c and c++ operators help - c++

can someone explain to me why the following results in b = 13?
int a, b, c;
a = 1|2|4;
b = 8;
c = 2;
b |= a;
b&= ~c;

It is using binary manipultaors. (Assuming ints are 1 byte, and use Two's complement for storage, etc.)
a = 1|2|4 means a = 00000001 or 00000010 or 00000100, which is 00000111, or 7.
b = 8 means b = 00001000.
c = 2 means c = 00000010.
b |= a means b = b | a which means b = 00001000 or 00000111, which is 00001111, or 15.
~c means not c, which is 11111101.
b &= ~c means b = b & ~c, which means b = 00001111 and 11111101, which is 00001101, or 13.

http://www.cs.cf.ac.uk/Dave/C/node13.html

a = 1|2|4
= 0b001
| 0b010
| 0b100
= 0b111
= 7
b = 8 = 0b1000
c = 2 = 0b10
b|a = 0b1000
| 0b0111
= 0b1111 = 15
~c = 0b111...1101
(b|a) & ~c = 0b00..001111
& 0b11..111101
= 0b00..001101
= 13

lets go into binary mode:
a = 0111 (7 in decimal)
b = 1000 (8)
c = 0010 (2)
then we OR b with a to get b = 1111 (15)
c = 0010 and ~c = 1101
finally b is anded with negated c which gives us c = 1101 (13)

hint: Convert decimal to binary and give it a shot.. maybe... just maybe you'll figure it all out by yourself

a = 1 | 2 | 4;
Assigns the value 7 to a. This is because you are performing a bitwise OR operation on the constants 1, 2 and 4. Since the binary representation of each of these is 1, 10 and 100 respectively, you get 111 which is 7.
b |= a;
This ORs b and a and assigns the result to b. Since b's binary representation is now 111 and a's binary representation is 1000 (8), you end up with 1111 or 15.
b &= ~c;
The ~c in this expression means the bitwise negation of c. This essentially flips all 0's to 1's and vice versa in the binary representation of c. This means c switches from 10 to 111...11101.
After negating c, there is a bitwise AND operation between b and c. This means only bits that are 1 in both b and c remain 1, all others equal 0. Since b is now 1111 and c is all 1's except in the second lowest order bit, all of b's bits remain 1 except the 2 bit.
The result of flipping b's 2 bit is the same as if you simply subtracted 2 from its value. Since its current value is 15, and 15-2 = 13, the assignment results in b == 13.

Related

Walkthrough: sum 2 integers using bit manipulation

I am trying to understand the logic behind the following code which sums 2 integers using bit manipulation:
def sum(a, b):
while b != 0:
carry = a & b
a = a ^ b
b = carry << 1
return a
As an example I used: a = 11 and b = 7
11 in binary representation is 1011
7 in binary representation is 0111
Then I walked through the algorithm:
iter #1: a = 1011, b = 0111
carry = 0011 (3 decimal)
a = 1100 (12 decimal)
b = 0110 (6 decimal)
iter #2: a = 1100, b = 0110
carry = 0100 (4 decimal)
a = 1010 (10 decimal)
b = 1000 (8 decimal)
iter #3: a = 1010, b = 1000
carry = 1000 (8 decimal)
a = 00010 (2 decimal)
b = 10000 (16 decimal)
iter #4: a = 00010, b = 10000
carry = 00000 (0 decimal)
a = 10010 (18 decimal)
b = 00000 (0 decimal)
We Done (because b is now 0).
As we can see, in all iterations a+b is always 18 which is the right answer.
However I failed to understand what is actually happens here. The value of a is going down and down with each iteration until suddenly pops to 18 in the last iteration. Also, can we learn anything from the value of the carry during the process?
I would love to understand the intuition behind this.
Thanks to #WJS answer I think I got it.
let's add 11 and 7 as before, but let's do it in the following order:
First, calculate it without the carry.
Second, calculate only the carry.
Then add both parts.
01011
00111
-----
01100 (neglecting carry)
00110 (finding only the carry)
-----
10010 (sum)
Now, to find the first part, how can we get rid of the carry bits? with XOR.
To find the second part, we use AND and then shift it 1 bit left to place it "under" the right bit.
Now all we have to do is sum both parts. The whole point is not using + operator so how can we do that? Recursion!
We assign the first part to a and the second part to b and we repeat this process until b=0 which means we are done.
Perhaps if you take a simpler example it will help.
a = 11
b = 11
a & b == 11 since AND returns 1's where both bits in the same
position are 1. These are the carry bits.
Now get rid of the the carry locations using exclusive or
a = a ^ b == 00
But a `carry` would cause addition to add bits one position to
the left so shift the carry bits left by 1 bit.
b = carry << 1 = 110
now repeat the process
carry = a & b = 0 & 110 == 0 no more carries
b = carry << 1 == 0
done.
11 + 11 = 110 = 3 + 3 = 6
Understanding the roles of (AND) & and (XOR) ^ are key. Applying those to slightly more complex examples should help. But ignore the interim decimal values as they don't help much. Think only about what is happening in binary.
I think this is easy to understand if you look at what happens with individual bits.
First step is calculating carry which only happens in binary when both bits are 1, so a&b calculates that for every bit. Then bitwise addition is happening via XOR (ignoring carry), and XOR works because:
0+0=0 (==0^0)
1+0=1 (==1^0)
1+1=0 (==1^1, generates carry bit which we ignore)
Next step is to shift carry to the left (<<1), move it to b and repeat until carry is empty.

