa****************************@Utel.no> =?iso-8859-1?q?Nils_O=2E_Sel=E5sdal?= <NO*@Utel.no> writes: On Sun, 17 Oct 2004 00:24:10 +0000, Dik T. Winter wrote:
.... Compilers tend to be very good at just that. Using the tmp variable
will much more likely get the optimal sequence:
load a to r1
load b to r2
store r2 to a
store r1 to b
looking at compiler output using a
inline void swap(int *a,int *b);
For 4 real world examples I've tested now, the compiler
optimizes away the entire swap altogether, it just seems to
reorder things so the swap isn't even needed :)
Oh, yes, for inlined functions that is pretty probable. I have worked
with tuning an Algol 68 compiler that with the probably tuning after
a switch still did know what was where, and optimized branches to
store the same thing in the same place. But that was only 1970s
technology.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/
On Sat, 16 Oct 2004 12:53:25 -0400, "Method Man" <a@b.c> wrote in
comp.lang.c:
"Abhishek Jha" <ab***********@yahoo.co.in> wrote in message
news:60**************************@posting.google.c om... hello all,
this has been an interesting topic since long. you can swap two
numbers (say a and b) using temp(as third variable). you can also swap
without using third variable.like[ a=a+b;b=a-b;a=a-b; ]This was also
done in three statements. Can anyon suggest this swapping in anything
less then that.
[ a=a+b;b=a-b;a=a-b; ] is a bad way to swap two numbers due to possible
overflow. A better method without using a temporary variable is to use the
XOR trick[*] (which works for non-integral types as well). But, the best
way is probably just to use a standard temporary variable and let the
compiler do the optimization.
[*] a^=b; b^=a; a^=b;
Except of course that it does not work for non-integer types at all,
and may generate a trap representation and thus undefined behavior on
signed integer types.
Works like a charm on all unsigned integer types, though.
--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
"Jarmo" <ja***@jarmo.com> wrote
Trite academic exercises aside, who needs a 'clever' way to swap two
numbers?
consider this
int gcf(int x, int y)
{
if(x < y)
swap(x, y);
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
And this
int gcf(int x, int y)
{
int temp;
if(x < y)
{
temp = x;
x = y;
y = temp;
}
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
Which one is clearer?
Malcolm wrote:
"Jarmo" <ja***@jarmo.com> wrote
Trite academic exercises aside, who needs a 'clever' way to swap two
numbers?
consider this
int gcf(int x, int y)
{
if(x < y)
swap(x, y);
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
And this
int gcf(int x, int y)
{
int temp;
if(x < y)
{
temp = x;
x = y;
y = temp;
}
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
Which one is clearer?
My first impulse after scanning the code, was to say the second one.
The second one, is entirely defined within your post.
I was scanning the code quickly.
When I saw the function call, without even reading it,
my mind said
"put that on that back burner and let's see
what the rest of this code is all about"
If I had been reading more slowly, I would have read the name
of the function and guessed what it meant.
Now that I am really taking my time,
I even realise that swap() is a macro and not a fuction.
If I were debugging the code,
I would have to look up the definition of swap().
swap() is easy to read, but this:
temp = x;
x = y;
y = temp;
is very easy to read.
--
pete
Jack Klein wrote: "Method Man" <a@b.c> wrote:
.... snip ...
[*] a^=b; b^=a; a^=b;
Except of course that it does not work for non-integer types at all,
and may generate a trap representation and thus undefined behavior
on signed integer types.
Works like a charm on all unsigned integer types, though.
Except when told to swap an item with itself, when it has a
penchant for zeroing the whole mess in disgust.
--
"I support the Red Sox and any team that beats the Yankees"
"Any baby snookums can be a Yankee fan, it takes real moral
fiber to be a Red Sox fan"
"CBFalconer" <cb********@yahoo.com> wrote in message
news:41***************@yahoo.com... --
"I support the Red Sox and any team that beats the Yankees"
"Any baby snookums can be a Yankee fan, it takes real moral
fiber to be a Red Sox fan"
Catch yesterday's game? Yikes!
-Mike
(Another NYY hater). Go Sox! :-)
hello malcolm,
You have written nice function to swap, but what is this swap().
"Malcolm" <ma*****@55bank.freeserve.co.uk> wrote in message news:<ck**********@newsg1.svr.pol.co.uk>... "Jarmo" <ja***@jarmo.com> wrote
Trite academic exercises aside, who needs a 'clever' way to swap two
numbers?
consider this
int gcf(int x, int y)
{
if(x < y)
swap(x, y);
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
And this
int gcf(int x, int y)
{
int temp;
if(x < y)
{
temp = x;
x = y;
y = temp;
}
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
Which one is clearer?
Abhishek Jha <ab***********@yahoo.co.in> scribbled the following: hello malcolm,
You have written nice function to swap, but what is this swap().
And besides, how could it ever work when you pass the ints themselves
and not pointers to them?
Perhaps it's not a function at all, but instead a macro.
"Malcolm" <ma*****@55bank.freeserve.co.uk> wrote in message news:<ck**********@newsg1.svr.pol.co.uk>... int gcf(int x, int y)
{
if(x < y)
swap(x, y);
if(x % y == 0)
return y;
else
return gcf(y, x % y);
}
--
/-- Joona Palaste (pa*****@cc.helsinki.fi) ------------- Finland --------\
\-------------------------------------------------------- rules! --------/
"A friend of mine is into Voodoo Acupuncture. You don't have to go into her
office. You'll just be walking down the street and... ohh, that's much better!"
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