Question

The following Pseudo and JavaScript code is a extract from the implementation of a algorithm , i want to convert it to C++ .

Pseudo Code :

for b from 0 to 2|R| do
for i from 0 to |R| do
if BIT-AT(b, i) = 1 then // b’s bit at index i

JavaScript Code :

for (var b = 0; b < Math.pow(2, orders[r].length); b++) // use b's bits for directions
   {
   for (var i = 0; i < orders[r].length; i++)
    {
    if (((b >> i) & 1) == 1) {  // is b's bit at index i on? 

I don’t understand what is happening in the last line of this code , What Should be the C++ code for the above given JavaScript code . So far what i have written is :

for (int b = 0; b < pow(2, orders.at(r).size()); b++) 
{
  for (int i = 0; i < orders.at(r).size(); i++)
    {
     if (((b >> i) & 1) == 1)***//This line is not doing what it is supposed to do according to pseudo code*** 

The last line is giving me segmentation fault .

—
Edit:I apologize the problem was somewhere else , This code works fine .

Answer 1

(((b >> i) & 1) == 1)
     |     |
     |     |
     |    bitwise AND between the result of the shift and number 1.
     |
    shift b by i bits to the right

After that the result is compared with the number 1.

So if, for example, b is 8, and i is 2, it will do the following:

1. shift 8 (which is 00001000) by 2 bits to the right. The result will be 00000100.
2. apply the bitwise AND: 00000100 BITWISE_AND 00000001, the result will be 0.
3. Compare it with 1. Since 0 =/= 1, you will not enter that last if.

As for the logic behind this, the code ((b >> i) & 1) == 1) returns true if the bit number i of the b variable is 1, and false otherwise.

And I believe that c++ code will be the same, with the exception that we don’t have Math class in c++, and you’ll have to replace vars with the corresponding types.