CODEWITHSUNDEEP
c++binarytwos-complement
Areyana
Areyana

Reputation: 385

Convert negative binary number to decimal

For example:

string binaryValue = "11111111111111111111111111111011" // -5

I need to convert this string to decimal representatin of this number.

stoi(binaryValue, nullptr, 2)

Will throw exception on this case. So how can i do this in c++ ? String or int doesn't matter.

Upvotes: 2

Views: 3218

Answers (2)

nima moradi
nima moradi

Reputation: 2618

as you probably know number are stored as Twos Complement
to convert it using simple pseudocode

flip numbers 0->1, 1->0 from left to write util you find last 1 in string don't toggle this one

this will be your answer 0000000000000000000000000101=5


here is the code brouht from https://www.geeksforgeeks.org/efficient-method-2s-complement-binary-string/

#include<bits/stdc++.h>
using namespace std;


string findTwoscomplement(string str)
{


  int n = str.length();


// Traverse the string to get first '1' from
// the last of string
int i;
for (i = n ; i >= 0 ; i--)
    if (str[i] == '1')
        break;

// If there exists no '1' concat 1 at the
// starting of string
if (i == 0)
    return '1' + str;

// Continue traversal after the position of
// first '1'
for (int k = i-1 ; k >= 0; k--)
{
    //Just flip the values
    if (str[k] == '1')
        str[k] = '0';
    else
        str[k] = '1';
}

// return the modified string
return str;;
}



int main()
{
    string str = "11111111111111111111111111111011";
    cout << findTwoscomplement(str);
//now you convert it to decimal if you want
    cout<<"Hello World";
    cout << stoul( findTwoscomplement(str),nullptr,2);
        return 0;


     }

preview at https://onlinegdb.com/SyFYLVtdf

Upvotes: 0

Mike Kinghan
Mike Kinghan

Reputation: 61202

See the documentation of: 

int  std::stoi( const std::string& str, std::size_t* pos = 0, int base = 10 );

in particular:

The valid integer value [of str] consists of the following parts:

  • (optional) plus or minus sign

...

...

If the minus sign was part of the input sequence, the numeric value calculated from the sequence of digits is negated as if by unary minus in the result type.

Exceptions

  • std::invalid_argument if no conversion could be performed

  • std::out_of_range if the converted value would fall out of the range of the result type...

In the absence of a preceding minus-sign, the string:

std::string binaryValue = "11111111111111111111111111111011";

will be interpreted in a the call:

std::stoi(binaryValue, nullptr, 2);

as a non-negative integer value in base-2 representation. But as such, it is out of range, so std::out_of_range is thrown:

To represent -5 as a string that your std::stoi call will convert as you expect, use:

std::string const binaryValue = "-101";

Live demo

If you don't want to prefix a minus sign to a non-negative base-2 numeral, or cannot do so in your real-world situation, but wish to interpret "11111111111111111111111111111011" as the two's complement representation of a signed integer using the std::sto* API, then you must first convert the string to an unsigned integer of a wide-enough type, and then convert that unsigned value to a signed one. E.g.

#include <string>
#include <iostream>

int main()
{
    auto ul = std::stoul("11111111111111111111111111111011",nullptr,2);
    std::cout << ul << std::endl;
    int i = ul;
    std::cout << i << std::endl;
    return 0;
}

Live demo

Upvotes: 3

Related Questions