Unable to extract nibbles out of a word

Question

I am trying to extract 4-bits from a 16-bit binary string, i.e nibbles out of a word Can anyone tell me what is wrong with this program?

#include <sstream>
#include <iomanip>
#include <math.h>
#include<iostream>

using namespace std;

int main()
{
    std::string aBinaryIPAddress = "1100110011001100";

    std::string digit0 = aBinaryIPAddress & 0x0f;
    cout << "digit0 : " << digit0 << endl;

    std::string digit1 = (aBinaryIPAddress >>  4) 0x0f;
    cout << "digit1 : " << digit1 << endl;

    std::string digit2 = (aBinaryIPAddress >>  8) 0x0f;
    cout << "digit2 : " << digit2 << endl;

    std::string digit3 = (aBinaryIPAddress >> 12) 0x0f;
    cout << "digit3 : " << digit3 << endl;

    return 0;
}

I am getting the following error:

 changes.cpp: In function `int main()':
 changes.cpp:117: error: invalid operands of types `char*' and `int' to binary `
 operator>>'
 changes.cpp:117: error: parse error before numeric constant

Try using [`std::bitset`](http://en.cppreference.com/w/cpp/utility/bitset) or a binary literal. — David G, Nov 05 '13 at 14:12
What makes you think you can perform integer bit-wise arithmetic on an `std::string`? — Lightness Races in Orbit, Nov 05 '13 at 14:12
`aBinaryIPAddress` is not a binary string. it is a string of characters. — akonsu, Nov 05 '13 at 14:12
@akonsu: More than that, it's an object encapsulating the abstraction of a string of characters. — Lightness Races in Orbit, Nov 05 '13 at 14:12

Sergey Kalinichenko · Accepted Answer · 2013-11-05T14:39:51.580

2

If you are manipulating a string, you should be using substr, not "shift and mask" technique: & and >> operators are undefined for strings and ints.

Here is how to do it with substr:

#include <iostream>
#include <string>
using namespace std;

int main() {
    string aBinaryIPAddress = "0101100111101101";
    size_t len = aBinaryIPAddress.size();
    for (int i = 0 ; i != 4 ; i++) {
        cout << "Digit " << i << ": " << aBinaryIPAddress.substr(len-4*(i+1), 4) << endl;
    }
    return 0;
}

This prints

Digit 0: 1101
Digit 1: 1110
Digit 2: 1001
Digit 3: 0101

Demo on ideone.

If you need four individual variables, "unroll" the loop, like this:

string d0 = aBinaryIPAddress.substr(len-4, 4);
string d1 = aBinaryIPAddress.substr(len-8, 4);
string d2 = aBinaryIPAddress.substr(len-12, 4);
string d3 = aBinaryIPAddress.substr(len-16, 4);

edited Nov 05 '13 at 14:39

answered Nov 05 '13 at 14:20

Sergey Kalinichenko

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Thanks for this dude, but I am having a group of binary words i,e 16-bits number stored in a char pointer around 128-bits – Prats Nov 05 '13 at 14:26
So for each word I need to extract a nibble, it will not be possible to use sub string on each word – Prats Nov 05 '13 at 14:27
@Prats Why not? Make a `string` out of your `char*` (assuming that it is formatted correctly), and then proceed with `substr`. – Sergey Kalinichenko Nov 05 '13 at 14:28
How can I take each of these digit0, digit1, digit3,.. into variables each time I extract from a word, I need to do furthur calculations on these nibbles. – Prats Nov 05 '13 at 14:31
@Prats If you need to take them into individual variables, replace `i` with a number `0`, `1`, `2`, or `3`, and use `string d0 = aBinaryIPAddress.substr(len-4, 4)`, `string d1 = aBinaryIPAddress.substr(len-8, 4)`, `string d2 = aBinaryIPAddress.substr(len-12, 4)`, etc. – Sergey Kalinichenko Nov 05 '13 at 14:34
Could you please tell how? – Prats Nov 05 '13 at 14:37
Say, if I have two words i.e, 32-bits string, than will I have two for loops. One for extracting words and the other for extracting nibble? – Prats Nov 05 '13 at 14:45
@Prats You can do it this way, but that is not necessary: you can continue on with the sequence of `substr` arguments - `len-16`, `len-20`, `len-24`, `len-28`, and `len-32` to extract all nibbles without an outer loop. – Sergey Kalinichenko Nov 05 '13 at 14:53
Thanks dude for giving a solution, I was thinking the other way – Prats Nov 05 '13 at 14:58
Also, is it safe extracting nibbles using string operations or using bitwise operators? – Prats Nov 05 '13 at 14:58
Hey, what if I want to extract nibbles from first not from the last? – Prats Nov 05 '13 at 15:02
1

@Prats Then instead of going backwards (i.e. `substr(len-4, 4)`, `substr(len-8, 4)`, ...) you'd go forward (i.e. `substr(0, 4)`, `substr(4, 4)`, `substr(12, 4)`, and so on). – Sergey Kalinichenko Nov 05 '13 at 15:07

score 0 · Answer 2 · answered Nov 05 '13 at 14:12

You can not convert from an integer to string the way you do. You can use stringstream:

#include <iomanip>
......
std::string convert_int_to_hex_stiring(int val) {
  std::stringstream ss;
  ss << std::hex << val;
  return val;
}

Before you use the function above you will have to read integers from parts of the input string. Problem is that the bitwise operations are not defined on a string.

score 0 · Answer 3 · answered Nov 05 '13 at 14:13

0

the type of aBinaryIPAddress should be a number rather than a string

something like

unsigned int aBinaryIPAddress = 0b1100110011001100;

should work

answered Nov 05 '13 at 14:13

Alix Martin

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1
5

does'nt works, gives following error: error: invalid conversion from 'unsigned int' to 'const char*' [-fpermissive] – Prats Nov 05 '13 at 14:20
the types of digit0123 also need to be changed – Alix Martin Nov 05 '13 at 14:48

score 0 · Answer 4 · answered Nov 05 '13 at 14:16

You have a std::string object that holds 16 characters, each with the value '0' or '1'. If you want to look at "nibbles" taken from there, just pull out the 4-character groups that you need:

std::string digit0 = aBinaryIPAddress.substr(12,4);
std::cout << digit0 << '\n';

That's just text manipulation; if you want to get values, it's straightforward (and a useful exercise) to convert the characters in digit0 into a numeric value..

score -1 · Answer 5 · answered Nov 05 '13 at 14:14

-1

It is obvious that the problem is in statements like this

std::string digit0 = aBinaryIPAddress & 0x0f;

Do you understand that std::string is not a number?!

answered Nov 05 '13 at 14:14

Vlad from Moscow

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Yes I understand that string is not a number, but how to use it for extracting a nibble – Prats Nov 05 '13 at 14:22

Unable to extract nibbles out of a word

5 Answers5