Types & Operators
CAS CS210
Ying Ye
Boston University
Outline
 Endianness
 Bitwise operators
 Arithmetic operators
 Data lab assignment
Endianness
 Endianness is the same as byte order, refering to how bytes
are ordered within a single large data item( e.g. int, short, long).
 Little endian:
stores the least significant byte first
 Big endian:
stores the most significant byte first
Endianness
int x = 0x44332211
0
4
44
33
22
11
big endian
0
4
11
22
little endian
33
44
Bitwise operators
 wget http://cs-people.bu.edu/yingy/bitwise.c
 Open the file and replace all /*TODO*/ with the expected
expressions
 Compile and run
Bitwise operators
 Output
Bitwise operators
 Bit mask
 wget http://cs-people.bu.edu/yingy/mask.c
 replace all /*TODO*/ with the expected expressions
 Output
Arithmetic operators
 In just one statement, make a == 2, b == 3
 wget http://cs-people.bu.edu/yingy/math.c
 replace the /*TODO*/ with the expected statement
A possible answer: a = ++b, b++;
Data lab assignment
 Example: For a one byte unsigned variable, calculate the
number of ones in its binary representation. (e.g. variable =
0x03, number of ones: 2)
int Count(unsigned char v)
{
int num = 0;
while(v){
num += v & 0x01;
v >>= 1;
}
return num;
}
Data lab assignment
Challenge!
Can you find an algorithm as fast as possible to solve this
counting problem.
Data lab assignment
 Faster
int Count(unsigned char v)
{
int num = 0;
while(v){
Can we solve it
even faster than
this code?
v &= (v - 1);
num++;
Maybe......
}
return num;
}
Based on the fact: for a variable v = xxxx1000(x can be 1 or 0),
v - 1 == xxxx0111, so v & (v - 1) == xxxx0000, the '1' in the rightmost is
erased. The number of this operation needed to make v == 0 is the
number of '1's in v.