Отправлено: 28.05.14 12:24. Заголовок: Bit operations

The task is follow: Implement function unsigned getbits( unsigned x, int p, int n ) which would return a bitfield containing bits of x starting from position p and of length n. It is assumed that bit in position 0 is the rightmost bit. Do not make any assumptions about the validity of p and n values.
I really dont get the idea of that task but I have some raw material which I have modified:

Can somebody give me tips or code how is it done ?

#include <stdio.h>

void showbits(unsigned int x)
{
int i;
int n=2; /*Length*/
for(i=(sizeof(int)*n)-1; i>=0; i--)
(x&(1<<i))?putchar('1'):putchar('0');

printf("\n");
}

int main()
{
int j = 14;
printf("The decimal %d is equal to binary - ", j);
showbits(j);
return 0;
}

 Отправлено: 28.05.14 12:42. Заголовок: This assignment is t..

This assignment is taken from book "The C programming language" by Brian W.Kernigham and Dennis M. Ritchie. See the chapter whether bitwise operators are described.

 Отправлено: 28.05.14 13:15. Заголовок: /* getbits: get n bi..

/* getbits: get n bits from position p */
unsigned getbits(unsigned x, int p, int n)
{
return (x >> (p+1-n)) & ~(~0 << n);
}

So really, that´s the whole program ?

 Отправлено: 28.05.14 13:26. Заголовок: That is the function..

That is the function.

 Отправлено: 28.05.14 13:41. Заголовок: Doesn´t work on..

Doesn´t work on Code::Blocks ... the only include needed is stdio.h which I included , am I missing something ?
If someone is used to Code::Blocks then the error is " undefined reference to `WinMain@16' "

Or do I have to value the variables p and n ?
Sorry for the dumb questions!

 Отправлено: 28.05.14 14:01. Заголовок: You have to compile ..

You have to compile the program as a console application instead of a Windows application.

 Отправлено: 28.05.14 14:11. Заголовок: Havent really done t..

Havent really done that before. Can you explain more ?
Google didn´t give any specific answer.

 Отправлено: 28.05.14 14:22. Заголовок: I have no Code::Bloc..

I have no Code::Blocks. So I do not know how to set properties of a project. Try to find this yourself.

 Отправлено: 29.05.14 16:30. Заголовок: The int main functio..

The int main function is missing, thats why the program would not start.

Doesnt it suppose to be like that:
#include <stdio.h>

unsigned getbits(unsigned x, int p, int n)
{
return (x >> (p+1-n)) & ~(~0 << n);
}
int main()
{
getbits(x);
printf("%d",x);
return 0;
}

I am struggling with declaring x ( dont know where to do that)

 Отправлено: 29.05.14 16:44. Заголовок: You shou;d eneter so..

You have to enter some values for x, p and n and then output the result. For example

 
 int main( void ) 
 { 
    int x; 
    int p; 
    int n; 
  
    // enter x, p, and n using scanf 
    // and output the return value of the function call 
 } 
 

 Отправлено: 29.05.14 16:55. Заголовок: Applying the getbits..

Applying the getbits function in the main() part is probably faulty.

int main()
{
int x, p, n;
printf("Insert bits\n");
scanf("%d",&x);
printf("Insert position\n");
scanf("%d",&p);
printf("Insert length\n");
scanf("%d",&n);
getbits(x,p,n);
printf("%d",x);
return 0;}

It returns me the same value as user enters.

 Отправлено: 29.05.14 17:02. Заголовок: Write printf( "..

Write

 
 printf( "\n%d\n", getbits( x, p, n ) ); 
 

 Отправлено: 29.05.14 17:08. Заголовок: Hmmmm...every time t..

Hmmmm...every time the output is 0(zero)

 Отправлено: 29.05.14 17:34. Заголовок: Write the functiopn ..

Write the functiopn the following way

unsigned getbits(unsigned x, int p, int n)
{
return ( x >> p ) & ~( ~0 << n );
}

It is supposed that p starts from 0.