What operation does the following ‘C’ statement perform?
star = star ^ 0b00100100;
(A) Toggles bits 2 and 5 of the variable star.
(B) Clears all bits except bits 2 and 5 of the variable star.
(C) Sets all bits except bits 2 and 5 of the variable star.
(D) Multiplies value in the variable star with 0b00100100.
I'm still clueless about this. Can someone help me out?
XOR operator (also called "logical addition") is defined like this:
a b a^b
-----------
0 0 0
0 1 1
1 0 1
1 1 0
So a^0 leaves a intact while a^1 toggles it.
For multiple-bit values, the operation is performed bitwise, i.e. between corresponding bits of the operands.
If you know how XOR works, and you know that ^ is XOR in C, then this should be pretty simple. You should know that XOR will flip bits where 1 is set, bits 2 and 5 of 0b00100100 are set, therefore it will flip those bits.
From an "during the test" standpoint, let's say you need to prove this to yourself, you really don't need to know the initial value of star to answer the question, If you know how ^ works then just throw anything in there:
00100100
^10101010 (star's made up value)
---------
10001110 (star's new value)
bit position: | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0
|---|---|---|---|---|---|---|---
star's new v: | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 0
|---|---|---|---|---|---|---|---
star's old v: | 1 | 0 | 1 | 0 | 1 | 0 | 1 | 0
Then check your answers again, did it:
(A) Toggles bits 2 and 5 of the variable star. (Yes)
(B) Clears all bits except bits 2 and 5 of the variable star. (Nope)
(C) Sets all bits except bits 2 and 5 of the variable star. (Nope)
(D) Multiplies value in the variable star with 0b00100100. (36x170 = 142? Nope)
It is (A) toggles bits 2 and 5.
The following is the truth table for the XOR operation:
x y x^y
0 0 0
1 0 1
0 1 1
1 1 0
You can see from the table that x XOR 0 = x and x XOR 1 = !x.
XOR is a bitwise operation, so it operates on individual bits. Therefore if you XOR star with some constant, it will toggle the 1 bits in the constant.
You can find some explanation e.g. here.
The exclusive OR has this truth table:
A B A^B
-----------
1 1 0
1 0 1
0 1 1
0 0 0
We can see that if B is true (1) then A is flipped (toggled), and if it's false (0) A is left alone. So the answer is (A).
XOR operator returns 0 if both inputs are same otherwise returns 1 if both inputs are different.For Example the Given Truth Table :-
a=1 b=1 => a^b=0,
a=0 b=0 => a^b=0,
a=0 b=1 => a^b=1,
a=1 b=0 => a^b=1.
well xor is binary operator that work on bits of 2 nos.
rule of xoring:for same bit ans is 0 and for different bit ans is 1
let
a= 1 0 1 0 1 1
b= 0 1 1 0 1 0
--------------
c= 1 1 0 0 0 1
--------------
compare bit of a and b bit by bit
if same put 0 else put 1
xor is basically used to find the unique in given set of duplicate no.
just xor all nos. and u will get the unique one(if only single unique is present)
Related
I want to check if the LSB is 0.
if(some_size_t & 1){} works fine
But why is if(some_size_t & 0){//This parts is unreachable} never reachable?
Because in order to ever get the value 1 i.e. true both operands for the logical and bitwise & i.e. AND operator have to be 1. Its so called truth table is
op1 | op2 | op1 AND op2
=====================
0 | 0 | 0
1 | 0 | 0
0 | 1 | 0
1 | 1 | 1
Because your value, e.g. op2, 0 has only zeros, no ones, you will always get only zeros as a result, no matter the other operand. And 0 will evaluate to false. It's what we call a contradiction in logic, often noted with a up side down T or \bot in latex.
As then the if condition is always false, the code in its body will never be executed, i.e. is unreachable.
I have a question: Is bitwise anding transitive, particularly in C and C++?
Say res=(1 & 2 & 3 & 4), is this same as res1=(1&2) and res2=(3&4) and
res= (res1 & res2). Will this be same?
Yes, bitwise AND is transitive as you've used the term.
It's perhaps easier to think of things as a stack of bits. So if we have four 4-bit numbers, we can do something like this:
A = 0xB;
B = 0x3;
C = 0x1;
D = 0xf;
If we simply stack them up:
A 1 0 1 1
B 0 0 1 1
C 0 0 0 1
D 1 1 1 1
Then the result of a bitwise AND looks at one column at a time, and produces a 1 for that column if and only if there's a 1 for every line in that column, so in the case above, we get: 0 0 0 1, because the last column is the only one that's all ones.
If we split that in half to get:
A 1 0 1 1
B 0 0 1 1
A&B 0 0 1 1
And:
C 0 0 0 1
D 1 1 1 1
C&D 0 0 0 1
Then and those intermediate results:
A&B 0 0 1 1
C&D 0 0 0 1
End 0 0 0 1
Our result is still going to be the same--anywhere there's a zero in a column, that'll produce a zero in the intermediate result, which will produce a zero in the final result.
The term you're looking for is associative. We generally wouldn't call a binary operator "transitive". And yes, & and | are both associative, by default. Obviously, you could overload the operators to be something nonsensical, but the default implementations will be associative. To see this, consider one-bit values a, b, and c and note that
(a & b) & c == a & (b & c)
because both will be 1 if and only if all three inputs are 1. And this is the operation that is being applied pointwise to each bit in your integer values. The same is true of |, simply replacing 1 with 0.
There are also some issues to consider if your integers are signed, as the behavior is dependent on the underlying bit representation.
Suppose I have a register(qs) of 3 qubits (first 2 being used solely for control, the last one is the input) . The first two control qubits are in the |+> state and the state of the 3rd input is unknown. Let it be a|0> + b|1>.