0 minus 0 gives carryout of 1 in adder-subtractor circuit

In this adder-subtractor design with the "M" input as the flag for subtraction, 0 minus 0 seems to provide the incorrect Cout. Let's assume that we're only using one full adder here (ignore A1/B1, A2/B2, A3/B3) for simplicity, and M=1, A0=0, A1=0:
The full adder will get the inputs of:
0 (B0) XOR 1 (M) = 1
0 (A0) = 0
1 (M) = 1
This results in 1+1=0, with Cout = 1 - but Cout should equal 0 for a full adder:
I think inverting the final Cout will provide the correct result, but everywhere I look online for this adder-subtractor circuit has no inverter for the final Cout. Is this circuit supposed to have an inverter at the final Cout to fix this problem?
The carry out equal to 1 is perfectly normal in this case.
When you work with unsigned logic the carry out is used as an overflow flag: assuming you're working with 4-bits operands, the operation:
a = 1000, b = 1001 (Decimal a = 8, b = 9)
1000 +
1001 =
--------
1 0001
produces a carry out of 1'b1 because the result of 8+9 cannot be represented on 4 bits.
On the other hand, when working with signed logic the carry out signal loses its 'overflow' meaning. Let's make an example:
a = 0111, b = 0010 (Decimal a = 7, b = 2)
0111 +
0010 =
--------
0 1001
In this case the result is 1001, that is -7 in two's complement. It's obvious that we had an overflow, since we added two positive numbers and we got a negative one. The carry out, anyway, is equal to 0. As a last case, if we consider:
a = 1111, b = 0001 (Decimal a = -1, b = 1)
1111 +
0001 =
--------
1 0000
we see that even though the result is correct -1+1=0, the carry out is set.
To conclude, if you work in signed logic and you need to understand whether there was an overflow, you need to check the sign of the two operands against the result's one.
Both operands positive (MSB = 0) and result negative (MSB = 1): overflow
Both operands negative (MSB = 1) and result positive (MSB = 0): overflow
Any other case: no overflow

