C++ using AND operator in integer expression - c++

I'm reading some source code for designing an Octree and found this in the code. I have removed some elements for simplication, but can anyone explain what i&4 is supposed to evaluate to?
for (int i = 0; i < 8; i++)
{
float j = i&4 ? .5f : -.5f;
}

& is the bitwise AND Operator.
It just does a bitwise operation of the value stored in i AND 0x4.
It exactly just isolates the third bit as 2^2 = 4.
Your expression in the loop checks if third bit is set in i and assigns to j (which must be a float!) 0.5 or if not set -0.5

I am not sure, but it may evaluate the bitwise and operation of i and 4 (100), so any number which has a '1' in its third bit will be evaluted to true, otherwise false.
Ex:
5 (101) & 4 (100) = 100 (4) which is different from 0 so its true
8 (1000) & 4 (100) = 0000 (0) which is false

The & operator in this case is a bitwise AND. Since the second operand is 4, a power of 2, it evaluates to 4 when i has its second least-significant bit set, and to 0 otherwise.
The for loop takes i from 0 to 7, inclusive. Consider bit representations of i in this range:
0000 - 0
0001 - 1
0010 - 2
0011 - 3
0100 - 4
0101 - 5
0110 - 6
0111 - 7
^
|
This bit determines the result of i & 4
Therefore, the end result of the conditional is as follows: if the second bit is set (i.e. when i is 4, 5, 6, or 7), the result is 0.5f; otherwise, it is -0.5f.
For the given range of values, this expression can be rewritten as
float j = (i >= 4) ? .5f : -.5f;

i & 4 just evaluate to true when the value-4 bit is set. In your case this only happens in the second half of the loop. So the code could actually be rewritten:
for (int i = 0; i < 4; i++)
{
float j = -.5f;
}
for (int i = 4; i < 8; i++)
{
float j = .5f;
}

Related

How can I simulate binary values using a vector of booleans in C++?

I want to be able to retain the same amount of bits to my vector whilst still performing binary addition. For example.
int numOfBits = 4;
int myVecVal = 3;
vector< bool > myVec;
GetBinaryVector(&myVec,myVecVal, numOfBits);
and its output would be:
{0, 0, 1, 1}
I don't know how to make a function of GetBinaryVector though, any ideas?
This seems to work (although the article I added in initial comment seem to suggest you only have byte level access):
void GetBinaryVector(vector<bool> *v, int val, int bits) {
v->resize(bits);
for(int i = 0; i < bits; i++) {
(*v)[bits - 1 - i] = (val >> i) & 0x1;
}
}
The left hand side sets the i'th least significant bit which is index bits - 1 - i. The right hand side isolates the i'th least significant bit by bit shifting the value down i'th bit and masking everything but the least significant bit.
In your example val = 8, bits = 15. In the first iteration i = 0: we have (*v)[15 - 1 - 0] = (8 >> 0) & 0x1. 8 is binary 1000 and shifting it down 0 is 1000. 1000 & 0x1 is 0. Let's jump to i = 4: (*v)[15 - 1 - 4] = (8 >> 4) & 0x1. 1000 >> 4 is 1 and 1 & 0x1 is 1, so we set (*v)[10] = 1. The resulting vector is { 0, ..., 0, 1, 0, 0, 0 }

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

What is the logic to use bitwise operation in generating subsequences? [duplicate]

