Sams Teach Yourself C in 21 Days

Working with Memory 583

20

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

mal 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 (deci-

mal 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 nplaces has the same effect

as multiplying it by 2n, and right-shifting an integer has the same effect as dividing it by

2 n. 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 divi-

sion 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. Listing 20.7 demonstrates the shift operators.

LISTING20.7 shiftit.c. Using the shift operators

1: /* Demonstrating the shift operators. */

2:

3: #include <stdio.h>

4:

5: int main( void )

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: }

INPUT

32 448201x-CH20 8/13/02 11:16 AM Page 583