RELATION BETWEEN OPERATORS

[ishwar mehta]

HI FRIENDS,SOMEWHERE i have Seen that

Expressions (x%8) and (x&7) in C , are equivalent.. that is
if i write a program like


int main ()
{
int x=50;
printf("%d\n", x%8);
printf("%d\n", x&7);
return 0 ;
}
output:
2
2

SO Frnd,plz help me to figure out ,what is the exact relation betwwen
the operators % and & .

plzzzzzzz.......

 

[Dr Nick]

HI FRIENDS,SOMEWHERE i have Seen that

In your homework assignment?

Expressions (x%8) and (x&7) in C , are equivalent.. that is

They aren't.

if i write a program like


int main ()
{
int x=50;

try it with x=-50 and you'll see the difference.

printf("%d\n", x%8);
printf("%d\n", x&7);
return 0 ;
}
output:
2
2

SO Frnd,plz help me to figure out ,what is the exact relation betwwen
the operators % and & .

What does the % operator do?

What does the & operator do?

Why will they give the same results for some range of values (NB, this
is maths rather than C).

plzzzzzzz.......

Can you take that bee out of here please, it's driving me nuts.

 

[BartC]

HI FRIENDS,SOMEWHERE i have Seen that

Expressions (x%8) and (x&7) in C , are equivalent.. that is
if i write a program like


int main ()
{
int x=50;
printf("%d\n", x%8);
printf("%d\n", x&7);
return 0 ;
}
output:
2
2

SO Frnd,plz help me to figure out ,what is the exact relation betwwen
the operators % and & .

The results of A%N and A&(N-1) will generally be different.

But if N is a power of 2 as in your example, eg 2**M, then it's not
difficult to see why they might be expected to be the same (both isolate the
last M bits of A).

 

[Nick Keighley]

HI FRIENDS,SOMEWHERE i have Seen that

Expressions  (x%8) and (x&7)  in C , are equivalent.. that is
 if i write a program like

int main ()
{
   int x=50;
 printf("%d\n", x%8);
 printf("%d\n", x&7);
 return 0 ;}

output:
2
2

try it with more numbers

SO Frnd,plz help me to figure out ,what is the exact relation betwwen
the  operators  % and & .

learn binary

 

[Ben Bacarisse]

The results of A%N and A&(N-1) will generally be different.

But if N is a power of 2 as in your example, eg 2**M, then it's not
difficult to see why they might be expected to be the same (both
isolate the last M bits of A).

But they don't both do that. % is defined (in C99) as an essentially
arithmetic operation, while & is defined in terms of the representation.
This is most clearly seen when A is negative. -2 % 8 is -2 no matter
what representation is used for negative numbers, but -2 & 7 is either
6, 5 or 2 depending on which of the three representations for negative
numbers is in use.

I know you were thinking of positive N, but I think it confuses the
issue to look at the cases when they *are* the same. The relationship
between things that have some similarity is almost always made most
clear by pointing out the differences.

 

[John Bode]

There is no relationship between the two. One's an arithmetic operation, the other's a bitwise operation. For some combinations of operands they will produce the same results, but that's by coincidence, not design.

 

[Keith Thompson]

There is no relationship between the two. One's an arithmetic
operation, the other's a bitwise operation. For some combinations of
operands they will produce the same results, but that's by
coincidence, not design.

The question was about the relationship between (x%8) and (x&7), but you
can't tell that by reading your answer. Please quote enough context
from the parent article so your answer makes sense by itself. (And
don't delete the "Re: " from the subject header.)

 

[John Bode]

The question was about the relationship between (x%8) and (x&7), but you
can't tell that by reading your answer.  Please quote enough context
from the parent article so your answer makes sense by itself.  (And
don't delete the "Re: " from the subject header.)

--
Keith Thompson (The_Other_Keith) (e-mail address removed)  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Blame Google Groups' mobile interface; I *thought* it was quoting
context, but it turned out it wasn't. As lame as GG can be, it's even
lamer on Android.

 

[Peter Nilsson]

If x is an unsigned type, or signed with a non-negative value, yes.

The % operator yields 'what remains' from division; the & operator
yields
'what remains' if you exclude certain value bits.

But they don't both do that. % is defined (in C99) as an essentially
arithmetic operation,

No less in C90.

while & is defined in terms of the representation.

Yes and no. It operates on value and sign bits, but not padding bits.
One can think of x % 1000 as a decimal-wise masking of value places,
so
there is a relation between &1, &3, &7, ... and %10, %100, %1000...

This is most clearly seen when A is negative.

In the general case, C programmers shouldn't be using bitwise
operators
on signed types. If we limited the discussion to unsigned types, then
in that (ideal) environment, your point is of no consequence.

Under C90, division needn't round towards zero, so even for %, there
is a case to limit use to non-negative integers for consistent
results.

Of course, the real question is when to use &7 and when to use %8.
The answer to that depends on context. If you want the remainder when
dividing by 8, then use %. If you want the lower 3 bits, then use &.

Whilst division can be expensive on some CPUs, most modern compilers
are more than good enough to optimise %8 without programmer
assistance.

If you see &7 used in place of %8, then you're just witnessing someone
using something that was clever and critcal 40+ years ago, not
something
that's clever or critical now. Or, if there's a case where the
compiler
isn't optimising %8 to &7, then it's more likely the code isn't using
the most practical type for the operation at hand (e.g. using int
rather
than size_t.)