Using Shift Operators in C
 


Two shift operators shift the bits in an integer variable by a specified number of positions. The << operator shifts bits to the left, and the >> operator shifts bits to the right. The syntax for these binary operators is

x << n

and

x >> n

Each operator shifts the bits in x by n positions in the specified direction. For a right shift, zeros are placed in the n high-order bits of the variable; for a left shift, zeros are placed in the n low-order bits of the variable. Here are a few examples:

Binary 00001100 (decimal 12) right-shifted by 2 evaluates to binary 00000011 (decimal 3).

Binary 00001100 (decimal 12) left-shifted by 3 evaluates to binary 01100000 (decimal 96).

Binary 00001100 (decimal 12) right-shifted by 3 evaluates to binary 00000001 (decimal 1).

Binary 00110000 (decimal 48) left-shifted by 3 evaluates to binary 10000000 (decimal 128).

Under certain circumstances, the shift operators can be used to multiply and divide an integer variable by a power of 2. Left-shifting an integer by n places has the same effect as multiplying it by 2n, and right-shifting an integer has the same effect as dividing it by 2n. The results of a left-shift multiplication are accurate only if there is no overflow--that is, if no bits are "lost" by being shifted out of the high-order positions. A right-shift division is an integer division, in which any fractional part of the result is lost. For example, if you right-shift the value 5 (binary 00000101) by one place, intending to divide by 2, the result is 2 (binary 00000010) instead of the correct 2.5, because the fractional part (the .5) is lost. Sample program demonstrates the shift operators.

Using the shift operators.

1:  /* Demonstrating the shift operators. */
2:
3:  #include <stdio.h>
4:
5:  main()
6:  {
7:       unsigned int y, x = 255;
8:       int count;
9:
10:      printf("Decimal\t\tshift left by\tresult\n");
11:
12:      for (count = 1; count < 8; count++)
13:      {
14:          y = x << count;
15:          printf("%d\t\t%d\t\t%d\n", x, count, y);
16:      }
17:      printf("\n\nDecimal\t\tshift right by\tresult\n");
18:
19:      for (count = 1; count < 8; count++)
20:      {
21:           y = x >> count;
22:           printf("%d\t\t%d\t\t%d\n", x, count, y);
23:      }
24:      return(0);
25: }
Decimal         shift left by   result
255             1               254
255             2               252
255             3               248
255             4               240
255             5               224
255             6               192
255             7               128
Decimal         shift right by  result
255             1               127
255             2               63
255             3               31
255             4               15
255             5               7
255             6               3
255             7               1
 
 
 
 
 
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