2's Complement binary to decimal conversion

gbilios · #1 (**permalink**) March 11th, 2004, 10:16 PM

Can someone please help me work out how I can wite in C++, a function to convert a 2's complement binary number to its equivalent decimal number, which is in the range -128 to 127??

gbilios

George N. Bilios

davekw7x · #2 (**permalink**) March 12th, 2004, 12:30 PM

I'm not sure what you mean. Do you mean that you want to extract the decimal digits from a number that is the value of a variable in C or C++? Or what?

printf() in C or C++ (or cout << in C++ ) does the job. However, if this is a programming exercise, maybe you could give us a clue as to what you really want to do.

Dave

nikolai · #3 (**permalink**) March 12th, 2004, 03:56 PM

Sounds to me like you have a string representation of a binary number, e.g. "10010101", and you need to convert this to either an decimal integer (or a string representation of one).

Either way, it sounds like a homework assignment to me, so you should double check with your teacher and/or school to verify that it's okay for you to get help from these forums. Most schools/teachers restrict the places a student is allowed to get help for solving programming problems.

Take care,

Nik
http://www.bigaction.org/

gbilios · #4 (**permalink**) March 13th, 2004, 06:34 AM

This is what i want to do:
2's complement binary numbers are entered (exactly 8-bits) at the command line. Then
the decimal output will be between -128 to +127 rather than 0 to 255.

George N. Bilios

Babmi1960 · #5 (**permalink**) February 5th, 2009, 02:33 PM

If the number is positive then the binary representation is the number
eg 111 = 7, 101 = 5 etc ...
if the number is negative the most signigicant bit will be set to a one
if the word is a 16 bit integer then bit 15 will be set to 1 -

You need to flip all the bits that are one to 0 and all the bits that are 0 to 1's

eg: 1111 1111 1111 0110
flip 0000 0000 0000 1001 = 9

add 1
=10
and make it negative

-10

Or

1111 1111 1111 0110

Exclusive Or with

1111 1111 1111 1111

0000 0000 0000 1001

add 1

and make negative

Hope that helps!

bert · #6 (**permalink**) March 15th, 2009, 11:34 AM

Hope this helps

Code:

#include <iostream>
#include <cassert>
#include <iomanip>
#include <cstring>
using std::setw;
using std::cout;
using std::endl;

const int BITS = 8;

int convert(char binary[]) {
    int power = 128;
    int sum = 0;
    assert(strlen(binary) == 8);

    for ( int i=0; i<BITS; ++i) {
        if ( i==0 && binary[i]!='0') {
            sum = -128;
        }
        else {
            sum += (binary[i]-'0')*power;
        }
        power /= 2;
    }
    return sum;
}
int main()
{
    char bin1[BITS+1] = "00000011";
    cout <<"Binary: "<< bin1 <<endl<<"Dec:    "<< setw(8) << convert(bin1) << endl<< endl;
    
    char bin2[BITS+1] = "10000000";
    cout <<"Binary: "<< bin2 <<endl<<"Dec:    "<< setw(8) << convert(bin2) << endl<< endl;

    char bin3[BITS+1] = "01111111";
    cout <<"Binary: "<< bin3 <<endl<<"Dec:    "<< setw(8) << convert(bin3) << endl<< endl;

return 0
}

star5 · #7 (**permalink**) April 21st, 2010, 06:12 AM

can you please tell me the code to convert decimal to 2's complement ,thanks

RustyOnRampage · #8 (**permalink**) February 25th, 2013, 09:46 AM

thnx a lot, but I,m really confused with the purpose of (binary[i]-'0') in the following statement sum =sum+ (binary[i]-'0')*power;.