ch2-getbits (get last bits of integer starting at specified position)

Chapter_2     Exercise_2-5

bitstrings     Exercise_2-6

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getbits.c     K&R, p. 49         download

#include <stdio.h> // for printf(), scanf(), putchar()

#define LENGTH 100 // bits

// get the last n bits from x starting at position p:
unsigned getbits(unsigned x, int p, int n);
// reverse s[], knowing its length:
void reverse(char s[], int len);
// get the bits of x into bits[], return length of bits[] (no of bits):
int bitstring (char bits[], unsigned x);

int main()
{
char bits[LENGTH];
unsigned x;
int len, p, n, diff;

printf("Give a positive integer: ");
scanf("%u", &x);

len = bitstring(bits, x);

printf("%s\n", bits);

printf("Return last 0 <= n <= %d bits ", len);
printf("starting at position max(0, n-1) <= p <=%d:\n", len-1);
printf ("n = ");
scanf("%d", &n);
printf ("p = ");
scanf("%d", &p);

x = getbits(x,p,n);
len = bitstring(bits,x);
diff = n - len;
while (diff > 0)
{
putchar('0');
diff--;
}
printf("%s\n", bits);

return 0;
}
// get the last n bits from x starting at position p:
unsigned getbits(unsigned x, int p, int n)
{
return (x >> (p+1-n)) & ~(~0 << n);
}
// reverse s[], knowing its length:
void reverse(char s[], int len)
{
int i = 0, j = len-1;
char temp;

while (i < j)
{
temp = s[i];
s[i] = s[j];
s[j] = temp;
i++;
j--;
}
}
// get the bits of x into bits[], return length of bits[] (no of bits):
int bitstring (char bits[], unsigned x)
{
int i = 0;

if (x == 0)
{
bits[i++] = '0';
bits[i] = '\0';

return i;
}

while (x > 0)
{ // last bit gives the parity:
bits[i++] = '0' + x % 2; // 0 for even, 1 for odd
x /= 2; // x = x / 2; // lose last bit
}
bits[i] = '\0'; // end the string

reverse(bits, i);

return i;
}
/*
gcc getbits.c -o getbits
./getbits
Give a positive integer: 74
1001010
Return last 0 <= n <= 7 bits starting at position max(0, n-1) <= p <=6:
n = 3
p = 4
010

./getbits
Give a positive integer: 74
1001010
Return last 0 <= n <= 7 bits starting at position max(0, n-1) <= p <=6:
n = 3
p = 5
001

./getbits
Give a positive integer: 97
1100001
Return last 0 <= n <= 7 bits starting at position max(0, n-1) <= p <=6:
n = 3
p = 4
000

./getbits
Give a positive integer: 97
1100001
Return last 0 <= n <= 7 bits starting at position max(0, n-1) <= p <=6:
n = 4
p = 3
0001

./getbits
Give a positive integer: 97
1100001
Return last 0 <= n <= 7 bits starting at position max(0, n-1) <= p <=6:
n = 5
p = 5
10000
*/

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Chapter_2     Exercise_2-5

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bitstrings     Exercise_2-6