BITWISE SHIFT question

[HARDCORECODER]

ok here is the question. I want to exract the first 4 bits in a int
so let say

int b = somenumber;
int result= 0;
result = b << 4;

if I got this right result should contain the 4 bits that were shifter
to the left right? :) and if I'm wrong how can i get an x number of
bits into a variable?

[noone]

On Wed, 12 Apr 2006 22:52:04 -0700, HARDCORECODER wrote:

ok here is the question. I want to exract the first 4 bits in a int so
let say

int b = somenumber;
int result= 0;
result = b << 4;

if I got this right result should contain the 4 bits that were shifter to
the left right? :) and if I'm wrong how can i get an x number of bits into
a variable?

"first four bits" is ambiguous...and depends upon whether your system is
big or little endian...and as everyone knows, little endian machines suck!

but, to answer your question, shift left increases the value by a power
of two and shift right decreases by a power of two....so, to extract bit
fields out of a number with bits numbers 2^n n=[0..15] use

bits1to3=(x>>1) &0x0f; // bits 1 2 3 4

bits12to15=(x>> 12)&0x0f; // bits 12 13 14 15

to extract bit fields you should shift right then use a bit mask
containing all ones.

[Maxim Yegorushkin]

noone wrote:

On Wed, 12 Apr 2006 22:52:04 -0700, HARDCORECODER wrote:

ok here is the question. I want to exract the first 4 bits in a int so
let say

int b = somenumber;
int result= 0;
result = b << 4;

if I got this right result should contain the 4 bits that were shifter to
the left right? :) and if I'm wrong how can i get an x number of bits into
a variable?

"first four bits" is ambiguous...and depends upon whether your system is
big or little endian...and as everyone knows, little endian machines suck!

In C and C++ bits are counted right to left no matter what hardware bit
order is. N's bit thus can be extracted by shifting 1 right N times and
using the value as a bit-mask:

unsigned x;
unsigned n;
unsigned bit_n = 0 != x & (1 << n);

Don't forget that in C and C++ we count from 0, so that the lowest
order bit has index 0.

[Michiel.Salters]

HARDCORECODER wrote:

ok here is the question. I want to exract the first 4 bits in a int
so let say

int b = somenumber;
int result= 0;
result = b << 4;

if I got this right result should contain the 4 bits that were shifter
to the left right? :) and if I'm wrong how can i get an x number of
bits into a variable?

Nope. b << 4 just means b * 16, and b >> 4 means b / 16.

What are "the first four bits" anyway? Those with the lowest value?
Or the highest? And what if there are padding bits?

HTH,
Michiel Salters

[Victor Bazarov]

Maxim Yegorushkin wrote:

[...]
In C and C++ bits are counted right to left no matter what hardware
bit order is. N's bit thus can be extracted by shifting 1 right N
You mean "by shifting 1 *LEFT* N times", of course...
times and using the value as a bit-mask:

unsigned x;
unsigned n;
unsigned bit_n = 0 != x & (1 << n);
And here you're shifting _left_, BTW.

Don't forget that in C and C++ we count from 0, so that the lowest
order bit has index 0.

V
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[Victor Bazarov]

Mi************* @tomtom.com wrote:

[..]
What are "the first four bits" anyway? Those with the lowest value?
Or the highest? And what if there are padding bits?

"The first four bits" are the bits numbered 0 through 3, no? Aren't
any "first" items always the ones with lower indices? How can they be
the "highest"?

V
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[Markus Schoder]

Victor Bazarov wrote:

Mi************* @tomtom.com wrote:

[..]
What are "the first four bits" anyway? Those with the lowest value?
Or the highest? And what if there are padding bits?

"The first four bits" are the bits numbered 0 through 3, no? Aren't
any "first" items always the ones with lower indices? How can they be
the "highest"?

If you write done a number which digits do you write down _first_?

[HARDCORECODER]

ok I meant the first 4 bits....bit 0,1,2,3. I want to exract these and
place them and place them into another variable.

for example noone wrote:

bits1to3=(x>>1) &0x0f; // bits 1 2 3 4

bits12to15=(x>> 12)&0x0f; // bits 12 13 14 15

to extract bit fields you should shift right then use a bit mask
containing all ones.

I beleive this is correct! but anyone else have suggestions?

[HARDCORECODER]

lets say I have a 32-bit int.

int x =555;

I want to extract the first 4 bits. thats bit 0,1,2,3 and place them
into another variable.

noone wrote this anwser:

int bits1to3=(x>>1) &0x0f; // bits 1 2 3 4

or

int bits12to15=(x>> 12)&0x0f; // bits 12 13 14 15

to extract bit fields you should shift right then use a bit mask
containing all ones.

I think he is right what do you think?

[Victor Bazarov]

Markus Schoder wrote:

Victor Bazarov wrote:

Mi************* @tomtom.com wrote:

[..]
What are "the first four bits" anyway? Those with the lowest value?
Or the highest? And what if there are padding bits?

"The first four bits" are the bits numbered 0 through 3, no? Aren't
any "first" items always the ones with lower indices? How can they
be the "highest"?

If you write done a number which digits do you write down _first_?

How is that relevant? If I write a number with leading zeros, would you
count those zeros as "first"? Are "00010" and "010" the same or not?
When you put on your pants, do you start with the left led or the right?

V
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