some questions about "bitset" class


A

arnuld

i am confused on some aspects of bitset class:

/* C++ Primer 4/e
* chapter 3
*
* exercise 3.23
*
*/

#include <iostream>
#include <string>
#include <bitset>

int main()
{
std::bitset<64> bitvec(32);
std::bitset<32> bv(1010101);

std::string bstr;
std::bitset<8> bsv(bstr);

std::cout << "std::bitset<64> bitvec(32): " << bitvec << std::endl;
std::cout << "std::bitset<32> bv(1010101): " << bv << std::endl;
std::cout << "std::bitset<8> bsv(bstr): " << bsv << std::endl;

std::cout << "\nprinting bits of \"std::bitset<64> bitvec(32)\": " <<
std::endl; for(size_t ix=0; ix != bitvec.size(); ++ix)
{
std::cout << bitvec[ix] << " ";
}

std::cout << std::endl;

return 0;
}


======== OUTPUT ============
{[email protected] cpp }% g++ -ansi -pedantic -Wall -Wextra ex_03-23.cpp
{[email protected] cpp }% ./a.out
std::bitset<64> bitvec(32):
0000000000000000000000000000000000000000000000000000000000100000
std::bitset<32> bv(1010101): 00000000000011110110100110110101
std::bitset<8> bsv(bstr): 00000000

printing bits of "std::bitset<64> bitvec(32)": 0 0 0 0 0 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0
{[email protected] cpp }%


(1) "std::bitset<64> bitvec(32)" is representing integer 32 in 64
bit-pattern, right?

compare (1) this with (2):

(2) in "bitset<n> b", "b" has n bits, each bit is zero. In "bitset<n>
b(u)", "b" is a copy of the unsigned long value u, then what is the use of
"n" ?


(3) the output of "std::bitset<32> bv(1010101)" has no "1010101"
bit-pattern in it. why ? Is it printing the "1010101" in 32 bit-pattern.

(4) "std::bitset<64> bitvec(32)" has 2 outputs, one is direct using
"std::cout" and another is using "for" loop. why the output is reversed in
these 2 cases? which one output is the correct representation of
bit-pattern ?
 
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G

Guest

i am confused on some aspects of bitset class:

/* C++ Primer 4/e
* chapter 3
*
* exercise 3.23
*
*/

#include <iostream>
#include <string>
#include <bitset>

int main()
{
std::bitset<64> bitvec(32);
std::bitset<32> bv(1010101);

std::string bstr;
std::bitset<8> bsv(bstr);

std::cout << "std::bitset<64> bitvec(32): " << bitvec << std::endl;
std::cout << "std::bitset<32> bv(1010101): " << bv << std::endl;
std::cout << "std::bitset<8> bsv(bstr): " << bsv << std::endl;

std::cout << "\nprinting bits of \"std::bitset<64> bitvec(32)\": " <<
std::endl; for(size_t ix=0; ix != bitvec.size(); ++ix)
{
std::cout << bitvec[ix] << " ";
}

std::cout << std::endl;

return 0;
}


======== OUTPUT ============
{[email protected] cpp }% g++ -ansi -pedantic -Wall -Wextra ex_03-23.cpp
{[email protected] cpp }% ./a.out
std::bitset<64> bitvec(32):
0000000000000000000000000000000000000000000000000000000000100000
std::bitset<32> bv(1010101): 00000000000011110110100110110101
std::bitset<8> bsv(bstr): 00000000

printing bits of "std::bitset<64> bitvec(32)": 0 0 0 0 0 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0
{[email protected] cpp }%


(1) "std::bitset<64> bitvec(32)" is representing integer 32 in 64
Yes

compare (1) this with (2):

(2) in "bitset<n> b", "b" has n bits, each bit is zero. In "bitset<n>
b(u)", "b" is a copy of the unsigned long value u, then what is the use of
"n" ?

Set u = 1, then there are lots of ways to store this value. You can use
a bitset with only 1 bit (std::bitset<1> b1(1);) or you can use 10 bits
(std::bitset<10> b10(1);). The first will have the binary representation
"1", the second "0000000001". The value supplied (u) is only the initial
value while the number of bits (n) determine the number of different
values possible to store.
(3) the output of "std::bitset<32> bv(1010101)" has no "1010101"
bit-pattern in it. why ? Is it printing the "1010101" in 32 bit-pattern.

Because the value (u) is a long, and the decimal number 1010101 does not
have the binary representation 1010101, but rather the one printed by
your code.
(4) "std::bitset<64> bitvec(32)" has 2 outputs, one is direct using
"std::cout" and another is using "for" loop. why the output is reversed in
these 2 cases? which one output is the correct representation of
bit-pattern ?

It depends on in which end of the output you put the most significant
digit. Normally we put the most significant digit to the left (first)
and the least significant digit to the right (last) (such as in the
number 1234, the digit telling how many thousands is to the left) and
this is what std::cout do. With your loop however you put the least
significant digit first and the most significant digit last.
 
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I

Ian Collins

arnuld wrote:

======== OUTPUT ============
{[email protected] cpp }% g++ -ansi -pedantic -Wall -Wextra ex_03-23.cpp
{[email protected] cpp }% ./a.out
std::bitset<64> bitvec(32):
0000000000000000000000000000000000000000000000000000000000100000
std::bitset<32> bv(1010101): 00000000000011110110100110110101
std::bitset<8> bsv(bstr): 00000000

printing bits of "std::bitset<64> bitvec(32)": 0 0 0 0 0 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0
{[email protected] cpp }%


(1) "std::bitset<64> bitvec(32)" is representing integer 32 in 64
bit-pattern, right?
Yes, the << operator outputs the bits left to right.
compare (1) this with (2):

(2) in "bitset<n> b", "b" has n bits, each bit is zero. In "bitset<n>
b(u)", "b" is a copy of the unsigned long value u, then what is the use of
"n" ?
Bits can be set or shifted into the hight positions.
(3) the output of "std::bitset<32> bv(1010101)" has no "1010101"
bit-pattern in it. why ? Is it printing the "1010101" in 32 bit-pattern.
You forgot to quote the string.
(4) "std::bitset<64> bitvec(32)" has 2 outputs, one is direct using
"std::cout" and another is using "for" loop. why the output is reversed in
these 2 cases? which one output is the correct representation of
bit-pattern ?
They both are, the former outputs the MSB first, in the latter, you
output the LSB first.
 

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