Lexicographical comparison of integers

I want to compare two small (<=20) sets of integers (1..20) lexicographically.
The sets are represented by single integers, e.g.
1, 2, 4, 6
will be represented as
... 0 1 0 1 0 1 1
(... 7 6 5 4 3 2 1)
So where there's a 1 the number is present in the set.
Could someone verify if this code is correct?
bool less_than(unsigned a, unsigned b) {
unsigned tmp = a ^ b;
tmp = tmp & (~tmp + 1); //first difference isolated
return (tmp & a) && (__builtin_clz(b) < __builtin_clz(tmp));
}
The __builtin_clz part is for the case when b is a prefix of a.
The case of an empty set is handled elsewhere (__builtin_clz is undefined for 0).
EDIT:
bool less_than(unsigned a, unsigned b) {
unsigned tmp = a ^ b;
tmp &= -tmp; //first difference isolated
return ((tmp & a) && (__builtin_clz(b) < __builtin_clz(tmp)))
|| (__builtin_clz(a) > __builtin_clz(tmp));
}
and
bool less_than_better(unsigned a, unsigned b) {
unsigned tmp = a ^ b;
tmp &= -tmp; //first difference isolated
return ((tmp & a) && tmp < b) || tmp > a;
}
appear to be both correct.
(Tested versus a naive implementation using std::lexicographical_compare on tens of millions of randomized tests)
The second one is more portable though since it doesn't use __builtin_clz.
The difference in speed on my machine is negligible (the second one being ~2% faster), however on machines without __builtin_clz as one processor instruction (e.g. BSR on x86) the difference will probably be huge.
It's not correct in the case that a == 0. This should return true unless b == 0, but since tmp & a will be false regardless of the value of tmp (which will be the lowest-order 1-bit in b), the function will return false.
a should be "less than" b if:
1. `a` is a proper prefix of `b`, or
2. The lowest-order bit of `a^b` is in `a`.
The first condition also handles the case where a is the empty set and b is not. (This is slightly different from your formulation, which is "(The lowest-order bit of a^b is in a) and not (b is a proper prefix of a).)
A simple test of the case "a is a proper prefix of b", given the fact that we have the the lowest-order bit of a^b in tmp, is tmp > a. That avoids the use of __builtin_clz [Note 1].
Also, you could write
tmp = tmp & (~tmp + 1);
as
tmp &= -tmp;
but I think that most C compilers will find that optimization on their own. [Note 2].
Applying those optimizations, the result would be (untested):
bool less_than(unsigned a, unsigned b) {
unsigned tmp = a ^ b;
tmp &= -tmp; //first difference isolated
return tmp > a || tmp & a;
}
Notes
This is worth doing because (1) even though __builtin_clz is builtin, it is not necessarily super-fast; and (2) it may not be present if you're compiling with a compiler other than gcc or clang.
-tmp is guaranteed to be the 2s-complement negative of tmp if tmp is an unsigned type, even if the underlying implementation is not 2s-complement. See ยง6.2.6.2/1 (the range of an unsigned type is 0..2N-1 for some integer N) and &6.3.1.3/2 (a negative value is converted to an unsigned integer type by repeatedly adding 2N until the value is in range.
Here's a listing calculating all combinations for 2-bit inputs:
#include <stdio.h>
bool less_than(unsigned a, unsigned b) {
unsigned tmp = a ^ b;
tmp = tmp & (~tmp + 1); //first difference isolated
return (tmp & a) && (__builtin_clz(b) < __builtin_clz(tmp));
}
#define BITPATTERN "%d%d%d"
#define BYTETOBITS(byte) \
(byte & 0x04 ? 1 : 0), \
(byte & 0x02 ? 1 : 0), \
(byte & 0x01 ? 1 : 0)
int main(int argc, char** argv) {
for ( int a = 0; a < 4; a ++ )
for ( int b = 0; b < 4; b ++)
printf("a: "BITPATTERN" b: "BITPATTERN": %d\n",
BYTETOBITS(a), BYTETOBITS(b), less_than(a,b)
);
}
And here's the output:
a: 000 b: 000: 0
a: 000 b: 001: 0
a: 000 b: 010: 0
a: 000 b: 011: 0
a: 001 b: 000: 0
a: 001 b: 001: 0
a: 001 b: 010: 1
a: 001 b: 011: 0
a: 010 b: 000: 0
a: 010 b: 001: 0
a: 010 b: 010: 0
a: 010 b: 011: 0
a: 011 b: 000: 0
a: 011 b: 001: 0
a: 011 b: 010: 1
a: 011 b: 011: 0
It doesn't seem to look correct..

What's the insight behind A XOR B in bitwise operation?

What I know for A XOR B operation is that the output is 1 if A != B, and 0 if A == B. However, I have no insight about this operation when A and B are not binary.
For example, if A = 1, B = 3, then A XOR B = 2; also, if A = 2, B = 3, then A XOR B = 1. Is there any pattern to the XOR operation for non-binary values?
I have a good understanding of boolean mathematics, so I already understand how XOR works. What I am asking is that how do you, for example, predict the outcome of A XOR B without going through the manual calculation, if A and B are not binaries? Let's pretend that 2 XOR 3 = 1 is not just a mathematical artifact.
Thanks!
Just look at the binary representations of the numbers, and perform the following rules on each bit:
0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0
So, 1 XOR 3 is:
1 = 001
3 = 011
XOR = 010 = 2
To convert a (decimal) number to binary, repeatedly divide by two until you get to 0, and then the remainders in reverse order is the binary number:
To convert it back, repeatedly subtract it by the largest power of two that's no bigger than it until you get to 0, having each position in the binary number corresponding to the powers you subtracted by set to 1 (the left-most position corresponds to the 0-th power):
(Images reference)
xor on integers and other data is simply xor of the individual bits:
A: 0|0|0|1 = 1
B: 0|0|1|1 = 3
=======
A^B: 0|0|1|0 = 2
^-- Each column is a single bit xor
When you use bit operations on numbers that are more than one bit, it simply performs the operation on each corresponding bit in the inputs, and that becomes the corresponding bit in the output. So:
A = 1 = 00000001
B = 3 = 00000011
--------
result= 00000010 = 2
A = 2 = 00000010
B = 3 = 00000011
--------
result= 00000001 = 1
The result has a 0 bit wherever the input bits were the same, a 1 bit wherever they were different.
You use the same method when performing AND and OR on integers.

what this "if(k.c[3] & c)" part of code doing?