Now I apply CCNOT(qs[0],qs[1],qs[2]) so their combined state becomes 0.5(a,b,a,b,a,b,b,a) in Transposed matrix form [Please correct if I'm wrong here] . Now I apply S-gate to the 3rd qubit which transforms |1> -> i|1> .
I am unable to guess the state of the combined state of 'qs' now.
What I thought:
One logic is to multiply every state by 'i' if it has the form|XY1> so the combined state becomes 0.5(a,ib,a,ib,a,ib,b,ia) [Transposed]
Another logic is to find tensor product of (I x I x S) since I'm not changing the first 2 qubits. Performing this yields a different result which is 0.5(a,b,a,b,ia,ib,ib,ia) [Transposed] [Again, correct me if I'm wrong].
Which is the correct output after passing through S-gate (if any) ?
The first two qubits can't start in |+> state, since |+> is a single-qubit state. I assume that the starting state of the first two qubits in the register is 0.5 (|00> + |01> + |10> + |11>).
Both approaches are correct, because they are different ways to represent the same transformation. The first answer 0.5(a,ib,a,ib,a,ib,b,ia) [Transposed] is correct. Your second answer 0.5(a,b,a,b,ia,ib,ib,ia) [Transposed] seems to be obtained by multiplying by S x I x I, i.e., applying S gate on the first qubit instead of the third one.
The tensor product I x I x S can be calculated as tensor product of I x I (which is just a 4x4 identity matrix) and S. The result is an 8x8 matrix which consists of 16 copies of S matrix, multiplied by corresponding elements of I x I:
1 0 | 0 0 | 0 0 | 0 0
0 i | 0 0 | 0 0 | 0 0
- - - - - - - -
0 0 | 1 0 | 0 0 | 0 0
0 0 | 0 i | 0 0 | 0 0
- - - - - - - -
0 0 | 0 0 | 1 0 | 0 0
0 0 | 0 0 | 0 i | 0 0
- - - - - - - -
0 0 | 0 0 | 0 0 | 1 0
0 0 | 0 0 | 0 0 | 0 i
If you multiply the state of the qubits by this matrix, you'll get the same answer as in the first approach.
So I saw this code which printed out individual bits of any number.I do not understand why the individual bits are accessed and not the entire number itself
#include <stdio.h>
int main()
{
int x=10, b;
for(b=0; x!=0; x>>=1) {
printf("%d:%d\n", b, (x&1));
b++;
}
}
OUTPUT:
0:0
1:1
2:0
3:1
Please help me understand this piece of code.
In your code you are printing the value of X variable in binary. For this, your code, use logical operation as AND operator and right-shift.
In the loop condition, you displace the X variable one bit to the right.
for b = 0 you get x = 1010
for b = 1 you get x = 101
for b = 2 you get x = 10
for b = 3 you get x = 1
Then, in your print, show your loop iterator (b) and your X variable AND 1.
The AND operator get this values:
0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1
In your case, you have:
1010 AND (000)1 = 0
101 AND (00)1 = 1
10 AND (0)1 = 0
1 AND 1 = 1
There are two questions in your post: how to access an individual bit ? and how to select that bit ?
Concerning the first question, suppose you want to access the less significant bit (or, to make it simpler, the rightmmost bit), you can use a mask: suppose your data is b0011 for instance, you can mask with b0001 (i.e. 1 in decimal).
0 0 1 1
& 0 0 0 1
---------
0 0 0 1
The & operator is the bitwise and. If you look in your code sample, you have printf("%d:%d\n", b, (x&1)); in which you can see x & 1, i.e. print the rightmost bit of x.
Now comes the second question: how to put each bit in the rightmost position one after each other (said otherwise, how to select the bit to print) ? An easy solution is to shift your data of 1 position to the right each time you want to select the next bit (i.e. the bit to the left of the current one).
In C, you can shift using >>. For instance, if x is b0011, then x >> 1 is b0001 (in this case, you fill the leftmost position with zeros, but in some cases it might be trickier). If you look in you code sample, you have x>>=1 in the for-loop, which assigns x >> 1 in x.
Hence, suppose you take the previous example:
0 0 1 1 = x 0 0 0 1 = x
& 0 0 0 1 & 0 0 0 1
--------- x >> 1 = b0001 -> x ---------
0 0 0 1 0 0 0 1
and so one...
A last bonus point, the loop stopping condition is x != 0, this implies that you don't prints all bits of your data, but only the bits up to the leftmost 1 (included). For instance, in the above example, after printing the two rightmost bits, x becomes 0 and the loop exits.
I have been going over a perl book i have recently purchased, and while reading I noticed a block of code that confused me..
use integer;
$value = 257;
while($value){
unshift #digits, (0..9,a..f)[$value & 15];
$value /= 16;
}
print digits;
the book mentions the purpose was to reverse the order of digits. however, new to perl I am having trouble figuring out what [$value & 15] is doing.
It's a bitwise and operation.
What it's doing is performing a bitwise and using the value of 15 and whatever value is contained in $value.
The resulting value is the decimal value that corresponds to the result of a bitwise and with the lower 4 bits of the value.
Ex:
$value = 21
which has a binary representation of: 0b10101
Performing a bitwise and with 15 means that any bits in $value will be zeroed if they are either outside the lower 4 bit range, or contain no 1's in the lower 4 bits.
The result is:
0b10101
&
0b 1111
-------
0b00101 = 5
Looking up the truth tables for performing bitwise operations will help with stuff like this in the future, but when performing an AND with any value, the result is only true, when both bits are 1, 0 otherwise.
V1 | V2 | V1 & V2
-----------------
0 | 0 | 0
0 | 1 | 0
1 | 0 | 0
1 | 1 | 1