This question already has answers here:
What does the statement if (counter & (1<<j)) mean and how does it work?
(2 answers)
Closed last year.
I have an array, A=[1,2,3,4] (where n=4). I want to generate sub-sequences from this array.
Number of subsequences is (2^n -1)
Run from counter 0001 to 1111
for (int counter = 1; counter < 2^n; counter++)
{
for (int j = 0; j < n; j++)
{
Check if the jth bit in the counter is set. If set then print jth element from arr[]
if (counter & (1<<j))
cout << arr[j] << " ";
}
cout << endl;
}
}
can anyone explain me? How it works " counter & (1<
The logic goes like this. Say, like you put in the example, you have n = 4, and, to avoid confusion, let's say the array is A = [5, 7, 8, 9], for example. Then you want all the possible sequences containing some elements (at least one) from the original array. So you want a sequence containing only the first one, a sequence containing the first and the second, the first and the third, etc. Each printed sequence may or may not contain each of the elements in the array. So you could see it as something like this:
| 5 | 7 | 8 | 9 | Sequence
----------------- --------
| 1 | 0 | 0 | 0 | -> 5
| 1 | 1 | 0 | 0 | -> 5 7
| 1 | 0 | 1 | 0 | -> 5 8
...
That is, each sequence can be expressed as a list of bits, each indicating whether each member of the array is included.
In this loop:
for (int counter = 1; counter < 2^n; counter++)
The program iterates from 1 to 2^n - 1, that is 15 in this case. So the values that you get for counter are:
Dec Binary
--- ------
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
10 1010
11 1011
12 1100
13 1101
14 1110
15 1111
If you look closely, you will see that we have all the possible sequences of four elements composed of 0 and 1 in binary (except 0000, which would be the empty sequence and does not interest us). In this loop:
for (int j = 0; j < n; j++)
The program just goes through each bit of counter, from 0 (the rightmost) to n - 1 and whenever it finds a 1 it outputs the corresponding array element. The condition:
if (counter & (1<<j))
Simply computes the number 1<<j which is 1 plus j number of zeros at its right (so, for example, for j = 0 it would be 1 and for j = 2 it would be 100) and then a bitwise and operation, so it basically "filters" the j-th bit only (e.g. 1101 & 100 = 0100), and if the result is not zero then it means there was a one, and so arr[j] must be printed.
Obviously, the problem with the function is that it is limited to the number of bits that the variable counter can hold. That depends on its declared type and your architecture/compiler, but typically it will be either 32 or 64.

What do "Non-Power-Of-Two Textures" mean?

What do "Non-Power-Of-Two Textures" mean? I read this tutorial and I meet some binaries operations("<<", ">>", "^", "~"), but I don't understand what they are doing.
For example following code:
GLuint LTexture::powerOfTwo(GLuint num)
{
if (num != 0)
{
num--;
num |= (num >> 1); //Or first 2 bits
num |= (num >> 2); //Or next 2 bits
num |= (num >> 4); //Or next 4 bits
num |= (num >> 8); //Or next 8 bits
num |= (num >> 16); //Or next 16 bits
num++;
}
return num;
}
I very want to understand this operations. As well, I read this. Very short article. I want to see examples of using, but I not found. I did the test:
int a = 5;
a <<= 1; //a = 10
a = 5;
a <<= 2; //a = 20
a = 5;
a <<= 3; //a = 40
Okay, this like multiply on two, but
int a = 5;
a >>= 1; // a = 2 Whaat??
In C++, the <<= is the "left binary shift" assignment operator; the operand on the left is treated as a binary number, the bits are moved to the left, and zero bits are inserted on the right.
The >>= is the right binary shift; bits are moved to the right and "fall off" the right end, so it's like a division by 2 (for each bit) but with truncation. For negative signed integers, by the way, additional 1 bits are shifted in at the left end ("arithmetic right shift"), which may be surprising; for positive signed integers, or unsigned integers, 0 bits are shifted in at the left ("logical right shift").
"Powers of two" are the numbers created by successive doublings of 1: 2, 4, 8, 16, 32… Most graphics hardware prefers to work with texture maps which are powers of two in size.
As said in http://lazyfoo.net/tutorials/OpenGL/08_non_power_of_2_textures/index.php
powerOfTwo will take the argument and find nearest number that is power of two.
GLuint powerOfTwo( GLuint num );
/*
Pre Condition:
-None
Post Condition:
-Returns nearest power of two integer that is greater
Side Effects:
-None
*/
Let's test:
num=60 (decimal) and its binary is 111100
num--; .. 59 111011
num |= (num >> 1); //Or first 2 bits 011101 | 111011 = 111111
num |= (num >> 2); //Or next 2 bits 001111 | 111111 = 111111
num |= (num >> 4); //Or next 4 bits 000011 | 111111 = 111111
num |= (num >> 8); //Or next 8 bits 000000 | 111111 = 111111
num |= (num >> 16); //Or next 16 bits 000000 | 111111 = 111111
num++; ..63+1 = 64
output 64.
For num=5: num-1 =4 (binary 0100), after all num |= (num >> N) it will be 0111 or 7 decimal). Then num+1 is equal to 8.
As you should know the data in our computers is represented in the binary system, in which digits are either a 1 or a 0.
So for example number 10 decimal = 1010 binary. (1*2^3 + 0*2^2 + 1*2^1 + 0*2^0).
Let's go to the operations now.
Binary | OR means that wherever you have at least one 1 the output will be 1.
1010
| 0100
------
1110
~ NOT means negation i.e. all 0s become 1s and all 1s become 0s.
~ 1010
------
0101
^ XOR means you turn a pair of 1 and 0 into a 1. All other combinations leave a 0 as output.
1010
^ 0110
------
1100
Bit shift.
N >> x means we "slide" our number N, x bits to the right.
1010 >> 1 = 0101(0) // zero in the brackets is dropped,
since it goes out of the representation = 0101
1001 >> 1 = 0100(1) // (1) is dropped = 0100
<< behaves the same way, just the opposite direction.
1000 << 1 = 0001
Since in binary system numbers are represented as powers of 2, shifting a bit one or the other direction will result in multiplying or dividing by 2.
Let num = 36. First subtract 1, giving 35. In binary, this is 100011.
Right shift by 1 position gives 10001 (the rightmost digit disappears). Bitwise Or'ed with num gives:
100011
10001
-------
110011
Note that this ensures two 1's on the left.
Now right shift by 2 positions, giving 1100. Bitwise Or:
110011
1100
-------
111111
This ensures four 1's on the left.
And so on, until the value is completely filled with 1's from the leftmost.
Add 1 and you get 1000000, a power of 2.
This procedure always generates a power of two, and you can check that it is just above the initial value of num.

