Shift operator

Xiangbuilder · 10-03-2003, 10:03 PM

I have a troube in the program from <<thinking in c++>>,

Code:

int main ()
{
   unsigned int getval;
   cout<<"enter a number between 0 and 255:";
   cin>>getval;
   unsigned char a=getval;
   cout<<getval<<" in binary is: ";
   for(int i = 7; i>=0; i--)
      if(a&(1<<i))
         cout<<"1";
      else
         cout<<"0";
   cout<<endl;
}

the author said: "The << and >> provide bit-shifting behavior, but when they shift bits off the end of the number, those bits are lost (it¡¯s commonly said that they fall into the mythical bit bucket, a place where discarded bits end up, presumably so they can be reused...)."
I don't know what are the losted bits.
Could someone give a example about the bits losted to me?
Thank you.

QtCoder · 10-03-2003, 10:50 PM

Hi.

To truly understand where bits go when doing bitwise shifting, you should take a look at an assembly programming tutorial.

Although I can't remember all about assembly, I do remember that when you shift bits, the last bit to be shifted off ends up in the "carry register", a specific location in CPU. The bit that's shifted into the carry register can be used to perform complex arithmetic involving carrying a digit.

That's the extent of my knowledge. You'd be amazed how much you learn about C++ when you study assembly. If you truly like to know how things work, do a google for "assembly tutorial." I finally understand how dynamic memory allocation works, how arguments are actually passed to functions, how a return statement works! :-)

jinksys · 10-03-2003, 11:34 PM

Take this byte into consideration.

Code:

00100111

Let's say we did a left shift of 2 bits on this byte.
All bits shift to the left two spaces, creating the new
byte:

Code:

10011100

Remember the two zeros on the left of the byte before we shifted? They are now gone, lost forever, which is what we
wanted. The same is true when you do a right shift. Bits on the
right of the byte will be lost.

When the author of your book said:

Code:

it ¡s commonly said that they fall into the 
mythical bit bucket, a place where discarded bits 
end up, presumably so they can be reused...

He was trying to make a joke, I hope. Because what he said
is a total lie, and 100% false.

Xiangbuilder · 10-04-2003, 01:17 AM

Thank you.
I think I understand it now by your help.
Here is a program to show the result:

Code:

#include<iostream>
using namespace std;
void func (unsigned char a);
int main ()
{
   unsigned int getval;
   cout<<"enter a number between 0 and 255:";
   cin>>getval;
   unsigned char a=getval;
   cout<<getval<<" in binary is: ";
   func (a);
   a>>=2;
   cout<<"("<<getval<<">>2) in binary is: ";
   func (a);
} 
void func (unsigned char a) {
   for(int i=7; i>=0; i--)
      if(a&(1<<i))
         cout<<"1";
      else
         cout<<"0"; 
   cout<<endl; 
}

# ./rotation_a
enter a number between 0 and 255:39
39 in binary is: 00100111
(39>>2) in binary is: 00001001
#

jinksys · 10-04-2003, 02:07 PM

Looks like it works