#include<stdio.h>
#include<iostream.h>
main()
{
unsigned char c,i;
union temp
{
float f;
char c[4];
} k;
cin>>k.f;
c=128;
for(i=0;i<8;i++)
{
if(k.c[3] & c) cout<<'1';
else cout<<'0';
c=c>>1;
}
c=128;
cout<<'\n';
for(i=0;i<8;i++)
{
if(k.c[2] & c) cout<<'1';
else cout<<'0';
c=c>>1;
}
return 0;
}
if(k.c[2] & c)
That is called bitwise AND.
Illustration of bitwise AND
//illustration : mathematics of bitwise AND
a = 10110101 (binary representation)
b = 10011010 (binary representation)
c = a & b
= 10110101 & 10011010
= 10010000 (binary representation)
= 128 + 16 (decimal)
= 144 (decimal)
Bitwise AND uses this truth table:
X | Y | R = X & Y
---------
0 | 0 | 0
0 | 1 | 0
1 | 0 | 0
1 | 1 | 1
See these tutorials on bitwise AND:
Bitwise Operators in C and C++: A Tutorial
Bitwise AND operator &
A bitwise operation (AND in this case) perform a bit by bit operation between the 2 operands.
For example the & :
11010010 &
11000110 =
11000010
Bitwise Operation in your code
c = 128 therefore the binary representation is
c = 10000000
a & c will and every ith but if c with evert ith bit of a. Because c only has 1 in the MSB position (pos 7), so a & c will be non-zero if a has a 1 in its position 7 bit, if a has a 0 in pos bit, then a & c will be zero. This logic is used in the if block above. The if block is entered depending upon if the MSB (position 7 bit) of the byte is 1 or not.
Suppose a = ? ? ? ? ? ? ? ? where a ? is either 0 or 1
Then
a = ? ? ? ? ? ? ? ?
AND & & & & & & & &
c = 1 0 0 0 0 0 0 0
---------------
? 0 0 0 0 0 0 0
As 0 & ? = 0. So if the bit position 7 is 0 then answer is 0 is bit position 7 is 1 then answer is 1.
In each iteration c is shifted left one position, so the 1 in the c propagates left wise. So in each iteration masking with the other variable you are able to know if there is a 1 or a 0 at that position of the variable.
Use in your code
You have
union temp
{
float f;
char c[4];
} k;
Inside the union the float and the char c[4] share the same memory location (as the property of union).
Now, sizeof (f) = 4bytes) You assign k.f = 5345341 or whatever . When you access the array k.arr[0] it will access the 0th byte of the float f, when you do k.arr[1] it access the 1st byte of the float f . The array is not empty as both the float and the array points the same memory location but access differently. This is actually a mechanism to access the 4 bytes of float bytewise.
NOTE THAT k.arr[0] may address the last byte instead of 1st byte (as told above), this depends on the byte ordering of storage in memory (See little endian and big endian byte ordering for this)
Union k
+--------+--------+--------+--------+ --+
| arr[0] | arr[1] | arr[2] | arr[3] | |
+--------+--------+--------+--------+ |---> Shares same location (in little endian)
| float f | |
+-----------------------------------+ --+
Or the byte ordering could be reversed
Union k
+--------+--------+--------+--------+ --+
| arr[3] | arr[2] | arr[1] | arr[0] | |
+--------+--------+--------+--------+ |---> Shares same location (in big endian)
| float f | |
+-----------------------------------+ --+
Your code loops on this and shifts the c which propagates the only 1 in the c from bit 7 to bit 0 in one step at a time in each location, and the bitwise anding checks actually every bit position of the bytes of the float variable f, and prints a 1 if it is 1 else 0.
If you print all the 4 bytes of the float, then you can see the IEEE 754 representation.
c has single bit in it set. 128 is 10000000 in binary. if(k.c[2] & c) checks if that bit is set in k.c[2] as well. Then the bit in c is shifted around to check for other bits.
As result the program is made to display the binary representation of float it seems.