How does condition statement work with bit-wise operators?

I tried to understand how if condition work with bitwise operators.
A way to check if a number is even or odd can be done by:
#include <iostream>
#include <string>
using namespace std;
string test()
{
int i = 8; //a number
if(i & 1)
return "odd";
else
return "even";
}
int main ()
{
cout << test();
return 0;
}
The Part I don't understand is how the if condition work. In this case if i = 8 then the in If statement it is doing 1000 & 1 which should gives back 1000 which equal 8.
If i = 7, then in if statement it should be doing 111 & 1 which gives back 111 which equal 7
Why is it the case that if(8) will return "even" and if(7) return "odd"? I guess I want to understand what the if statement is checking to be True and what to be False when dealing with bit-wise operators.
Just A thought when I wrote this question down is it because it's actually doing
for 8: 1000 & 0001 which gives 0
for 7: 0111 & 0001 which gives 1?
Yes, you are right in the last part. Binary & and | are performed bit by bit. Since
1 & 1 == 1
1 & 0 == 0
0 & 1 == 0
0 & 0 == 0
we can see that:
8 & 1 == 1000 & 0001 == 0000
and
7 & 1 == 0111 & 0001 == 0001
Your test function does correctly compute whether a number is even or odd though, because a & 1 tests whether there is a 1 in the 1s place, which there only is for odd numbers.
Actually, in C, C++ and other major programming languages the & operator do AND operations in each bit for integral types. The nth bit in a bitwise AND is equal to 1 if and only if the nth bit of both operands are equal to 1.
For example:
8 & 1 =
1000 - 8
0001 - 1
----
0000 - 0
7 & 1 =
0111 - 7
0001 - 1
----
0001 - 1
7 & 5 =
0111 - 7
0101 - 5
----
0101 - 5
For this reason doing a bitwise AND between an even number and 1 will always be equal 0 because only odd numbers have their least significant bit equal to 1.
if(x) in C++ converts x to boolean. An integer is considered true iff it is nonzero.
Thus, all if(i & 1) is doing is checking to see if the least-significant bit is set in i. If it is set, i&1 will be nonzero; if it is not set, i&1 will be zero.
The least significant bit is set in an integer iff that integer is odd, so thus i&1 is nonzero iff i is odd.
What you say the code is doing is actually how bit-wise operators are supposed to work. In your example of (8 & 1):
1000 & 0001 = 0000
because in the first value, the last bit is set to 0, while in the second value, the last bit is set to 1. 0 & 1 = 0.
0111 & 0001 = 0001
In both values, the last bit is set to 1, so the result is 1 since 1 & 1 = 1.
The expression i & 1, where i is an int, has type int. Its value is 1 or 0, depending on the value of the low bit of i. In the statement if(i & 1), the result of that expression is converted to bool, following the usual rule for integer types: 0 becomes false and non-zero becomes true.