Using bitshift in assigning a number to a variable?

[bollod]

Is there any benefit to declaring a variable:

x = (1<<12);

instead of:

x = 4096;

(besides the geek appeal of course ;-) )

I have seen these kind of declarations come up in some code I have been
fiddling with and was wondering if there was any tangible benefit, (I have
also seen stuff declared as hex, is there any benefit to that either).

 

[bollod]

It's easier to visualise the binary representation
of the values that way,
which is especially good if your numbers are masks.

That makes sense.

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

 

[pete]

Is there any benefit to declaring a variable:

x = (1<<12);

instead of:

x = 4096;

(besides the geek appeal of course ;-) )

I have seen these kind of declarations come up
in some code I have been
fiddling with and was wondering if there was any tangible benefit,
(I have also seen stuff declared as hex,
is there any benefit to that either).

It's easier to visualise the binary representation
of the values that way,
which is especially good if your numbers are masks.

 

[pete]

That makes sense.

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

'10101010' looks more like 0xaa to me.

The shift notation is better
when the right operand of the shift operator
is an enum or a macro identifier which indicates which flag
the mask applies to.

 

[Robert Bachmann]

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

I would write
#define SOME_MASK (1<<7 | 1<<5 | 1<<3 | 1<<1)
or
const int SOME_MASK=1<<7 | 1<<5 | 1<<3 | 1<<1;

 

[those who know me have no need of my name]

in comp.lang.c i read:

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

correction: foo=0xAA. also foo=0252 and foo=strtol("10101010", 0, 2),
though the later cannot be used at file scope.

 

[Darrell Grainger]

Is there any benefit to declaring a variable:

x = (1<<12);

instead of:

x = 4096;

(besides the geek appeal of course ;-) )

I have seen these kind of declarations come up in some code I have been
fiddling with and was wondering if there was any tangible benefit, (I have
also seen stuff declared as hex, is there any benefit to that either).

I'll answer the easier question first, there is a benefit to hexidecimal
notation. When I am doing bit twiddling it is easier to visualize the bit
pattern what using hexidecimal notation. One digit in hexidecimal notation
is exactly 4 bits.

The harder question is why use (1<<12) rather than 4096. If they used
0x1000 it would be more obvious. I'd guess that 99% of the time this is
just more readable. Another possible option is that someone was tuning the
code and found a 'trick' for a particular processor/compiler.

It is not impossible that a compiler will take:

x = (1<<12);

and optimize it to a STORE IMMEDIATE 1 and SHIFT opcode. The cycle count
for this might be faster than trying to assign 4096. This is a stretch but
not impossible. Mind you, I'd put a comment next to the code in case a
revision in the compiler or a port to a different architecture made this
tweek disappear.

 

[Sean Kenwrick]

That makes sense.

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

x=2<<6|2<<4|2<<2|2;

or

x=10<<4|10;

or

x=5<<5|5<<1;

or

x=85<<1;


But now its just getting silly.....

Sean

 

[Richard Heathfield]

That makes sense.

What would you do if you needed to create the mask '10101010' (besides
foo=170 or foo=0xA4)?

#define l )*2+1
#define O )*2
#define b1 ((((((((0

unsigned char foo = b1 l O l O l O l O ;

 

[Robert Bachmann]

Is there any benefit to declaring a variable:

x = (1<<12);

instead of:

x = 4096;

(besides the geek appeal of course ;-) )

If x is used as a "common variable" I can see no benefit.

But if x is used as some kind of bit-mask you got the benefit of
immediately seeing that the value of x is 1000000000000 in binary notation.

As (1<<n) can be read as 1 followed by n zero digits in binary notation.
For example: 1<<3 is 1000 in binary.

(I have also seen stuff declared as hex, is there any benefit to that either).

Hexadecimal notation is also very common when using bit-masks.
As Darrell Grainger already said:
| One digit in hexidecimal notation
| is exactly 4 bits.

So if you familiar with hexadecimal notation it's easy to see that
0x1000 (4096) is 1000000000000.

I also have seen code with bit-masks in octal notation (one digit in
octal notation represents 3 bits).

HTH,
Robert

 

[pete]

I would write
#define SOME_MASK (1<<7 | 1<<5 | 1<<3 | 1<<1)

Out of all of the various replies posted,
that form, shifts and bitwise ors,
is similar to macros that I've actually used.

 

[Sean Kenwrick]

----- Original Message -----
From: "Richard Heathfield" <[email protected]>
Newsgroups: comp.lang.c
Sent: 27 December 2003 11:12
Subject: Re: Using bitshift in assigning a number to a variable?

#define l )*2+1
#define O )*2
#define b1 ((((((((0

unsigned char foo = b1 l O l O l O l O ;

Hey this is pretty neat way of representing binary in your 'C' code. I
would generalise it a bit further by allowing 16 and 32 bit values as well,
something like:

#define bin32bit ((((((((((((((((((((((((((((((((0
#define bin16bit ((((((((((((((((0
#define bin8bit ((((((((0

#define O )<<1
#define I )<<1+1


Then

x=bin8bit I O I O I O I O

or

x = bin16bit I O I O I O I O I O I O I O I O

nice!

Sean

 

[Richard Heathfield]

----- Original Message -----
From: "Richard Heathfield" <[email protected]>
Newsgroups: comp.lang.c
Sent: 27 December 2003 11:12
Subject: Re: Using bitshift in assigning a number to a variable?



Hey this is pretty neat way of representing binary in your 'C' code. I
would generalise it a bit further by allowing 16 and 32 bit values as
well,

Yes, that's why I called it b1 rather than b - to allow for b2 and b4 (and,
if you have bigger integral types, b8 and so on as well).

See "Expert C Programming (Deep C Secrets)" by Peter van der Linden, p203,
for the original idea. All I did was hack the names.