How to retrieve first few bits from a bitset?

Question

If I have a bitset like:

std::bitset<8> bs = 00000101;

how can I only retrieve the bitset "101" from bs? To make things simpler, I already know I will need the first three bits.

With @Baum's help I have something like this so far:

std::bitset<8> bs = 00000101;
int off = 3; // the number of bits I would like
std::string offStr;  // final substring of bitset I wanted
for (std::size_t i = 0; i < off; ++i)
{
      offStr += bs[i];
}
return offStr; // resulting substring

Can you give us a higher-level picture of what you are trying to accomplish? Using bitmaps in this way is strange. — QuestionC, Jul 21 '15 at 15:27
@kmk09k: For your (deleted) answer, you probably want `'0' + bs[i]`. — Jarod42, Jul 21 '15 at 15:42

score 0 · Answer 1 · answered Jul 21 '15 at 16:31

It will work if you

put the right value in the bitset (i.e. you want 5_10 = 101_2 but you do octal 0000101_8 = 65_10), and
properly add the representation to your string.

Try the following:

std::bitset<8> bs(5);// = 00000101;
int off = 3; // the number of bits I would like
std::string offStr;  // final substring of bitset I wanted
for (std::size_t i = 0; i < off; ++i)
{
      offStr += (bs[i] ? "1" : "0");
}

Note, though, that if you are using std::string::operator+= the bits will be in the wrong order, so either change your loop, or better, pre-allocate the string and use operator[].

score 0 · Answer 2 · edited Feb 27 '21 at 05:09

0

You can try the following as well:

using namespace std;   
bitset<8> bs(5);// = 00000101;
int off = 3; // the number of bits I would like   
string offStr = bs.to_string().substr(8-off,off);  // final substring of bitset I wanted

edited Feb 27 '21 at 05:09

Sabito stands with Ukraine

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answered Feb 27 '21 at 03:03

Akhil Trivedi

1
1

Recommending the use of `using namespace std;` is very risky, here on Stack Overflow. See [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/q/1452721/10871073) I strongly suggest you edit/improve your code, or you are likely to attract downvotes. – Adrian Mole Feb 27 '21 at 20:15

paladin · Answer 3 · 2021-02-27T04:57:11.153

#include <iostream>

unsigned int clip(unsigned int bitset, const unsigned int offset){
    static const unsigned int integer_bitsize = sizeof(unsigned int) * 8;
    if(offset >= integer_bitsize) return bitset;
    bitset <<= (integer_bitsize - offset);
    bitset >>= (integer_bitsize - offset);
    return bitset;
}

int main(int argc, char *argv[]){
    int a = clip(5, 3);
    int b = clip(13, 3);
    std::cout << a << " " << b << std::endl;
    return 0;
}

Using the shift operator works great. Just make sure to use unsigned int.

How to retrieve first few bits from a bitset?

3 Answers3