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  1. #include<stdio.h>
  2.  
  3. unsigned getbits(unsigned x, int p, int n)
  4. {
  5. 	/* Build a mask consisting of 'n' bits */
  6. 	/* ...111 */
  7. 	int all_ones = ~0;
  8. 	/* ...111000 */
  9. 	int all_ones_shifted_left = all_ones << n;
  10. 	/* ...000111 */
  11. 	int mask = ~all_ones_shifted_left;
  12.  
  13. 	/* Shift 'x' down to discard lower bits we don't care about */
  14. 	/* How many bits do we discard? */
  15. 	int shift_down = p + 1 - n;
  16. 	/* ...76543210 -> ...765432 */
  17. 	unsigned int x_shifted_down = x >> shift_down;
  18.  
  19. 	/* Use the mask to discard the upper bits we don't care about */	
  20. 	return x_shifted_down & mask;
  21. }
  22.  
  23. int main(void)
  24. {
  25. 	/* Decimal 8 is binary 1000 */
  26. 	unsigned x = 8;
  27. 	/* Position 4 starts at ...[43210 */
  28. 	int p = 4;
  29. 	/* 3 bits means the range is ...[432]10 */
  30. 	int n = 3;
  31.  
  32. 	printf("%d", getbits(x, p, n));
  33. 	return 0;
  34. }
Success #stdin #stdout 0.01s 1720KB
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https://ideone.com/XwzQi
language:
C (gcc 8.3)
created:
12 years ago
visibility:
public

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